Thread: Newbie question about if()

I'm working through the C tutorial (if statements) and I want to use an if() statement to check the user input and act on it accordingly.

It seems that I always hit the final 'else' statement even though the character stored in the variable is correct (namely A or S in my case). Is there something odd about evaluating char variables as opposed to int etc?

Any pointers would be helpful. Code attached for reference.

Code:

void askuser_fn()
{
   /* Define our variables here */
   char user_input[1];

   clear_screen

   /* Ask the user what to do */
   printf( "Would you like to add or subtract?\n\n" );
   printf( "Enter A for Add and S for Subtract: " );
   scanf ( "%s", &user_input );

   /* Let's see what it is that the user wants us to do */
   if ( user_input == "A" ) {
      addition_fn();
   }
      else if ( user_input == "S" ) {
         subtraction_fn();
      }
         else printf( "Sorry you entered an invalid option. Option entered was: %s\n", &user_input );
}

"A" is a string, and you can't compare strings with == (the address of the string literal will actually be compared here). Use 'A' (single quotes) if you want to compare against the character A. A char array of size 1 is quite redundant as well. :P

That's because you are trying to do a string comparison on a character field. Try using single quotes, not double quotes. Also, user_input is a string array. Try using user_input[0] or *user_input.

Code:

 /* Let's see what it is that the user wants us to do */
   if ( user_input[0] == 'A' ) {
      addition_fn();
   }
      else if ( user_input[0] == 'S' ) {
         subtraction_fn();
      }
         else printf( "Sorry you entered an invalid option. Option entered was: %s\n", &user_input );
}

Thanks for the replies. I'll go try it out.

With regards to the string array etc - this is due to my getting ahead of myself as I haven't learnt them yet. Thanks for the tips though.

Code:

   scanf ( "%s", &user_input );

And you dont' need & here.

Okay so I got it working and I'm adding some more "checking".

I found that this works:

Code:

   if ( ( user_input[0] == 'A' ) || ( user_input[0] == 'a' ) ) {

However if I try this:

Code:

   if ( user_input[0] == 'A'  || 'a'  ) {

Then it doesn't in a sense. I find that no matter what I enter, this first IF entry is always evaluated as TRUE, even if I type 'X' into the input. Perhaps I need to brush up on my boolean logic.

@Bayint Naung
Thanks for the tip.

What is being evaluated as true is not the first part, but the second part.

Code:

if ( user_input[0] == 'A' || 'a' ) {

'a' will always be true, so when you say || 'a' you are always making a true statement.

It's called operator precedence.

Originally Posted by samf

'a' will always be true, so when you say || 'a' you are always making a true statement.

I still don't understand why - but as I'm still a noob, I'll use the first example of code and get back to the tutorials.

Thanks for helping nonetheless.

Try looking here...

Cprogramming.com Tutorial: If Statements

Yep that's what I'm working through - but it would seem that I've missed a trick.

The penny will drop at some point! :-)

There's a few things wrong with that program.

Code:

askuser_fn()
{
   /* No need to specify an array size if it's only a single character. */
   char user_input; 
   clear_screen; 
   /* You terminate statements with a semicolon, silly. */

   printf("Would you like to add or subtract?\n\n");
   printf("(A)dd (S)ubtract: ");
   /* That's how the classics did it. */
   scanf("%c", &user_input);
   /* Since the program expects a single character, use %c instead of %s. */

   if (user_input == "A" || user_input == "a")
         addition_fn();
   /* Don't use braces if it's only a single statement. Just indent. */
   
   if (user_input == "S" || user_input == "s")
         subtraction_fn();

   else 
         printf("Sorry you entered an invalid option. Option entered was: %c\n", user_input);
}