Programming Fundamentals

Transcription

1 Programming Fundamentals 1

2 Session Plan Introduction to Data Structures Introduction to Arrays Arrays and Functions Introduction to Strings Strings and Functions Handling Multidimensional Arrays 2 Today's session we will be discussing the various data structures 2

3 Data structure Data Struture is the logical or mathematical model of a particular organization of data mirror the actual relationships of the data Examples of data structures: Arrays linked lists Stacks Queues trees c 3 Data structures are "an organization of information, usually in memory, for better algorithm efficiency." 1 Examples of data structures are arrays, queues, dictionaries, hash tables, and collections. The key component of this definition is related to "algorithm efficiency." What does a data structure have to do with algorithm efficiency? As you will see in this set of articles, plenty. Arrays serve as a good starting point for demonstrating how data structure choice affects algorithm choice, and vice versa The logical or mathematical model of a particular organization of data is called a data structure. While solving a programming problem, once the algorithm has been developed, the next step is to determine the data structures to be used to implement the algorithm. Two things should be considered in selecting a data structure. 1. The structure should mirror the actual relationships of the data in the real world. 2. The structure should be simple so that processing data is effective. Data structure are classified as either linear or nonlinear. A data structure is said to be linear if its data have a linear relationship. One way of achieving the linear relationship in a data structure is to represent the data in sequential memory locations. These linear structures are called arrays. 3

4 Data structure.. contd... A data structure is linear if elements are accessed in a single direction Example: Arrays Lists Stacks strings A data structure is nonlinear if its elements are accessed through different directions Example: Trees : Are used to represent data containing a hierarchical relationship between elements 4 4

5 How to select a data structure? Two things should be considered in selecting a data structure The structure should mirror the actual relationships of the data in the real world. The structure should be simple so that processing data is effective. 5 5

6 Arrays Arrays are linear data structures The linear relationships between the elements is represented by means of sequential memory locations. 6 A very familiar concept to most any programmer is that of an array, and they are a common part of our everyday programming lives. We use them so often that we probably forget what they really are. Arrays are a data structure that is used to store a group of objects of the same type sequentially in memory. All the elements of an array must be the same data type. Arrays are "a set of items which are randomly accessible by numeric index." Arrays are typically laid out in memory as a contiguous chunk of blocks, where each block is of the same data type. Because the memory is laid out as a contiguous set of memory chunks, arrays are very fast for accessing items by index. Arrays are a great choice of data structure when the number of elements it contains is known ahead of time. An array is a fixed collection of same data-type that are stored contiguously and are accessible by an index. What this means is that an array consists of a set of physically contiguous memory locations. These memory locations can all be addressed with one name the name of the array. Each location in the array is referred to by the subscript of the array. In C, the syntax to declare an array is as follows. int aiarrayofintegers[10]; This statement declares an integer array named aiarrayofintegers, consisting of 10 elements. Each of these 10 elements can only contain an integer value. The index of the first element of an array in C is 0. That is, the ten elements of the array can be referenced as aiarrayofintegers[0], aiarrayofintegers[1], aiarrayofintegers[2],..., aiarrayofintegers[9]. These values may be accessed as shown below. ivar1 = aiarrayofintegers[0]; Note that in the declaration statement, aiarrayofintegers[10]means that there are 10 elements in the array; but in an assignment statement, aiarrayofintegers[10]refers to the 11th element of a 10-element array. That is, if there are ten elements in an array, the subscript of the last element of the array is 9, and not 10. This is a common programming mistake and results in some indeterminate value being returned by aiarrayofintegers[10], which is very likely to cause program failure. In the declaration statement, the subscript can be a named constant; however, it cannot be a variable. 6

7 Example Read the marks of 2 subjects; find their average; print the marks of that subject as queried by user. 7 write the program. #include <stdio.h> int main() int imark1, imark2, inumber; float faveragemarks; printf("\nenter marks for the subject1 : "); scanf("%d", &imark1); printf("\nenter marks for the subject2 : "); scanf("%d", &imark2); faveragemarks = (imark1 + imark2) / 2.0; printf("\naverage is: %f", faveragemarks); printf("\nenter subject number whose marks you want"); scanf("%d", &inumber); if (inumber == 1) printf("\nmarks of subject 1 is: %d", imark1); else if (inumber == 2) printf("\nmarks of subject 2 is: %d", imark2); else printf("\ninvalid Number"); 7

8 Array Sequence of elements A R R A Y In d iv id u al d ata o b jects o f id entical n atu re : : : : : : : : Indexing Store an element Retrieve an element 8 Now lets try and extend the previous problem to accepts marks in five subjects for a student and display the average. It should then be able to display marks for any subject that the user wants. Consider solving this problem without the use of arrays. #include <stdio.h> main() int imark1;int imark2;int imark3;int imark4;int ichoice; float faverage; printf("\n Enter the four marks"); scanf("%d%d%d%d",&imark1,&imark2,&imark3,&imark4); faverage = (imark1 + imark2 + imark3 +imark4)/4.0; printf("\n Average mark is %f", faverage); printf("\n Do you want to see the Mark? ); printf("\n 1. Mark1"); printf("\n 2. Mark2"); printf("\n 3. Mark3"); printf("\n 4. Mark4"); printf("\n scanf("%d", &ichoice); switch (ichoice) Enter your choice"); case 1: printf("\n Mark1 = %d", imark1); break; case 2: printf("\n Mark2 = %d", imark2); break; case 3: printf("\n Mark3 = %d", imark3); break; case 4: printf("\n Mark4 = %d", imark4); break; default: printf( \n Invalid choice. Please enter your choice again ); break; 8

