Programming Review: Functions, pointers and strings.

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1 Programming Review: Functions, pointers and strings

2 Pointers int nums[] = {1, 2, 3}; char str[] = "moshe"; int* q = nums; char* p = str; 1 2 3 q num m o s h e \0 p str

3 Pointers int nums[] = {1, 2, 3}; char str[] = "moshe"; int* q = nums; char* p = str; 1 2 3 (q+1) (p+3) m o s h e \0 str

4 Pointers p[i] *(p+i) 1 2 3 m o s h e \0 q p q[1]q[2]q[0] p[1]p[4] p[0] *p

5 Pointers and Functions If we want to change a variable inside a function, we must pass it a pointer to the variable (its address) The function will “fill” this address with the right value Example: Swap void swap(int *x, int *y) { int temp = *x; *x = *y; *y = temp; }

6 Pointers and Functions Variables that are defined inside the function “die” when the function ends!!! char* func() { char str[LENGTH + 1];... return str; } str doesn’t exist outside the function’s body

7 1.What is wrong here? int main() { int n = 3; multBy3(n); printf(“n=%d”,n); } void multBy3(int n) { int num = n; num *= 3; n = num; }

8 2.What is wrong here? int main() { int n = 3; multBy3(&n); printf(“n=%d”,n); } void multBy3(int *n) { int num = n; num *= 3; n = num; }

9 3.What is wrong here? int main() { int n = 3; multBy3(&n); printf(“n=%d”,n); } void multBy3(int *n) { int num = *n; num *= 3; n = num; }

10 4.What is wrong here? int main() { int n = 3; multBy3(&n); printf(“n=%d”,n); } void multBy3(int *n) { int num = *n; num *= 3; n = # }

11 References 1. Define a variable in the main 2. Pass its address to the function 3. The function fills the address with a value 4. The main can use it as a normal variable int main() { int num; }

12 References 1. Define a variable in the main 2. Pass its address to the function 3. The function fills the address with a value 4. The main can use it as a normal variable Int main() { int num; multBy3(&num); }

13 References 1. Define a variable in the main 2. Pass its address to the function 3. The function fills the address with a value 4. The main can use it as a normal variable void multBy3(int *n) { (*n) *=3; } Int main() { int num; multBy3(&num); }

14 References 1. Define a variable in the main 2. Pass its address to the function 3. The function fills the address with a value 4. The main can use it as a normal variable Int main() { int num; multBy3(&num); printf(“num=%d”,num); }

15 Exercise with pointers and strings Implement the following function: char* str_any(char *str1, char *str2);  Input – two strings str1, str2  Output – pointer to the first occurrence in str1 of any of the characters contained in str2

16 Exercise (cont.) Write a program that accepts a string from the user and replaces all punctuation signs (,.;:!? ) with spaces

17 Solution (str_any.c) char* str_any(char* str1, char* str2) { while (*str1 != '\0') { if (strchr(str2, *str1) != NULL) { return str1; } ++str1; } return NULL; }

18 Solution int main(void) { char* punc = ".,;:!?"; char s[MAX_LENGTH + 1]; char *p; printf("Please enter a line of text\n"); scanf("%100s", s); for (p = str_any(s, punc); p != NULL; p = str_any(p + 1, punc)) { *p = ' '; } printf("Resulting string is:\n%s\n", s); return 0; }

19 Command line arguments Command line arguments are arguments for the main function  Recall that main is basically a function  It can receive arguments like other functions  The ‘calling function’ in this case is the operating system, or another program

20 ‘main’ prototype When we want main to accept command line arguments, we must define it like this  argc holds the number of arguments that were entered by the caller  argv is an array of pointers to char – an array of strings – holding the text values of the arguments The first argument is always the program’s name int main(int argc, char* argv[])

21 ‘main’ prototype int main(int argc, char* argv[]) argc : 3 argv : 1 7 8 \0 p r o g n a m e \0 t e x t \0

22 Example /* This program displays its command-line arguments */ #include int main(int argc, char *argv[]) { int i; printf("The program's command line arguments are: \n"); for (i = 0; i < argc; ++i) { printf("%s\n", argv[i]); } return 0; }

23 Specifying the arguments In Visual Studio: Project  Settings  Debug, in the ‘program arguments’ field

24 Specifying the arguments We can also specify the arguments directly, by using the Windows console (Start  Run…, then type ‘cmd’ and drag the executable into the window. Then type the arguments and )

25 Helper functions – atoi/atof Command line arguments are received in the form of strings These functions are used when we want to transform them into numbers For example – atof(“13.5”) returns the number 13.5. Must #include int atoi(char s[]); double atof(char s[]);

26 Exercise Write a program that accepts two numbers as command line arguments, representing a rectangle’s height and width (as floating- point numbers). The program should display the rectangle’s area and perimeter

27 Solution (args_rectangle.c) int main(int argc, char* argv[]) { double width, height; if (argc != 3) { printf("Wrong number of arguments!\n"); return 1; } width = atof(argv[1]); height = atof(argv[2]); printf("The rectangle's area is %g\n", width * height); printf("The rectangle's perimeter is %g\n", 2 * (width + height)); return 0; }