How do you use a string array in a conditional statement in C?

Question

I want to use a string array in an if statement to test whether the input string matches any of the strings in the array.

So far this is what I've tried:

void checkForError(char input[50])
{

    const char *input2[]={"print","loadStarter","terminate()"};

    if(input != input2)
    {
        printf("Error:Incorrect method '%s'.\n",input);
    }
    else
    {
        abort();
    }
}

And if I were to enter something in the array like "print" it would end up showing me:

Error:Incorrect method 'print'.

but when I try something not listed in the array like "g" it repeats the error message nonstop.

I was thinking perhaps something like this could work:

void checkForError(char input)
{
  if(strcmp(input,"print"))!=0 || strcmp(input,"loadStarter"))!=0 || strcmp(input,"terminate()")
  {
    printf("Error:Incorrect method '%s'.\n");
  }
  else
  {
    abort();
  }
}

But it turns out that actually doesn't work so what do I do?

"It repeats the error message nonstop". Impossible from only what you've shown; there is no loop in the code. Show the part where you call `checkForError` as well, please. — Karl Knechtel, Nov 27 '11 at 06:12

score 4 · Answer 1 · edited May 23 '17 at 12:27

4

You cannot compare strings (usefully) in C using == and !=; you must use library functions such as strcmp instead. See How to compare strings in an "if" statement? for details.

edited May 23 '17 at 12:27

Community

1
1

answered Nov 27 '11 at 05:02

score 2 · Accepted Answer · 2011-11-27T05:30:46.710

I think a good solution to your question would be to loop around your array, aborting on the first match.

void checkForError(char* input)
{
   const char *input2[]={"print","loadStarter","terminate()"};
   const int len = sizeof(input2)/sizeof(input2[0]);

   for (int i = 0; i < len ;i ++)
   {
       if (strcmp(input,input2[i]) == 0)
       {
          //I have matched the string input2[i]
          abort();
       }
    }

     // Nothing matched
     printf("Not found\n");
 }

This would also be easier to extend than any handcoded method.

Also, if you plan on doing a lot with these strings, and you have a limited number of strings, you should probably turn them into some sort of enum. This way you do not have to have strcmp scattered everywhere, and you can use a standard switch statement.

must be const int len = sizeof(input2)/sizeof(input2[0]); – Sergei Nikulov Nov 27 '11 at 05:29 — Sergei Nikulov, Nov 27 '11 at 05:29

MByD · Answer 3 · 2011-11-27T05:19:21.853

Your second thought is correct, you should not compare the strings using == operator, anyway, I'm not sure whether the rest is a typo or not, but it should be like that:

void checkForError(char * input)
//                      ^ note the * (pointer)
{
  if(strcmp(input,"print")!=0 || strcmp(input,"loadStarter")!=0 || strcmp(input,"terminate()") != 0)
//  you forgot brackets
  {
    printf("Error:Incorrect method '%s'.\n", input);
    //                                       ^ you forgot the "input" parameter
  }
  else
  {
    abort();
  }
}

score 0 · Answer 4 · answered Nov 27 '11 at 05:08

a better method would be to have a return value then have your error message depend on return value.

// return 1 when string is OK, 0 otherwise:

int checkForError(const char* input)
{
  if(!strcmp(input,"print")) || !strcmp(input,"loadStarter")0 || !strcmp(input,"terminate()")
  {
    return 1;
  }
  else
  {
    return 0;
  }
}

How do you use a string array in a conditional statement in C?

4 Answers4