Thread: Switching chars in file.

I have to make a program that takes the file that is firs command line argument and whenever it finds a char in that file that is same as the second command line argument it switches it with char that is third command line argument

Code:

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

int main(int argc, char* argv[])
{
    FILE *cit;
    char s[100];
    int i;

    cit=fopen(argv[1],"r");
    if (cit==NULL)
        return 1;

    fgets(s,100,cit);
    int d=strlen(s);

    for(i=0;i<d;i++)
    {
        if(s[i]==argv[2])
            s[i]=argv[3];
    }
    s[i]=0;
    fclose(cit);
    cit=fopen(argv[1],"w");
    if(cit==NULL)return 1;
    fputs(s,cit);
    fclose(cit);



    return 0;
}

When i run it using command prompt it shows no errors or something, yet nothing changes in file, probably because string remained the same after my for loop, but i can't tell what's wrong.

Originally Posted by Salem

No errors you say....

Code:

$ gcc -Wall bar.c
bar.c: In function ‘main’:
bar.c:19:16: warning: comparison between pointer and integer [enabled by default]
bar.c:20:17: warning: assignment makes integer from pointer without a cast [enabled by default]

Well, i didn't considered it as error since it appears just as a warning, i saw it, but i couldn't figure out what's the problem since both s[i] and argv[2] are chars, i don't know how one of them can be integer, is that the only problem with this code?

Originally Posted by laserlight

s is an array of 100 chars, therefore s[i] is a char (which is an integer type). argv is a pointer to a pointer, therefore -- contrary to your claim -- argv[2] is a pointer, not a char.


Other things to consider include:

• Check the return value of fgets.
• Check argc before using argv[1], argv[2] and argv[3].
• Unless you really want it there, remove the newline character from entering the input if it was stored.
• Instead of using the magic number 100, define a named constant.
• Unless the return value of 1 has some special significance here, return EXIT_FAILURE from <stdlib.h> instead.
• The return type of strlen is size_t, so consider using size_t instead of int to store the return value of strlen. If you do so, then i should also be a size_t.
• Use descriptive names: it is not clear to me why you choose "cit" and "d".

Thanks a lot for helping me understand this, im not really familiar with pointers (especially with "double pointers"), basically i compared a value of s[i] with the memory location of command line argument (that's how i understood it), but i fixed it.
As for the other things you pointed out, im just a beginner in programming the only thing im doing is trying to have a code that works, but i guess i should keep them in mind. And i choose "cit" and "d" because they are descriptive in my language. Thanks again.

Originally Posted by laserlight

What language might that be? The single character name "d" is unlikely to be descriptive in any language, except maybe the language of mathematics (e.g., if you are implementing an algorithm from a mathematics text that uses "d" as the conventional variable name). Such single character names are generally only used for loop/array indices, and even then they might not be used if a short yet descriptive alternative is apparent.

It's first letter of the word that means length, i guess i should use three letters just like i did for "cit".

You are right, i will keep that in mind for the next time.