9 Array -Contd..) Please refer to Notes page for more explanation on previous slide 9 Consider the Previous program written using array. An array is defined with this syntax. datatype arrayname[size]; int ID[30]; /* Could be used to store the ID numbers of students in a class */ float temperatures[31]; /* Could be used to store the daily temperatures in a month */ unsigned short int[52]; /* Holds 52 unsigned short integer values */ #include <stdio.h> main() int aimarksarray[4]; int ichoice; int Counter; int isum; isum = 0; ichoice = 0; printf("\n Enter the five marks"); fflush(stdin); for (Counter = 0; Counter < 4; Counter = Counter + 1) scanf("%d ", &aimarksarray[counter]); isum = isum + aimarksarray[counter]; printf("\naverage mark is %f", isum/4.0); do printf("\n Do you want to see the Mark? ); printf("\n 1. Mark1"); printf("\n 2. Mark2"); printf("\n 3. Mark3"); printf("\n 4. Mark4"); printf("\n Enter your choice"); fflush(stdin); scanf("%d", &ichoice); while (ichoice < 1 ichoice > 4); printf("\n Mark%d = %d",ichoice, aimarksarray[ichoice-1]); 9

10 Array-(Contd..) Please refer to Notes page for more explanation on previous slide 10 Note how the use of an array makes input, processing and output easier. Since an array consists of physically contiguous elements that can all be addressed using the name of the array and a subscript, manipulating the values in an array is not only more efficient, it is also more intuitive compared to working with a number of independent variables. When you declare an array, you declare the type of data it will hold and the size of the array. Once the memory for the array is allocated, it cannot be changed. If you want to set the value for an item in the array, you reference a pre-existing index in the array and specify its contents.if you want to increase the size of the array, you must create a new array with size n+1 and copy the contents of the old array into the new one 10

11 Example Read the marks of students in a class and then print the number of students for each possible total marks. marks are in the range Using Arrays Arrays in C are zero based. Suppose an array named myexample contains N elements. This array is indexed from 0 to (N-1). The first element of myexample is at index 0 and is accessed as myexample[0]. The second element is at index 1 and is accessed as myexample[1]. The last element is at index (N-1) and is accessed as myexample[n-1]. As a concrete example, suppose N equals 5 and that myexample will store integer data. int myexample[5]; /* Defines myexample to be of length 5 and to contain integer data */ myexample[0] /* First element of myexample */ myexample[1] /* Second element of myexample */ myexample[2] /* Third element of myexample */ myexample[3] /* Fourth element of myexample */ myexample[4] /* Fifth and final element of myexample */ Here is a sample program that calculates and stores the squares of the first one hundred positive integers. #include <stdio.h> int main() int square[100]; int i; /* loop index */; int k; /* the integer */ /* Calculate the squares */ for (i = 0; i < 100; i++) k = i + 1; /* i runs from 0 to 99, k runs from 1 to 100 */ square[i] = k*k; printf("the square of %d is %d\n",k,square[i]); return 0; 11

12 Solution #include <stdio.h> int main( ) int aiarrhist[101]; int isize; int icount; int imarks; printf("how many students are there?"); scanf("%d", &isize); for(icount = 0; icount < 101; icount++) iarrhist[icount] = 0; 12 You cannot initialize an array using a variable. For example: int x = 5; int ia[x]; This above example is illegal. C restricts the array initialization size to be constant. So is this legal? int ia[]; No. The array size is not known at compile time. How can we get around this? By using macros we can also make our program more readable! #define MAX_ARRAY_SIZE 5 /*... code... */ int ia[max_array_size]; Now if we wanted to change the array size, all we'd have to do is change the define statement! We can initialize the contents of the array int ia[5] = 0, 1, 3, 4; int ia[ ] = 0, 2, 1; Both of these work. The first one, ia is 20 bytes long with 16 bytes initialized to 0, 1, 3, 4. The second one is also valid, 12 bytes initialized to 0, 2, 1. Arrays are static memory allocated data structures. 12

13 Solution (contd ) for(icount = 0; icount < isize; icount++) scanf("%d", &imarks); aiarrhist[imarks] = aiarrhist[imarks] + 1; for(icount = 0; icount < 101; icount++) printf("\nthe no. of students with marks %d are %d", icount, aiarrhist[icount]); 13 13

14 Example Insert a element in the array 14 #include <stdio.h> void fvacceptarrvalues(int [],int); void fvinsarrelement(int aiarrdata[],int ipos,int inoofelements,int inewnumber); void fvdisparr(int aiarrdata[],int inoofelenents); int icount; int main( ) int aiarrdata[100]; int inoofelementsinarray; int inewnumber; int ipos; printf("how many numbers are there "); scanf("%d", &inoofelementsinarray); fvacceptarrvalues(aiarrdata,inoofelementsinarray); printf("enter the new number "); scanf("%d", &inewnumber); printf("where do you want to insert"); scanf("%d", &ipos); fvinsarrelement(aiarrdata,ipos,inoofelementsinarray,inewnumber) printf("the new data is:"); fvdisparr(aiarrdata,inoofelementsinarray); 14

