Unable to compare two char arrays against each other

Question

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;
}

You are comparing pointers but not the values. Use `strcmp`. — Mahesh, Oct 30 '15 at 18:26
Possible duplicate of [How do I properly compare strings in C?](http://stackoverflow.com/questions/8004237/how-do-i-properly-compare-strings-in-c) — Mahesh, Oct 30 '15 at 18:27
Why your Title say CHARS and you need to compare an Array of chars (string) ? **if(strcmp(string1,string2) == 0){printf("True");}else{printf("Flase");}** [Try this](http://ideone.com/yDRhCl) — Michi, Oct 30 '15 at 18:32
"Evaluate two strings against each other"? What is that supposed to mean? "Evaluate" does not mean "compare". — Thomas Padron-McCarthy, Oct 30 '15 at 18:41

score 4 · Answer 1 · answered Oct 30 '15 at 18:28

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);
}

score 2 · Accepted Answer · answered Oct 30 '15 at 18:43

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");
    }

I'm using a text file as a command line argument. The file reads like so: happy b00b face monster cake — Fullmetal_Alchemist_Fan, Oct 30 '15 at 20:24
so it should come out as happy yppah b00b b00b face ecaf monster retsnom cake ekac — Fullmetal_Alchemist_Fan, Oct 30 '15 at 20:25
@Fullmetal_Alchemist_Fan Looks good with my changes. Please update your question with your updated code (after the original code) and the input file. — dbush, Oct 30 '15 at 20:30
Okay, I'm an idiot. I forgot to assign s2 to reverse(s). It works now! Thanks! :D — Fullmetal_Alchemist_Fan, Oct 30 '15 at 20:36

Unable to compare two char arrays against each other

2 Answers2