Thread: I want this program to say the length of the longest common prefix.

I wrote this program to find the longest common prefix but for some reason it is failing some of the tests I am running on it. Any ideas why?

Code:

int longestCommonPrefix(const char *a, const char *b){
	int size,sizea, sizeb;
	int com=0;
	int i=0;
	sizea=strlen(a);
	sizeb=strlen(b);
	if(sizea==NULL||sizeb==NULL){//probably wrong
		return com;}
	if(sizea==0||sizeb==0){
		return com;}
	if(sizea>=sizeb){
		size=sizea;}
	if(sizea<sizeb){
		size=sizeb;}
	for(i=0; i<size; i++){
		if(a[i] != b[i]){
			return com;
			break;}
		com++;}
	return com;
}

Thank you. I forgot to say that, I am slightly new to asking for help with programming.
ok "foo", "bar" does not come back right and "", "bar" and "balls", "" those are the only ones that do not work. I tried to fix the issue of "" with this part of the code

Code:

Code:

    if(sizea==NULL||sizeb==NULL){//probably wrong
        return com;}
    if(sizea==0||sizeb==0){
        return com;}

What does strlen return? Does it make sense to compare it to NULL? It will probably work, depending on how NULL is defined, but it's not standard. Maybe you really want to check a and b to see if they are NULL before you use them, because you're doing the same thing here

Code:

if(sizea==0||sizeb==0){
        return com;}

I'd suggest learning to use your debugger and step through the code to see what's happening so that you actually understand it.

When I comment out the bizarre NULL comparison, it compiles with no warnings, or errors, and functions correctly for foo/bar = 0 , and bar /balls = 2. Here's the full code:

Code:

#include <stdio.h>
#include <string.h>

int longestCommonPrefix(const char *a, const char *b);

int main(void)
{
int c = longestCommonPrefix("bar","balls");
printf("longest prefix: - %i",c);
return 0;
}

int longestCommonPrefix(const char *a, const char *b)
{
    int size,sizea, sizeb;
    int com=0;
    int i=0;
    sizea=strlen(a);
    sizeb=strlen(b);
 //   if(sizea==NULL||sizeb==NULL){//probably wrong
 //       return com;}
    if(sizea==0||sizeb==0){
        return com;}
    if(sizea>=sizeb){
        size=sizea;}
    if(sizea<sizeb){
        size=sizeb;}
    for(i=0; i<size; i++){
        if(a[i] != b[i]){
            return com;
            break;}
        com++;}
    return com;
}

Ok I have made those changes but the program using my code to check to see if it is correct still says it is wrong for those cases only. And it still does not accept the "" one to be zero. I think it wants it to be returning null instead of 0. I will check, but still the foo and bar should be correct.

What can I say, it compiles with no warnings and functions as intended. I am able to put in a string with nothing, "" and it returns 0. I am using gcc. I am guessing you are on some windows based IDE. Have you tried just copying and pasting and compiling my working version? Good luck.

I am using GCC too. Yes I have tried using your code too. I think it a problem with the grade checker. Mine was working the same too. I will check and then keep you updated.

Ok it was a graders error. It is now fixed. Thanks for all the help guys.