15 Inserting Elements - Solution printf("how many numbers are there "); scanf("%d", &inoofelementsinarray); fvacceptarrvalues(aiarrdata,inoofelementsinarray); printf("enter the new number "); scanf("%d", &inewnumber); printf("where do you want to insert"); scanf("%d", &ipos); fvinsarrelement(aiarrdata,ipos,inoofelementsinarray,inewnumber); printf("the new data is:"); fvdisparr(aiarrdata,inoofelementsinarray); 15 Like simple value and variable, it is also possible to pass the value of an array element and even an entire array as an argument to a function. To pass an array element to a function, the array element is specified as an argument to the function in the normal fashion. Let us revisit our earlier example of finding average marks for student. We rewrite the example using the array variable. #include <stdio.h> float fgetaverage (int inum1, int inum2, int inum3); void main() int i; int aiarray[3]; float faverage; for (i = 0; i < 3; i++) scanf("%d", &aiarray[i]); faverage = fgetaverage(aiarray[0], aiarray[1], aiarray[2]); printf("average is %f", faverage); float fgetaverage (int inum1, int inum2, int inum3) float favg; favg = (inum1 +inum2 +inum3) /(3.0); return favg; 15

16 Inserting Elements Solution-(Contd..) Please refer to Notes page for more explanation on previous slide 16 In the above example, we have passed the array elements in the same fashions as we did for simple variables in our earlier example However, the above example can also be written more succinctly as shown below by passing the whole array to the function fgetaverage instead of passing each element of the array separately. #include <stdio.h> #define SIZE 3 float fgetaverage (int aiarr[], const int N); void main() int i; float faverage; int aiarray[size]; printf(" Please enter %d numbers",size); fflush(stdin); for (i = 0; i < SIZE; i++) scanf("%d", &aiarray[i]); faverage = fgetaverage(aiarray,size); printf("average is %f", faverage); float fgetaverage (int aiarr[], const int N) int isum; int i; float favg; isum = 0; for (i = 0; i < N; i++) isum = isum + aiarr[i]; favg = isum /(3.0); return favg; 16

17 Inserting Elements Solution-(Contd..) Please refer to Notes page for more explanation on previous slide 17 Observe the prototype declaration, function definition and function call for fgetaverage in the above program. For passing an entire array to a function, it is only required to list the name of the array, without any subscript, in the function call. In the function prototype and definition there is no mention about the size of the array. 17

18 Inserting Elements Solution-(Contd..) Please refer to Notes page for more explanation on previous slide 18 However, there is a major distinction that must always be kept in mind while dealing with array arguments. Before discussing further let us look at the following simple example. Consider an array of integers of size 10. We initialize all the elements of the array to 0. Then we change the value of those array elements to 1 who are occupying the even position in the array, i.e., with index 1, 3 5, 7, 9. Finally, print all the elements of the array after reinitialization. The program can be written as below. #include <stdio.h> # define SIZE 10 main( ) int i; int aiarray[size]; for (i = 0; i < SIZE; i++) aiarray[i] = 0; //initialization for (i = 0; i < SIZE; i++) if ( i %2!= 0) aiarray[i] = 1; //re-initialization for (i = 0; i < SIZE; i++) printf( array[%d] = %d \n, i, aiarray[i] ); 18

19 Inserting Elements Solution-(Contd..) Please refer to Notes page for more explanation on previous slide 19 Let us rewrite the above example using a self-explanatory function where the entire array has been passed in the argument. #include <stdio.h> # define SIZE 10 void vre_initialize( int aiarr[], int n) int i; for (i = 0; i < n; i++) if ( i %2!= 0) aiarr[i] = 1; main( ) int i; int aiarray[size]; for (i = 0; i < SIZE; i++) aiarray[i] = 0; //initialization vre_initialize(aiarray, SIZE); for (i = 0; i < SIZE; i++) printf( array[%d] = %d \n, i, aiarray[i]); The output of the above program after its execution demonstrates one major deviation from what we have been seeing so far. That is, the function vre_initialize changes the value of the original array aiarray that has been passed to it as argument. This change remains in effect even after the function vre_initialize has completed its execution and has returned to the calling routine (i.e., main( )) The following points have to be kept in mind when dealing with array arguments. In a function call (in C language), the values that are passed as arguments to the function are copied (known as Pass by value) into the formal parameters. 19

20 Inserting Elements Solution-(Contd..) Please refer to Notes page for more explanation on previous slide 20 However, when the entire array is passed as arguments, the entire contents of the array are NOT copied into the formal parameter array of the function. Instead, the formal parameter array holds the information describing WHERE in the computer s memory the actual array is located (known as Pass by Reference). Thus, any changes made to the formal parameter array by the function are reflected on the actual array passed to the function. Hence, when the function returns, the changes still remain in effect. Note: The preceding discussion applies only to the entire arrays that are passed as arguments, and not to the individual elements, whose values are copied into the corresponding formal parameters and therefore cannot be changed by the function. 20

