I do not understand where the error is.
The function should reverse a string.
Code:#include <string.h> char* reverse (char string []) { int cntr = 0, len = (strlen (string) - 1); char ch; while (cntr < len) { ch = string[cntr]; string[cntr] = string[len]; string[len] = ch; ++cntr; --len; } return string; } int main(){reverse("hello.");}
Last edited by benjaminp; 10-10-2012 at 12:06 PM.
Works fine for me. What input are you passing to the function? How is that string declared? Please provide more complete code in the future, along with what input causes it to crash.
I would bet you're doing something like:
The problem with that is that p is just a pointer. It points to a string constant/literal, which is typically placed in read-only memory (it's constant, you shouldn't be changing it). Try declaring it as an array and initialize it:Code:char *p = "Some string to reverse"; reverse(p);
That creates an array on the stack (writable memory) and initializes it with the value in the string literal, so you can reverse it in place.Code:char arr[] = "Some string to reverse"; reverse(arr);
String literals are non-modifiable arrays of char. Try copying the string literal to a "standard" (modifiable) array and pass that to the function
Note: your program has no output.Code:int main(){char a[]="hello.";reverse(a);}
the full code is:
Code:#include <stdio.h> #include <string.h> char* reverse (char string []); void print_string (char* string); int main (void) { print_string (reverse ("Hello.")); print_string ("\n"); return 0; } char* reverse (char string []) { int cntr = 0, len = (strlen (string) - 1); char ch; while (cntr < len) { ch = string[cntr]; string[cntr] = string[len]; string[len] = ch; printf ("string[cntr] == %c\nstring[len] == %c\ncntr == %i\nlen == %i\n\n", string[cntr], string[len], cntr, len); ++cntr; --len; } return string; } void print_string (char* string) { char c; int counter = 0; while ((c = putc (*(string + counter), stdout)) != '\0') ++counter; return; }
Yep, exactly what we said. You can't use a string literal. You must use a modifiable array of char. Don't pass "Hello." directly into your reverse function, copy it into a regular char array (make sure it's big enough for the whole string plus 1 byte for the null terminator).
Also, for print_string, you could just use printf("%s", string).
Right, well you have ben told that you can't pass a string literal to that function because you cant modify string literals. You have been shown what you may do instead.
Perhaps you could post code showing that you've done as has been explained.
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Now it is working properly:
Code:#include <stdio.h> #include <string.h> char* reverse (char string []); void print_string (char* string); int main (void) { char str [] = "Hello."; print_string (reverse (str)); print_string ("\n"); return 0; } char* reverse (char string []) { int cntr = 0, len = (strlen (string) - 1); char ch; while (cntr < len) { ch = string[cntr]; string[cntr] = string[len]; string[len] = ch; ++cntr; --len; } return string; } void print_string (char* string) { char c; int counter = 0; while ((c = putc (*(string + counter), stdout)) != '\0') ++counter; return; }
Last edited by benjaminp; 10-10-2012 at 12:42 PM.
Because the first option copies the data in the string literal to the array, then uses the array as function arguments;Originally Posted by benjaminp
and the second option uses the string literal itself (which is non-modifiable) as function arguments.
thanks. problem solved.
