arrays and strings week 3 Ritsumeikan University College of Information Science and Engineering Ian Piumarta 1 / 22 imperative programming review

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1 of char imperative week 3 and Ritsumeikan University College of Information Science and Engineering Ian Piumarta 1 / 22

2 : miscellaneous of char several library functions are have put or get in their name the convention is always get means to get a value into the program from some outside source put means to put a value from the program to some outside destination input (e.g., keyboard) get program put output (e.g., terminal) always explicitly initialise your variables otherwise they will have random values be careful that your recursive functions are guaranteed to terminate running out of stack space causes a fatal error (with unhelpful message) Ian Piumarta 2 / 22

3 : of char two : void (absence of any value) numeric (integer, floating-point) there is no Boolean type in C, and there are no Boolean values any numeric value equal to 0 represents false any non-zero value represents true if (0) doanything(); if (1) dosomething(); // this will never do anything // this will always do something Ian Piumarta 3 / 22

4 numeric of char integer typical name size official rules char 8 a character, typically 1 byte (typically 8 bits) short 16 at least 16 bits, not smaller than char int 32 at least 16 bits, not smaller than short long 64 at least 32 bits, not smaller than int long long 64 at least 64 bits floating-point float 32 double 63 long double 128 not smaller than double Ian Piumarta 4 / 22

5 numeric of char some useful char %c (as a character) int %i (integer) or %d (decimal integer) long %li or %ld float %f i indicates an integer, d means decimal integer integers are decimal by default, so i and d mean the same thing the l modifier indicates a longer version of what follows f indicates floating-point Ian Piumarta 5 / 22

6 data size to check the size of a data type (such as the above) use the sizeof operator sizeof(type) indicates the size (in bytes) of the given type of char e.g., on my (64-bit) computer: sizeof(char) == 1, sizeof(long) == 8 sizeof(short) == 2, sizeof(long long) == 8 sizeof(int) == 4 you can also use sizeof on any expression long l; if (4 == sizeof(l)) { printf("do I detect a 32-bit CPU?\n"); } Ian Piumarta 6 / 22

7 data type dangers always know and be thinking about the range of the integer type(s) you are using what is the result of running this program? int a; a= ; if ( ) puts("yes"); else puts("no"); if (a ) puts("yes"); else puts("no"); [ex1] of char always be thinking about the accuracy of the numeric type(s) you are using what is the result of running this program? float f= 0.0; // f == 0.0 if (f) printf("is it true?\n"); f += 0.1; // f == 0.1 f += 0.1; // f == 0.2 f += 0.1; // f == 0.3 f += 0.1; // f == 0.4 f += 0.1; // f == 0.5 f -= 0.5; // f == 0.0 if (f) printf("yes, it s true!\n"); [ex2] Ian Piumarta 7 / 22

8 an array is a sequential collection of values that all have the same type each value in the array is an element of the array of char to declare an array, write the type of each element the name of the array the number of elements in the array, inside [ square ] brackets int array[3]; declares an array called a containing three int elements a int int int memory Ian Piumarta 8 / 22

9 of char a is therefore a collection of several integers each of the integers inside a is referred to by its numeric index the indices start from 0 for an array of size N the valid indices are therefore 0 through N-1 a int int int a[0] a[1] a[2] to indicate a specific element, write the name of the array and the index of the element in [ square ] brackets this works both in expressions and on the left side of an assignment operator Ian Piumarta 9 / 22

10 of char int a[4], i= 0; while (i < 4) { a[i]= i*i*i; ++i; } printf("%d\n", a[3]); // assigning to an array element // using the value of an array element sizeof can be applied to an array too it tells you the size (in bytes) of the entire array if an int is 4 bytes wide, and we have int thearray[5]; then what is the sizeof(thearray)? Ian Piumarta 10 / 22

11 size of sizeof? how big can the size of an array become? depends on the computer s word size of char a 32-bit computer can address 2 32 bytes of memory an array (or any data structure) cannot therefore be larger than 2 32 bytes how large an integer do we need to represent the largest possible size? a 64-bit computer can (in theory) address 2 64 bytes of memory an array (or any data structure) cannot be as large as 2 64 bytes how large an integer do we need to represent the largest possible size? C defines a type called size_t an integer type wide enough for the largest possible size on the computer it is usually either an int or a long whichever is the smaller integer needed to hold the largest possible size size_t is defined in stddef.h stddef.h is included by many other header files too Ian Piumarta 11 / 22

