Finding the longest common prefix

[gizmo7008]

I am trying to find the longest common prefix of two words entered by the user. I don't understand what is going wrong. I keep getting this error when I compile:

prefix.c:11:13: error: expected declaration specifiers or '...' before '(' token

And this is my code:

#include <stdio.h>
#include <string.h>
#define MAXFORTHIS 100

void prefix((char x1[]; char x2[])
{
        char x3[MAXFORTHIS];
        int a = 0;
        int b = 0;
        int c = 0;
        a = strlen(x1);
        b = strlen(x2);

        if (a < b)
                c = a;
        else
                c = b;

        for(int i = 0; i < c; ++i)
        {
                while(x1[i] == x2[i])
                        x3[i] = x1;
                if (x[i] != x2[i])
                        break;
        }

        printf("%s",x3);
}

int main(void)
{
        char x1[MAXFORTHIS];
        char x2[MAXFORTHIS];

        for(int i = 0; i < 5; i++)
        {
                printf("Enter two words: ");
                scanf(x1,x2,MAXFORTHIS,stdin);
                x1[strlen(x1)-1] = '\0';
                x2[strlen(x2)-1] = '\0';

                printf("The longest common prefix of %s and %s is ", x1,x2);
                prefix(x1,x2);
        }

        return 0;
}

[Narue]

Your prefix() function signature is jacked up. Fix that and you'll get other errors that are easier to fix.

[gizmo7008]

What do you mean?

[Narue]

What do you mean?

If you don't see it after it being pointed out, I'd recommend re-reading your chapter on functions:

void prefix((char x1[]; char x2[])

Parameters are separated by a comma, not a semicolon, and you have a rogue opening paren.

[gizmo7008]

I didn't see that it was a semicolon. I changed it but I still get the same error. I don't know what you mean by rogue opening paren.

void prefix((char x1[],char x2[]))

Plus I get this error, I don't know what this means:

prefix.c:41:2: error: 'for' loop initial declarations are only allowed in C99 mode
prefix.c:41:2: note: use option -std=c99 or -std=gnu99 to compile your code

[gizmo7008]

I actually fixed the first error. But now I am getting:

prefix.c: In function 'prefix':
prefix.c:25:9: error: 'for' loop initial declarations are only allowed in C99 mode
prefix.c:25:9: note: use option -std=c99 or -std=gnu99 to compile your code
prefix.c:28:31: warning: assignment makes integer from pointer without a cast [enabled by default]
prefix.c: In function 'main':
prefix.c:41:2: error: 'for' loop initial declarations are only allowed in C99 mode

[Narue]

I don't know what you mean by rogue opening paren.

Count the parens. Two opening, one closing. Do I have to hold your hand the whole way?

Plus I get this error, I don't know what this means:

Prior to C99, you couldn't declare a variable in the initialization clause of a for loop. GCC doesn't enable C99 mode by default, yet your code uses that feature here:

for(int i = 0; i < c; ++i)

and here:

for(int i = 0; i < 5; i++)

[gizmo7008]

Okay, I understand that. I took the i out and declared it outside the loop. However, now that I did that to both my for loops, it compiles, but when I run the program it is just blank. It doesn't prompt the user anymore.

int i = 0;
for(i < c; ++i;)
        {
                while(x1[i] == x2[i])
                        x3[i] = x1[i];
                if (x1[i] != x2[i])
                        break;
        }

[gizmo7008]

Nevermind! I need to just declare i and put i = 0 in my for loop conditions. And it is fixed!

The program runs and compiles. When I enter the two words, it gives me a bunch of random stuff when printing.

Enter two words: economy ecology
The longest common prefix of  and § is hA(~·

[D33wakar]

The program runs and compiles. When I enter the two words, it gives me a bunch of random stuff when printing

That's because scanf is not used like this

scanf(x1,x2,MAXFORTHIS,stdin);

click here

[zeroliken]

Why not use scanf like this?

printf("Enter two words: ");
scanf("%s",x1);
scanf("%s",x2);

Also why use a while condition when an if else condition can be enough

for(i = 0; i < c; i++)
{
    if (x1[i] == x2[i]){
        x3[i] = x1[i];
}
else break;
}
printf("%s\n",x3);

[gizmo7008]

I got the garbage letters to go away, however it is cutting off the last letter when I run the program. Do I need to make the array bigger? Or is something else going wrong?

Enter two words: program problem
The longest common prefix of progra and proble is pro

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

void prefix(char x1[], char x2[])
{
        char x3[254];
        int a = 0;
        int b = 0;
        int c = 0;
        a = strlen(x1);
        b = strlen(x2);
        int i;

        if (a < b)
                c = a;
        else
                c = b;

        for(i = 0; i < c; ++i)
        {
               if(x1[i] == x2[i])
                        x3[i] = x1[i];
                else break;
        }

        printf("%s\n",x3);
}

int main(void)
{
        char x1[254];
        char x2[254];
        int i;

        for(i = 0; i < 1; i++)
        {
                printf("Enter two words: ");
                scanf("%s %s",x1,x2);
                x1[strlen(x1)-1] = '\0';
                x2[strlen(x2)-1] = '\0';

                printf("The longest common prefix of %s and %s is ", x1,x2);
                prefix(x1,x2);
        }

        return 0;
}

[WaltP]

Try outputting the words immediately after you input them.

[gizmo7008]

Okay, I did that and they are being read properly. I think it has something to do with when I add the null character, and it is taking away that last letter and replacing it with a null one.

How do I properly add a null character?

[gizmo7008]

I just need to change the - to a + and it works. Thanks everyone!

[WaltP]

Okay, I did that and they are being read properly. I think it has something to do with when I add the null character, and it is taking away that last letter and replacing it with a null one.

How do I properly add a null character?

What makes you think you have to add it? If you do a strlen() and it works, doesn't that mean there already is a null?

[gizmo7008]

I went back and figured that out and saw that I needed to make it a + and not a - when I was using strlen(x1)+1

[WaltP]

I went back and figured that out and saw that I needed to make it a + and not a - when I was using strlen(x1)+1

I repeat -- WHY?
Why do you now need to add a null after the null that's already there? That's what your + is doing.

Directly to the point: Why do you need to add the null at all?