21 Inserting Elements Solution-(Contd ) void fvacceptarrvalues(int aiarrdata[],int inoofelementsinarray) for(icount = 0; icount < inoofelementsinarray; icount++) scanf("%d", &aiarrdata[icount]); void fvinsarrelement(int aiarrdata[],int ipos,int inoofelementsinarray,int inewnumber) for(icount = inoofelementsinarray - 1; icount >= ipos-1; icount--) aiarrdata[icount+1] = aiarrdata[icount]; aiarrdata[ipos -1] = inewnumber;inoofelementsinarray = inoofelementsinarray +1; void fvdisparr(int aiarrdata[],int inoofelementsinarray) for(icount = 0; icount < inoofelementsinarray; icount++) printf("%d",aiarrdata[icount]); 21 When an array is passed into a function, the function receives not a copy the array, but instead the address of the first element of the array. The function receives a pointer to the start of the array. Any modifications made via this pointer will be seen in the calling program. Let's see how this works. Suppose the main program has an array of 10 integers and that a function has been written to double each of these. void doublethem(int a[], int size); int main() int myints[10] = 1,2,3,4,5,6,7,8,9,10; doublethem(myints, 10); return 0; void doublethem(int a[], int size) int i; for (i = 0; i < size; i++) a[i] = 2 * a[i]; Hence entire Arrays are passed by reference by default. But when individual array elements are passed as argument they could be passed by ref / by argument. 21

22 Inserting Elements Solution-(Contd ) Please refer to Notes page for more explanation on previous slide 22 Consider another example #include <stdio.h> #include <stdio.h> void fvchange(int aiarr1[],int aiarr20,int *aiarr21); main() int aiarr1[2],aiarr2[2]; aiarr1[0]=1; aiarr1[1]=2; aiarr2[0]=3; aiarr2[1]=4; printf("arr1 Elements %d-%d",aiarr1[0],aiarr1[1]); printf("\narr2 Elements %d-%d",aiarr2[0],aiarr2[1]); printf("\n"); fvchange(aiarr1,aiarr2[0],&aiarr2[1]); printf("arr1 Elements %d-%d",aiarr1[0],aiarr1[1]); printf("\narr2 Elements %d-%d",aiarr2[0],aiarr2[1]); void fvchange(int aiarr1[],int aiarr20,int *aiarr21) aiarr1[0]=5; aiarr1[1]=6; aiarr20=7; *aiarr21=8; In the example aiarr1 array is passed by reference,aiarr2[0] element is passed by value and aiarr2[1] element is passed by reference. 22

23 Arrays and Pointers int ia[6] = 0, 1, 2, 3, 4, 5; int *ip; ip = ia; /* equivalent to ip = &ia[0]; */ ip[3] = 32; ia 100 ia[0] ia[1] ia[2] ia[3] ia[4] ia[5] ip An array name is just a pointer to the beginning of the allocated memory space. Let's take this example and analyze it: int ia[6] = 0, 1, 2, 3, 4, 5; /* 1 */ int *ip; /* 2 */ ip = ia; /* equivalent to ip = &ia[0]; */ /* 3 */ ip[3] = 32; /* equivalent to ia[3] = 32; */ /* 4 */ ip++; /* ip now points to ia[1] */ /* 5 */ printf("%d ", *ip); /* prints 1 to the screen */ /* 6 */ ip[3] = 52; /* equivalent to ia[4] = 52 */ /* 7 */ Ok, so what's happening here? Let's break this down one line at a time. Refer to the line numbers on the side: 1.Initialize ia 2.Create ip: a pointer to an int 3.Assign ip pointer to ia. This is effectively assigning the pointer to point to the first position of the array. 4.Assign the fourth position in the array to 32. But how? ip is just a pointer?!?! But what is ia? Just a pointer! 5.Use pointer arithmetic to move the pointer over in memory to the next block. Using pointer arithmetic automatically calls sizeof(). 6.Prints ia[1] to the screen, which is 1 7.Sets ia[4] to 52. Why the fifth position? Because ip points to ia[1] from the ip++ line. Now it should be clear. Pointers and arrays have a special relationship because arrays are actually just a pointer to a block of memory! Point to be noted here is that Arrays are constant pointers. 23

24 Coping with inflexibilities of Arrays define constant for size recompile define a very big array wastage of space 24 Because names of arrays represents just a pointer to the beginning of the array, we have some limitations or "problems." 1.No Array Out of Bounds Checking. For example: int ia[2] = 0, 1; printf("%d ", ia[2]); The above code would result in runtime error, because you are trying to look at an area of memory not inside the array memory allocation. 2.Array Size Must be Constant or Known at Compile-time 3.Arrays Cannot be Copied or Compared. Why? Because they are pointers. 4.Array Index Type must be Integral. Another limitation comes with arrays being passed into functions. Take for example: void func(int ia[]) void func(int *ia) Both are the same declaration (you should know why by now). But why would this cause problems? Because only the pointer to the array is passed in, not the whole array. So what if you mistakenly did a sizeof(ia) inside func? Instead of returning the sizeof the whole array, it would only return the size of a single element in the array. 24

