I wrote a reverse function it works fine but having trouble solving valgrind throwing
Please adviceCode:#include <string.h> #include <stdio.h> #include <stdlib.h> char *strrev(char *str){ char *tmp = malloc(sizeof(char) * strlen(str)); memset(tmp,0,strlen(str)); for(int i=0, k=(int)strlen(str); i<=(int)strlen(str); i++, k--){ printf("%c\n", str[k]); strncat(tmp, &str[k], 1); } return tmp; } int main(){ char *str = strrev("ABCD"); printf("%s\n", str); free(str); return 0; }
Last edited by Mr.President; 08-22-2020 at 02:41 AM.
Eh. You need to account for the terminating null character. You only need to call strlen(str) once: store the result in a size_t variable. Your loop index variables should be size_t too. Finally, strncat is overkill and inefficient: just assign characters.
Another approach could be to implement an in-place string reverse, then call that in-place string reverse to implement this string copy and reverse.
By the way, I suggest you rename your strrev function: if I remember correctly, it is technically a reserved name (something to with the str prefix, I think, rather than there being such a standard function), and more practically it could conflict with a strrev provided as a standard library extension.
Last edited by laserlight; 08-22-2020 at 02:24 AM.
Look up a C++ Reference and learn How To Ask Questions The Smart WayOriginally Posted by Bjarne Stroustrup (2000-10-14)
yes thank you ! darn I missed the terminating 0/
All works fine now !Code:#include <string.h> #include <stdio.h> #include <stdlib.h> char *strrev(char *str){ size_t len = strlen(str); char *tmp = malloc(sizeof(char) * len + 1); memset(tmp, 0, len); for(size_t i=0, k=len-1; i<len; i++, k--){ tmp[i] = str[k]; } tmp[len] = 0; return tmp; } int main(){ char *str = strrev("ABCD"); printf("%s\n", str); free(str); return 0; }
Not that your strrev() is wrong, but I prefer to follow C's standard library philosophy and assume the arguments are correct and test/initialize them outside the funcion. I would implement strrev() this way:
If you pass invalid pointers you'll get a 'segmentation fault' or misbehavior the same way it happens to strlen() (passing a NULL pointer to srcp, for example).Code:// Notice: No malloc() and just one call to strlen(). char *strrev2( char *destp, const char *srcp ) { const char *p; char *q; // Points to source string NUL char. p = srcp + strlen( srcp ); // Initially the destination string is empty. *destp = 0; // if source string isn't empty... if ( *p-- ) { // Copy bytes in reverse order. q = destp; while ( p >= srcp ) *q++ = *p--; // puts the final NUL char at destination string. *q = 0; } // Returns destination string pointer. return destp; }
And you'll need to allocate the destination string before calling the function, the same way it happens with strcpy(), for example.
Reverse in-place using swaps:
Note that the null-termination step isn't necessary here since it's only the enclosing chars are being swapped out.Code:#include <string.h> char* reverse(char* text) { char* left = text; char* right = text + strlen(text); while(left < --right) { char saved = *left; *left++ = *right; *right = saved; } return text; } char* reversed(char* destination, const char* source, size_t max) { strncpy(destination, source, max); return reverse(destination); } #include <stdio.h> int main(void) { char buffer[1024]; for(;;) { printf("Input to reverse (press `enter` to exit) > "); fgets(buffer, sizeof(buffer), stdin); char* newline = strstr(buffer, "\n"); if(newline) *newline = 0; if(*buffer == 0) break; puts(reverse(buffer)); } }
Nice!
