Edit - Stack Overflow

You are not logged in. Your edit will be placed in a queue until it is peer reviewed.

We welcome edits that make the post easier to understand and more valuable for readers. Because community members review edits, please try to make the post substantially better than how you found it, for example, by fixing grammar or adding additional resources and hyperlinks.

Required fields*

Rev

Required fields*

How to debug this substring matching code?

I am a beginner in C programming, and this is a string matching code I wrote. It aims to find the positions where the substring t appears in the string s and print them. The use of pointers is required.This code scored 94 out of 100 in the OJ test.

#include <stdio.h>
#include <stdlib.h>

int main () {
    char *s = malloc(100005);
    char *t = malloc(100005);

    scanf ("%s%s", s, t);

    char *ptrS = s;
    char *ptrT = t;

    if (s == NULL) {
        free(t);
        return 1;
    }
    if (t == NULL) {
        free(s);
        return 1;
    }

    while ( *ptrS != '\0') {
        if (*ptrT == *ptrS) {
            ptrT++;
            ptrS++;
        } else if (*ptrS != *(ptrT = t)) {
            ptrS++;
        }

        if (*ptrT == '\0') {
            printf("%d ", (ptrS - s) - (ptrT - t));
            ptrS = ptrS - (ptrT - t) + 1;
            ptrT = t;
        }
    }

    free(t);
    free(s);

    return 0;
}

I have tried many test cases that I could think of, and it gives the correct results for all of them. I hope to find any bugs or any test cases that cause it to error.

Answer*

There are several logical errors in your code.

The first one is that before using the function `scanf` you shall check that pointers `s` and `t` are not null pointers that is that memory for the character arrays was successfully allocated.

You could write for example

char *s = NULL;
    char *t = NULL;

if ( ( s = malloc( 100005 ) ) != NULL && ( t = malloc( 100005 ) ) != NULL )
    {
        // process the arrays
    }

free( t );
    free( s );

In the call of `scanf` you should specify the field width like

if ( scanf( "%100004s %100004s", s, t ) == 2 )
    {
        // process the inputed strings
    }

The inner if statement in the else part of this if statement

if (*ptrT == *ptrS) {
        ptrT++;
        ptrS++;
    } else if (*ptrS != *(ptrT = t)) {
        ptrS++;
    }

must be written for example like

if (*ptrT == *ptrS)
    {
    	ptrT++;
    	ptrS++;
    }
    else if ( *ptrT != '\0' )
    {
    	ptrS = ptrS - ( ptrT - t ) + 1;
    	ptrT = t;
    }
    
    if (*ptrT == '\0')
    {
    	printf( "%td ", ( ptrS - s ) - ( ptrT - t ) );
    
    	ptrS = ptrS - ( ptrT - t ) + 1;
    	ptrT = t;
    }

That is the logic of the assignment of the pointer `PtrS` must be the same as inside this if statement

if (*ptrT == '\0') {
        printf("%d ", (ptrS - s) - (ptrT - t));
        ptrS = ptrS - (ptrT - t) + 1;
        ptrT = t;
    }

Bear in mind that there is a duplicated code in your program.

Difference of two pointers has type `ptrdiff_t`. So you need to use length modifier `t` in the format string of the call of `printf`

printf("%td ", (ptrS - s) - (ptrT - t));
            ^^^

And after the while loop you should insert the following call

putchar( '\n' );

Using such expressions like for example that

(ptrS - s) - (ptrT - t)

makes your code difficult to read. Usually such codes contain bugs as a rule.

Instead of your while loop I would write something like the following

for ( const char *ptrS = s; *ptrS != '\0'; ++ptrS )
    {
    	if ( *ptrS == *t )
    	{
    		const char *ptrT = t;
    
    		while ( *ptrT != '\0' && *ptrT == *( ptrS + ( ptrT - t ) ) )
    		{
    			++ptrT;
    		}
    
    		if ( *ptrT == '\0' )
    		{
    			printf( "%td ", ptrS - s );
    		}
    	}
    }

putchar( '\n' );

Within the loops there are used pointers to `const char` because the loops do not change the processed strings.

Pay attention to that instead of manually written loops you could use standard C function `strstr` together with `strchr`. And you could initially check that the length of the string stored in the array pointed to by the pointer `s` is not less than the length of the string stored in the array pointed to by the pointer `t`.

Edit Summary*

Cancel

The expression *( ptrS = ( ptrS - ( ptrT - t ) + 1 ) ) != *( ptrT = t ) is undefined behavior due to unsequenced writing and reading of ptrT.
– interjay
Commented Nov 26, 2024 at 15:29
@interjay Oh, you are right. I will update teh answer,
– Vlad from Moscow
Commented Nov 26, 2024 at 15:35
it's very helpful. thanks a lot for your detailed explanation@VladfromMoscow
– Elliott T
Commented Nov 26, 2024 at 16:14
@ElliottT No at all. We, beginners, should help each other.:)
– Vlad from Moscow
Commented Nov 26, 2024 at 16:16
@interjay How about the updated code snippet?
– Vlad from Moscow
Commented Nov 26, 2024 at 16:18

Add a comment |

Correct minor typos or mistakes
Clarify meaning without changing it
Add related resources or links
Always respect the author’s intent
Don’t use edits to reply to the author

create code fences with backticks ` or tildes ~
```
like so
```
add language identifier to highlight code
```python
def function(foo):
print(foo)
```
put returns between paragraphs
for linebreak add 2 spaces at end
_italic_ or **bold**
indent code by 4 spaces
backtick escapes `like _so_`
quote by placing > at start of line
to make links (use https whenever possible)

<https://example.com>

[example](https://example.com)

<a href="https://example.com">example</a>

formatting help »
answering help »

Collectives™ on Stack Overflow

How to debug this substring matching code?

Answer*