Lab Session # 7 Pointers. ALQUDS University Department of Computer Engineering

Transcription

1 2013/2014 Programming Fundamentals for Engineers Lab Lab Session # 7 Pointers ALQUDS University Department of Computer Engineering Objective: After completing this lab, the students should be able to: 1. Learn how to declare, initialize and use pointers. 2. Understand the relationship between pointers and arrays. 3. Deal with string objects and character pointers. Background: Pointer is a user defined data type which creates special types of variables which can hold the address of primitive data type like char, int, float, double or user defined data type like function etc. or derived data type like array, structure, union, enum. Pointers are widely used in programming; they are used to refer to memory location of another variable without using variable identifier itself. They are mainly used in linked lists and call-by-reference functions. Pointer Declaration Declaring pointers can be very confusing and difficult at times (working with structures and pointer to pointers). To declare pointer variable we need to use * operator (indirection/dereferencing operator) before the variable identifier and after data type. Pointer can only point to variable of the same data type. Syntax of pointer: The syntax of array in C is as follows: Datatype * identifier; Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 1

2 Example 7.1: Trace the following code and write down the results, start by drawing simple diagram that shows your work. char a = 'b'; char *ptr; printf("%c\n", a); ptr = &a; printf("%p\n", ptr); *ptr = 'd'; printf("%c\n", a); Note: & notation means address-of operand in this case &a means address-of a. Address operator Address operator (&) is used to get the address of the operand. For example if variable x is stored at location 100 of memory; &x will return 100. This operator is used to assign value to the pointer variable. It is important to remember that you MUST NOT use this operator for arrays, no matter what data type the array is holding. This is because array identifier (name) is pointer variable itself. When we call for ArrayA[2]; ArrayA returns the address of first item in that array so ArrayA[2] is the same as saying *(ArrayA+2); and will return the third item in that array. Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 2

3 Pointer arithmetic Pointers can be added and subtracted. However pointer arithmetic is quite meaningless unless performed on arrays. Addition and subtraction are mainly for moving forward and backward in an array. Note: you have to be very careful NOT to exceed the array elements when you use arithmetic; otherwise you will get horrible errors such as access violation. This error is caused because your code is trying to access a memory location which is registered to another program. Operator Result ++ Goes to the next memory location that the pointer is pointing to. -- Goes to the previous memory location that the pointer is pointing to. -= or - Subtracts value from pointer. += or + Adding to the pointer Example 7.2: Trace the following code and write the result start by drawing simple diagram that show your work. int ArrayA[3]=1,2,3; int *ptr; ptr=arraya; printf("address: %p - array value:%d \n",ptr,*ptr); ptr++; printf("address: %p - array value:%d \n",ptr,*ptr); Pointers and functions Pointers can be used with functions. The main use of pointers is call by reference functions. Call by reference function is a type of function that has pointer/s (reference) as parameters to that function. All calculation of that function will be directly performed on referred variables. Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 3

4 Example 7.3: Trace the following code and write the results, start by drawing simple diagram that show your work. void DoubleIt(int *num) *num*=2; int number=2; DoubleIt(&number); printf("%d",number); Pointer to Arrays Array identifier is a pointer itself. Therefore & notation shouldn t be used with arrays. When working with arrays and pointers always remember the following: Never use & for pointer variable pointing to an array. When passing array to function you don t need * for your declaration. Be VERY CAREFUL not to exceed number of elements your array holds when using pointer arithmetic to avoid errors. Pointer to Pointer Pointers can point to other pointers; there is no limit whatsoever on how many pointer to pointer links you can have in your program. It is entirely up to you and your programming skills to decide how far you can go before you get confused with the links. Here we will only look at simple pointer to pointer link. Pointing to pointer can be done exactly in the same way as normal pointer. Diagram below can help you understand pointer to pointer relationship char *ptra; char x= b ; char *ptrb; ptrb=&x; ptra=&ptrb; Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 4

5 ptra gives 100, *ptra gives 101, ptrb is 101,*ptrb gives b, **ptra gives b Comment: **ptra means value stored at memory location of value stored in memory location stored at PtrA Example 7.4: Trace the following code and write the results, start by drawing simple diagram that show your work, what does this program actually do? #define N 10 void reverse(int *p, int size); int array[n], c; for (c = 0; c < N; c++) scanf("%d", &array[c]); reverse(array, N); printf("reversed array elements are:\n"); for (c = 0; c < N; c++) printf("%d\n", array[c]); void reverse(int *p, int size) int end = size-1; for (int c = 0; c < size/2; c++) t = array[c]; array[c] = array[end]; array[end] = t; end--; Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 5

6 Post lab #7 1. State whether the following are true or false. a) Pointer variable can only contain an address. b) Address of the memory location can be assigned to ordinary variables. c) Pointer can refer to the content of the memory location by & operator. d) Size of the pointer variable is equivalent to the size of the data item it points. 2. What is the use of generic pointers? 3. What is the output of the following code? int n[25]; n[0]=100; n[24]=200; printf("\n%d,%d", *n, *(n+24)+*(n+0) ); 4. Given the following declaration: int a, *b = &a, **c = &b; What is the values stored in a, b, c after executing the following statements: a=4; **c=5; b = (int *)**c; 5. What is the output of the following code? char *str1= abcd ; char str2[]= abcd ; printf( %d %d %d, sizeof(str1),sizeof(str2), sizeof( abcd )); Dr. Labib Arafeh, Mr. Yacoub Sabatin, Eng. Allam Abu-Mwais, Eng. Nouraldeen Jibreen 6