25 Array as a data structure very fast at storing and getting from a particular index Slow in inserting and deleting elements Alternatives are there linked lists trees a a a NULL Address of third node 2 nd data element 25 Although arrays are extremely useful, one of the major disadvantages of using an array is insertion (deletion) of an element to (from) the array. Consider inserting an element in the beginning of an array (assume that enough memory is available for insertion). For achieving this, the value in every element must be moved to its subsequent element. Then the new value is inserted into the first position. Imagine such situation when the array size is large enough! Similar is the case for deleting an element from an array. An alternative is to use linked lists. A linked list(a linear data structure) consists of a series of nodes such that, each node has a data element and the address of the next node in the list. The following figure gives a schematic view of a link list with 5 nodes. The first node contains the information of element 1 and address of element 2, the second node contains the information of element 2 and address of node 3 and so on. To insert or delete a new node, only two nodes at most (the ones just before in case it is inserted after the first node and just after in case it is inserted before the last node) have to be modified suitably. 25

26 Strings Text processing word processors Searching databases Input/output 26 String (in C) is referred to a variable-length array of characters, defined by a starting point and by a string-termination character ( \0 ) marking the end. In computing applications where processing of textual data are involved, strings are useful in representing these textual data and hence, are valuable in low-level data structure. The difference between an array of characters and a string revolves around length. Both represents a contiguous area of memory, but the length of the array is set at the time that array is created, whereas the length of a string may change during the execution of a program. We need to reserve memory for a string, either during compile time by declaring an array of characters (will be followed in this course) or at execution time by invoking dynamic memory allocation function (beyond the scope of the course). Once the array is allocated, we can fill it with characters, starting at the beginning and ending with the string termination character ( \0 )or popularly known as the NULL character(i.e., having value 0). Without a string termination character, the string is same as an array of characters. Only with the string terminating character, we can consider the portion of the array from the beginning to the string termination character to contain meaningful information. 26

27 Definitions of strings char name[4] = xy ; Will create a string consisting of 4 memory locations. 1 st element stores x 2 nd element stores y 3 rd element stores \0 ; 4 th element is unused char n1[] = xy ; Will create a string consisting of 3 memory locations. 1 st element stores x 2 nd element stores y 3 rd element stores \0 ; char a[5]='h','e','l','l','o'; Will create a character array consisting of 5 memory locations. 27 Stings in C are stored as null character, '\0', terminated character arrays. This means that the length of a string is the number of characters it contains plus one to store the null character. Common string operations include finding lengths, copying, searching, replacing and counting the occurrences of specific characters and words. A string is just a character array with the convention that the end of the valid data is marked by a null '\0'. Now you should be able to see why you can read in a character string using scanf("%s", name) rather than scanf("%s",&name) - name is already a pointer variable. Manipulating strings is very much a matter of pointers and special string functions. For example, the strlen(str) function returns the number of characters in the string str. It does this simply by counting the number of characters up to the first null in the character array - so it is important that you are using a valid null-terminated string. Indeed this is important with all of the C string functions 27

28 Input/output of strings char yourname[20] scanf("%s",yourname); scanf( %s,&yourname[0]); /* yourname is a character array. yourname[0] specifies the first element of the array. &yourname[0] specifies the address of the first element of the array. In C, an array name by itself is shorthand for the address of the first element. So, yourname is equivalent to &yourname[0], which is what must be passed into scanf. */ 28 scanf will skip over white space such as blanks, tabs and newlines in the input stream. The exception is when trying to read single characters with the conversion specifier %c. In this case, white space is read in. So, it is more difficult to use scanf for single characters. Some more input functions getchar getchar reads a single character from standard input. Its prototype is: int getchar(); It returns int rather than char because the "end of file", EOF, character must be handled. EOF is an int and is too large to fit in a char variable. A newline character in C is '\n'. This designates the end of a line. gets gets reads a line of input into a character array. Its prototype is: char *gets(char *buffer); It returns a pointer to the character string if successful, or NULL if end of file is reached or if an error has occurred. The string is also read into the character array specified as an argument. The character string will be terminated be a "\0", which is standard for C. 28

29 Input/output of strings printf("hello World\n"); /* The format string contains only ordinary characters. Ordinary characters are output unmodified. A character string in C is of type "char *". */ printf("my name is %s\n",myname); printf("my first initial is %c\n",initial); /* The %s specifies that a character string will be output. */ /* The %c specifies that a character will be output. */ printf("hello %s or should I say %c\n",myname,initial); /* Multiple arguments of different types may be output. */ 29 putchar putchar writes a single character to standard output. Its prototype is: int putchar(int value); puts puts writes a line of output to standard output. Its prototype is: int puts(const char *buffer); Here is a simple program which echoes back everything that is typed in. Note that depending on which operating system you are using, EOF is entered by typing control-z or control-d. So, if you try running this code, type control-z or control-d to end execution #include <stdio.h> int main() int c; while ((c = getchar())!= EOF) putchar(c); return(0); It terminates the line with a newline, '\n'. It will return EOF is an error occurred. It will return a positive number on success. 29