12 size of sizeof? of char size_t is therefore guaranteed to be able to hold any array index use it for array index variables, and other things that measure size unless you know for certain that the maximum index/size fits a smaller integer problematic when passed to printf because we don t know whether to use %d or %ld (the difference between the two is very important) printf therefore provides a modifier z for size_t integers printf("%zd\n", sizeof(int)); the zd means whatever size of integer matches a size_t it should now be easy to see that the must be size_t library function arguments that represent sizes are of type size_t library function return values representing sizes are of type size_t Ian Piumarta 12 / 22

13 of char a string is a sequential collection of characters its representation in memory is a sequence of char integers the string "hello" is represented as mystring h e l l o a string in C is therefore an array of char in truth, there are no in C only of characters declare a string just like any other array, using char as the element type char mystring[10]; Ian Piumarta 13 / 22

14 of char the relationship between string and array of char is very strong in C you can initialise an array of char with a string literal char mystring[10]= "hello"; printf("%s\n", mystring); // %s tells printf to print a string how long is mystring? the size of the storage for the string sizeof(mystring) is 10 bytes the of the string stored there is only 3 characters try it yourself: the above program always prints abc how does printf, or any string function, know the of the string? Ian Piumarta 14 / 22

15 of char C explicitly marks the end of every string, using a sentinel character the sentinel character can never appear in the middle of a string but the sentinal must always be present to terminate the string the string terminator character has integer value 0 and is called NUL it is the only character whose Boolean interpretation is false in C it is written \0 mystring h e l l o \ NUL Ian Piumarta 15 / 22

16 to find the of a string count the number of characters before (not including) the terminating NUL of char the library function strlen(astring) does this for you the return value is a size, and therefore has type size_t the strlen function is declared in string.h char mystring[10]= "hello"; printf("mystring has size %zi\n", sizeof(mystring)); printf("mystring has %zi\n", strlen(mystring)); this has several critical implications, including a string always occupies one byte more than its e.g., the empty string (of 0) occupies one byte of storage when a string, you must always copy the NUL byte at the end too Ian Piumarta 16 / 22

17 string.h also declares strcpy(to, from) copies string from to string to (same order as assignment: assign right operand to left operand) of char char fromstring[10]= "abc"; char tostring[10]= "hello"; printf("%s\n", tostring); strcpy(tostring, fromstring); printf("%s\n", tostring); // => hello // => abc Ian Piumarta 17 / 22

18 of char fputs prints a string; the caller specifies the string to be printed the file to which it should be printed stdout is the file connected to the standard output (e.g., terminal) declared in stdio.h char mystring[20]= "hello, world\n"; fputs(mystring, stdout); fgets reads a string; the caller specifies the storage array into which the string should be read the size of that storage, in bytes the file from which the string should be read stdin is the file connected to the standard input (e.g., keyboard) string I/O char mystring[20]; fgets(mystring, sizeof(mystring), stdin); fputs("you entered: ", stdout); fputs(mystring, stdout); note: the string will always be properly terminated with a NUL Ian Piumarta 18 / 22

19 of char multiple indices in [ square ] brackets can follow an array name each additional index implies another dimension in the array consider char [3][10]; the type of is char [3][10] the type of [0] is char [10], as is the type of [1], and the type of [2] is therefore an array of size 3, in which each element [i] is an array of size 10, in which each element [i][j] is a single character Ian Piumarta 19 / 22

20 of char C has no array you can use an index that is larger than the size of the array you can use negative indices any and all is your responsibility; either design your program so it can never generate an illegal index, or write your own bounds check during every array access why does C behave like this? there sometimes are reasons to use indices that are negative, or too large we will understand why this is even possible in a few weeks time we will see some examples of legitimate uses towards the end of this course for now beware of an element outside the bounds of an array forgetting to account for the terminating NUL character in a string [ex3] [ex4] Ian Piumarta 20 / 22

21 of char what is wrong with this program? char a[3]= "abc"; char b[4]= "pqr"; char c[5]= "xyz"; printf("%s\n", a); strcpy(a, b); // copy b to a strcpy(b, c); // copy c to b Ian Piumarta 21 / 22

22 of char what is wrong with this program? char a[3]= "abc"; char b[4]= "pqr"; char c[5]= "xyz"; printf("%s\n", a); strcpy(a, b); // copy b to a strcpy(b, c); // copy c to b a is not a string there is no storage space for the terminating NUL printing the non-string a will cause unpredictable results, including nothing bad at all the terminal becoming unusable because of random characters being printed the program crashing strcpy will attempt to move 4 bytes from b to a the fourth byte is the terminating NUL for b a character will be written to the byte following the end of a the results are unpredictable (and may include the program crashing) (the final strcpy is fine, even though sizeof(b) < sizeof(c) strlen(c) is 3, so 4 bytes will be copied, and b has just enough space) Ian Piumarta 22 / 22