CODEWITHSUNDEEP
c
Fullmetal_Alchemist_Fan
Fullmetal_Alchemist_Fan

Reputation: 761

Unable to compare two char arrays against each other

I have a reverse function that I would like to use to compare two strings against each other, but it doesn't seem to work. I think that every time the string is iterated over it overwrites the value every time. I'm think that it needs to be stored in a separate array to be able to evaluate the two against each other. Here's what I have so far: 

int compare_strings(char [], char []);

char* reverse(char* str)
{
    int i, j;
    char temp;
    i=j=temp=0;

    j=strlen(str)-1;
    for (i=0; i<j; i++, j--)
    {
        temp=str[i];
        str[i]=str[j];
        str[j]=temp;
    }
    return str;
}
int main(int argc, char* argv []) {
    char word[100];
    char const* const fileName = argv[1];
    FILE *file = fopen(fileName,"r");
    char s[100];
    char formattedWord[100];
    while(fgets(word, 100, file)!=NULL){
        memset(s,'\0',sizeof s);
        char *token=strtok(word," ");
        while(token!=NULL){
            size_t n=strlen(s);
            sprintf(&s[n],"%s",token);
            token=strtok(NULL," ");

        }
        if (reverse(s) == s)
            {
                printf("%s\n", reverse(s));
            }    
        }
    fclose(file);

    return 0;
}

Upvotes: 1

Views: 105

Answers (2)

dbush
dbush

Reputation: 223689

There are three problems. First is this:

if (reverse(s) == s)

This is not how you compare two strings. You instead call strcmp, and if it returns 0 the two strings are the same.

However, the reverse function reverses the string in place. So with either method the result will always be true.

Inside of reverse, you want to allocate new memory and write the reversed string there, then return that new string.

char* reverse(char* str)
{
    int i, j;
    char *temp;
    i=j=0;

    temp = malloc(strlen(str) + 1);
    j=strlen(str)-1;
    for (i=0; j>=0; i++, j--)
    {
        temp[j]=str[i];
    }
    temp[strlen(str)] = '\0';
    return temp;
}

Now when you call reverse, you need to save off the return value so you can free that memory later.

    char *s2;
    ...
    s2 = reverse(s);
    if (!strcmp(s,s2))
        {
            printf("%s\n", s2);
        }
    free(s2);

Finally, fgets will append a newline to the end of the string to read if there's enough space for it. So you need to account for that when tokenizing the string:

    char *token=strtok(word," \n");
    while(token!=NULL){
        size_t n=strlen(s);
        sprintf(&s[n],"%s",token);
        token=strtok(NULL," \n");
    }

Upvotes: 2

Linus
Linus

Reputation: 1518

You can't do if (reverse(s) == s) because that will simply compare the pointer values. You must use a library function such as strcmp or to iterate through each char and compare them.

Something like this would be valid.

char *reverse_s = reverse(s);
if(!strcmp(reverse_s, s)) {
  printf("%s\n", reverse_s);
}

Upvotes: 4

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