30 Input/output of strings-(contd ) Please refer to Notes page for more explanation on previous slide 30 Here is the "echo" program implemented using the functions gets and puts. If you run this, remember to type control-z or control-d to end execution. #include <stdio.h> int main() char buffer[120]; /* Holds input and output strings */ char *pt; /* A pointer to datatype char */ int returncode; while ((pt = gets(buffer))!= NULL) returncode = puts(buffer); if (returncode == EOF) printf("error on output\n"); return(0); There are some very important things to note in the this example. First, gets returns a NULL pointer when EOF is reached. This is the condition that must be checked in while loop. Second, puts returns EOF on failure. A check is made on this, and an appropriate error message is output if an error has occurred. Finally, the character array buffer is declared to be of length 120. The function gets makes no check on the array size to see if it is large enough to hold the string entered. It is up to the programmer to create a character array that is large enough. If the entered string is larger than the array, gets will still put the string in memory starting at the location of the first byte of the designated array. Since the string is longer than the array, the result is that memory past the end of the array will be overwritten with the entered string. This will cause either erratic behavior or failure (core dump) of the program. 30

31 String Handling functions There is no basic data type for a string in C. Instead, strings in C are implemented as an array of characters. There is an extensive set of library functions for manipulating strings available in string.h Length of string strlen function Concatenate two strings strcat function Compare two strings strcmp function Copy a string to another strcpy function Next Token of string strtok function Replace/substitute a string Find substring in string 31 The strlen function takes as parameter the string whose length has to be calculated and return the number of characters (excluding \0 ) The strcmp function takes 2 strings, to be compared.as parameters and does a character by character comparison. It returns a integer value which is 0 if the strings are equal >0 if the string 1 is greater than string 2 (ascii difference of the chars where mismatch is found) <0 if the string 1 is less than string 2 (ascii difference of the chars where mismatch is found) The strcpy function takes 2 string arguments. The first parameter is the destination string and the second parameter is the source string. Care need to be taken that the destination string needs to be large enough to store the source string. The function does not validate this while copying Some more String handling functions are listed in the slide. 31

32 String Handling functions-(contd..) Please refer to Notes page for more explanation on previous slide 32 Functions Function Signature Description strchr() char* strchr(const char* s, int c) Returns a pointer to the first occurrence of c in string s. Returns NULL if c is not found in s. strncat() char* strncat(char* s1, const char* s2, size_t n) Tacks on the first n characters of s2 onto s1. A pointer to s1 is returned. strncmp() int strncmp(const char* s1, const char* s2, size_t n) Compares s1 with the first n characters of s2. Returns a negative, zero, or positive integer (just like strcmp). strncpy() char* strncpy(char* s1, const char*s2, size_t n) Copies the first n characters of s2 into the first n characters of s1. strrchr() char* strrchr(const char*s, int c) Returns a pointer to the last occurrence of c in string s. (Compare with strchr().) strstr() char* strstr(const char* s1, const char* s2) Returns the address of the first occurence of string s2 that is also in string s1. Returns NULL if s2 is not found in s1. 32

33 String Length Function strlen() - Returns the number of characters in the string s, starting at s[0] and ending before the first NULL. Prototype declaration - size_t strlen(const char* s) #include <stdio.h> #include <string.h> int main() char name[80],int length; printf("enter your name: "); gets(name); length = strlen(name); printf("your name has %d characters\n", length); return 0; 33 User defined function equivalent to strlen is :- int istr_len(char acstring[]) int iindex ; iindex = 0; while(acstring[iindex]!= \0 ) return iindex; iindex++; 33

34 String Compare Function strcmp() - Compares s1 and s2 alphabetically. Returns a negative, zero, or positive number depending on whether s1 is before, the same as, or after s2 if you were alphabetizing s1 and s2. Prototype declaration - int strcmp(const char* s1, const char* s2) #include <stdio.h> #include <string.h> void main() char first[80], second[80]; printf("enter a string: "); gets(first); printf("enter another string: "); gets(second) ; if (strcmp(first, second) == 0) /* comparison of the form if(first==second) is not valid*/ puts("the two strings are equal"); else puts("the two strings are not equal"); return 0; 34 The strcmp Function The strcmp function is used to compare two strings together. The variable name of an array points to the base address of that array. Therefore, if we try to compare two strings using the condition if (first == second) We would be comparing two addresses, which would obviously never be the same as it's not possible to store two values in the same location. Code the user defined version of the same function. 34

35 String Copy Function strcpy() - Copies s2 into s1, and returns a point to s1. Prototype declaration - char* strcpy(char* s1, const char* s2) #include <stdio.h> #include <string.h> void main() char first[80], second[80]; printf("enter a string: "); gets(first); strcpy(second, first); /* an assignment of the form second=first is invalid */ printf("first: %s, and second: %s\n", first, second); return 0; 35 The strcpy Function The strcpy function is used to copy one string to another. Examples firstname= "Arnold"; /* Illegal */ strcpy(firstname, Arnold ) ; /* valid */ lastname= "Schwarznegger"; /* Illegal */ strcpy(lastname, Schwarznegger ) ; /* valid */ 35

36 String Concatenate Function strcat() - Adds string s2 onto the end of s1 (concatenation). Prototype declaration - char* strcat(char* s1, const char* s2) #include <stdio.h> #include <string.h> void main() char first[80], second[80]; printf("enter a string: "); gets(first); printf("enter another string: "); gets(second); strcat(first, second); /* adding using first=first+second */ is invalid */ printf("the two strings joined together: %s\n", first); return 0; 36 The strcat Function The strcat function is used to join one string to another. Exmaple fullname= "Mr"+firstname +lastname; /* Illegal */ strcat(fullname, Mr ); strcat(fullname,firstname); strcat(fullname,lastname); 36

