C Isogram tests pass locally but fail online

[gberenzy]

C Isogram tests pass locally but fail online after CLI submission

2/21/23: I appear to currently be having this same issue as issue #56 despite it being a closed issue. Tests pass locally, but fail online after CLI submission. Double checked, it is not a TEST_IGNORE issue. Note that #56 cites the same exercise, Isogram, for the C track.

Submitted files:

isogram.c

#include "isogram.h"

bool is_isogram(const char phrase[])
{
    // Check if NULL
    if (phrase == NULL)
    {
        return false;
    }

    // Create working copy of phrase
    char *input = malloc(strlen(phrase) + 1);
    strcpy(input, phrase);
    
    // Create counter, set to zero
    int counter[26];
    for (int i = 0; i < 26; i++)
    {
        counter[i] = 0;
    }

    // Convert to lowercase
    for (int i = 0; phrase[i] != '\0'; i++)
    {
        input[i] = tolower(phrase[i]);
        if (input[i] != ' ' && input[i] != '-')
        {
            counter[input[i] - 97]++;
        }
        if (counter[input[i] - 97] > 1)
        {
            return false;
        }
    }

    // Free copy
    free(input);

    return true;
}

isogram.h

#ifndef ISOGRAM_H
#define ISOGRAM_H

#include <ctype.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

bool is_isogram(const char phrase[]);

#endif

[gberenzy]

See local tests here:

[iHiD]

Hi. Thanks for posting. It seems there are comments on the GH Issue so I’ll let the conversation continue there, but if it doesn’t get resolved, please post here.

[gberenzy]

Solved on Github! Shout out to siebenschlaefer catching it! See comment.

for (int i = 0; phrase[i] != '\0'; i++)
    {
        input[i] = tolower(phrase[i]);
        if (input[i] != ' ' && input[i] != '-')
        {
            counter[input[i] - 97]++;
        }
        if (counter[input[i] - 97] > 1) // This accesses memory out of bounds when character is not alphabetical
        {
            return false;
        }
    }

Solution:

for (int i = 0; phrase[i] != '\0'; i++)
    {
        input[i] = tolower(phrase[i]);

        if (input[i] != ' ' && input[i] != '-')
        {
            counter[input[i] - 97]++;
     
            if (counter[input[i] - 97] > 1) 
            {
                return false;
            }
        }
    }

[astyanax]

I’m not sure if all/most/many of the exercises support this, but I’ve noticed that the makefile provided usually contains a memcheck command, which is helpful for such cases. External tooling like valgrind or clang sanitizers can help you identify many memory-related errors as well. Welcome to the struggle!

p.s. This is why “It works on my machine!” is never a good answer!

[gberenzy]

@astyanax I ran memcheck and all the tests passed. I wonder if valgrind would have done a more thorough job. Ideally memcheck would have caught the potential seg fault.

p.s. This is why “It works on my machine!” is never a good answer!

Haha sure. I will say Exercism does request users submit an issue if tests pass locally but fail online after confirming it’s not an IGNORE_TEST issue, hence this post!