Programming

1. Programming Review: Functions, pointers and strings

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

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

4. 1 2 3 Pointers p[0] *p p[i] *(p+i) p[1] p[4] q[0] q[1] q[2] m o s h e \0 q 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. int main() { int n = 3; multBy3(n); printf(“n=%d”,n); } void multBy3(int n) { int num = n; num *= 3; n = num;} 1.What is wrong here?

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

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

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

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

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

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

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

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 userand 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 int main(int argc, char* argv[]) • 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

21. ‘main’ prototype int main(int argc, char* argv[]) argc : 3 argv : progname\0 text\0 178\0

22. Example /* This program displays its command-line arguments */ #include <stdio.h> 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 <Enter>)

25. Helper functions – atoi/atof int atoi(char s[]); double atof(char s[]); • 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 <stdlib.h>

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; }