37 String Tokenizer function strtok() - to find the next token in a string Prototype declaration - char* strcat(char* s1, char* s2) #include <stdio.h> #include <string.h> void main() char line[80], char *delimiters = " ", char *token; gets(line); token = strtok(line, delimiters); while (token!= NULL) puts(token); /* Get the next word */ token = strtok(null, delimiters); 37 The strtok Function The strtok function is used to find the next token in a string. The token is specified by a list of possible delimiters. 37

38 Example: Palindrome Check whether the user entered string is a palindrome or not 38 38

39 Solution #include <stdio.h> int icheck_palindrome(char acstring[], int istringlen); int istr_len(char [ ]); int main ( ) char acstring[100]; int istrlen, iindex, int ipalindrome; printf("enter the string"); gets(acstring); istrlen=istr_len(acstring); ipalindrome=icheck_palindrome(acstring, istrlen); if (ipalindrome == 1) printf("a palindrome"); else printf("not a palindrome"); 39 39

40 Solution (contd ) int icheck_palindrome(char acstring[ ], int istrlen) int ipalindrome; int iindex; ipalindrome = 1; for(iindex=0; iindex<istrlen/2; iindex++) if (acstring[iindex]!= acstring[istrlen - iindex - 1]) ipalindrome = 0; return(ipalindrome); 40 40

41 Example Reverse the user entered string and then print #include <stdio.h> void vreverse_string(char[ ]); int istr_len(char [ ]); int main( ) char acstring[100]; char acreversestring[100]; int istrlen; printf("enter the string"); gets(acstring); istrlen=istr_len(acstring); vreverse_string(acstring,istrlen); printf( The reversed string is %s \n, acstring); 41 41

42 Solution #include <stdio.h> void vreverse_string(char[ ]); int istr_len(char [ ]); int main( ) char acstring[100]; char acreversestring[100]; int istrlen; printf("enter the string"); gets(acstring); istrlen=istr_len(acstring); vreverse_string(acstring,istrlen); printf( The reversed string is %s \n, acstring); 42 42

43 Solution (contd ) void vreverse_string(char acstring[ ], int istrlen) int iindex; char ctemp; for(iindex = 0; iindex < istrlen/2; iindex++) ctemp = acstring[iindex]; acstring[iindex] = sstring[istrlen - iindex - 1]; sstring[istrlen - iindex -1] = ctemp 43 43

44 Multidimensional arrays Declaration int a2[3][2]; -/* a2 is an array of 3 elements, each of which is an array of 2 int reserves memory for 3*2 ~ 6 element / integer values * float temperatures[12][31]; /* Used to store temperature data for a year */ float altitude[100][100]; /* Used to store the altitudes of grid points of a 100 by 100 mile square */ Memory Allocation a2 a2[0][0] a2[0][1] a2[0][2] a2[1][0] a2[1][1] a2[2][2] Accessing elements int i, j; for(i = 0; i < 5; i++) for(j = 0; j < 7; j++) a2[i][j] = 0; 44 C does not have true multidimensional arrays. However, because of the generality of C's type system, you can have arrays of arrays, which are almost as good. (This should not surprise you; you can have arrays of anything, so why not arrays of arrays?) Just as we declared a two-dimensional array using two pairs of brackets, we access its individual elements using two pairs of brackets: Make sure that you remember to put each subscript in its own, correct pair of brackets. Neither int a2[5, 7]; /* XXX WRONG */ nor a2[i, j] = 0; /* XXX WRONG */ nor a2[j][i] = 0; /* XXX WRONG */ would do anything remotely like what you wanted. While declaring arrays, there are times when you don't know the size, so you'd want to use an unsized array. However, you must specify every dimension size except the left one. Like this: int array2d[][5]; 44

45 Multidimensional arrays-(contd ) Please refer to Notes page for more explanation on previous slide 45 Consider an example:- #include <stdio.h> void main() int first[3][4] = 0,1,2,3,4,5,6,7,8,9,10,11; int second[3][4] = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11; /* a clearer definition than the first */ int third[][5] = 0,1,2,3,4; /* third[] only has one index of 1 */ int fourth[][6] = 0,1,2,3,4,5, 6,7,8,9,10,11; /* fourth[] has 2 indices - 0 or 1 */ int i,j; int fifth[5][4]; int sixth[2][6]; int seventh[2][3]; for(i=0 ; i<5 ; i++) for(j=0 ; j<4 ; j++) fifth[i][j] = i * 4 + j; for(i=0 ; i<2 ; i++) printf("enter 6 integers separated by spaces: "); for(j=0 ; j<6 ; j++) scanf("%d", &sixth[i][j]); printf("\n"); printf("you entered:\n"); for(i=0 ; i<2 ; i++) for(j=0 ; j<6 ; j++) printf("%d ", sixth[i][j]); printf("\n"); seventh[0][0] = 0; seventh[0][1] = 1; seventh[0][2] = 2; seventh[1][0] = 3; seventh[1][1] = 4; seventh[1][2] = 5; return 0; 45

46 Multidimensional arrays-(contd ) Please refer to Notes page for more explanation on previous slide 46 If you ever wanted to find out how much memory your arrays occupy (1D or multidimensional), you can use the sizeof operator. Example for accessing multidimesional array elements A common way to access the elements of a multidimensional arrays is with nested for loops. #define MAXI 50 #define MAXJ 75 int i; int j; float values[maxi][maxj]; for (i = 0; i < MAXI; i++) for (j = 0; j < MAXJ; j++) values[i][j] = whatever; Important Note About Array Dimensions The C language performs no error checking on array bounds. If you define an array with 50 elements and you attempt to access element 50 (the 51st element), or any out of bounds index, the compiler issues no warnings. It is the programmer's task alone to check that all attempts to access or write to arrays are done only at valid array indexes. Writing or reading past the end of arrays is a common programming bug and can be hard to isolate. 46

47 Multidimensional arrays-(contd ) Please refer to Notes page for more explanation on previous slide 47 What will happen if a program accesses past the end of an array? Suppose a program has the following code. int val; int buffer[10]; val = buffer[10]; /* Bug, remember that the indexes of buffer run from 0 to 9. */ What value will be in val? Whatever happens to be in memory at the location right after the end of the array. This value could be anything. Worse yet, the program may continue to run with the incorrect value and no warnings are issued. What will happen if a program writes past the end of an array? Suppose a program has the following code. int buffer[10]; buffer[593] = 99; The value of 99 will be written at the memory location, buffer "buffer" is a pointer to the beginning of the array. buffer is pointer arithmetic for the address equal to the starting address of the array plus the size of 593 integers. The overwriting of the value at this memory location will change the value of whatever variable is stored there. Some other variable may have its value changed unintentionally. If the program writes unintentionally to memory locations that not valid, the program may crash. 47

48 Multidimensional arrays-(contd ) Please refer to Notes page for more explanation on previous slide 48 The most common cause of writing/reading to invalid array indexes are errors in loop limits. int i; float b[10]; for (i < 0 ; i <= 10; i++) b[i] = 3.14 * i * i; This loop should use "<" rather than "<=" The most straightforward way of passing a multidimensional array to a function is to declare it in exactly the same way in the function as it was declared in the caller. If we were to call func(a2); then we might declare func(int a[5][7])... and it's clear that the array type which the caller passes is the same as the type which the function func accepts. 48

49 Multidimensional arrays-(contd ) Please refer to Notes page for more explanation on previous slide 49 While passing single dimensional arrays as parameters to functions, the formal parameters to the function definition were only the array name[]. Size was omitted because it was decided by the parent function which calls this function. But when we look at passing 2-dimensional arrays as parameters to the function the formal parameters to the function definition needs to be array name[][<size>. We can only omit the row dimension and need to specify the column dimension. With multidimensional arrays being passed as parameters to functions, we need to specify all but the first dimension of the array. 49

50 Multidimensional arrays-(contd ) Please refer to Notes page for more explanation on previous slide 50 Another important point to understand here is what is common between arrays and pointers. When we pass arrays as arguments to functions what is actually been passed is the starting address to the array (ie the address of the first element of the array) 50

51 Multidimensional Arrays and functions Passing Arrays as arguments to functions Declaration Definition Invocation func(int arr[][7]) func(int arr[][7]) func(arr) Write a program to accept values into a 2-dimensional array (arr) and display the row totals display the column totals transpose the elements of the array. 51 The most straightforward way of passing a multidimensional array to a function is to declare it in exactly the same way in the function as it was declared in the caller. If we were to call func(a2); then we might declare func(int a[5][7])... and it's clear that the array type which the caller passes is the same as the type which the function func accepts. Concluding, the formal parameters declaration within the function definition must include explicit size specifications in all of the subscript positions except the first 51

52 Array of strings Declarations char caname[5][10]; caname[0] - Creates a 2D array of characters with 5 rows, each row having 10 characters - 1 st single dimensional character array, which can be used to hold 10 characters or a string of 9 characters caname points the 1 st element of the array ie caname[0][0]; caname 52 An array of strings is just a two dimensional array of characters. Consider this: char names[7][6] = "Ryan", "Tom", "Chad", "Jackie", "Tara", "Lori", "Kelly" ; This gives you an array of seven names. The first number in brackets says how many strings there will be, and the second number is the amount of characters any of the strings can have. If you left out the numbers in the brackets, the compiler would figure it out for you using the longest name length as the maximum length (the second number in brackets). To reference a string, only use the first dimension. So the statement printf("%s", names[0]); would print the name Ryan on the screen. Do not reference a string like this names[3][0]. That refers to the character 'J' not the string "Jackie". 52

53 Example #include <stdio.h> #include <string.h> void main() char *mess[] = "Pen", "Pencil", "Scale", "Sharpner", "Scissor" ; printf("%s",mess[0]); printf("%c",mess[3][4]); Consider the declaration above and answer the questions Is it a valid declaration? mess[0] pointer to the string Pen Character at mess[3][4] is? 53 The declaration is valid. Each the string is referred to using mess[0] ->Pen,mess[1]-> Pencil and so on mess[3] -> Sharpner Mess[3][0]->S mess[3][1]->h..hence mess[3][4] -> p 53

54 Summary Definition of strings Input/output of strings String functions Multidimensional Arrays 54 54

55 Thank You! 55 55