-2
if (class == 'a','A')

I'm trying to continue only if the user inputs 'a' or 'A'. Every time I build it, it warns "left hand operand of comma has no effects".

I'm very new to C, so I apologize if I'm not specific enough or missing something obvious.

Jeremy
  • 181
  • 1
  • 2
  • 10

4 Answers4

4

that's not how to do it , you should do instead 

if(class == 'a' || class == 'A')
Farouq Jouti
  • 1,657
  • 9
  • 15
  • Thank you very much! Do you know why It worked the other way, even though it was wrong? – Jeremy Oct 08 '14 at 21:10
  • 3
    @Jeremy: It didn't *work*, but it did compile. The `,` is a *comma operator*. Consult your C text book to find out what that means. This: `(class == 'a', 'A')` is equivalent to: `((class == 'a'), 'A')`. – Keith Thompson Oct 08 '14 at 21:14
4

Change the condition to

if ( class == 'a' || class == 'A' )

As for your original expression

if (class == 'a','A')

then there is used the comma operator and the value of expression is 'A'. As 'A' is not equal to zero then the condition will be always equal to true. The expression before the comma is discarded.

Vlad from Moscow
  • 301,070
  • 26
  • 186
  • 335
1

Whilst both current answers are correct, I think it may be more idiomatic to do:

#include <ctype.h>
...
if (tolower(class) == 'a')
{
    ...
}
abligh
  • 24,573
  • 4
  • 47
  • 84
0

In if (class == 'a','A'), the left half is the comparison class == 'a' and may evaluate to either true or false (in the boolean sense¹). The right half is just 'A', which is not equal to 0 and thus true. The left half gets evaluated but the result is not used. Then the right half is evaluated, and its "result" is 'A', which is true. Hence, the code acts as if you wrote

if (true)
   ...

You can check this by using a '0' at the right half:

if (class == 'a', 0)
    printf ("if-statement is true\n");
else
    printf ("if-statement is false\n");

and running this (after coercing the compiler to compile it) will indeed show the "false" line.

The compiler complains because the left half class == 'a' "does nothing": it has no side effect on class or on the result of the if statement. Thus, the compiler assumes this is a mistake and therefore issues the warning. It is a warning and not an error, because the syntax is correct. For comparison, you are allowed to write something similar such as

if (class = 'a', 'A')
    printf ("class == a,A seems to be true\n");
else
    printf ("That didn't work.\n");

which has a valid side effect (class gets the value for the character constant 'a') -- but it also will always state that it appears to be true, because the second half is what is passed on to the if statement.

¹ For C, I use true and false in the boolean sense only, as C has by default no concept of these named values without including stdbool.h (this defines the constants by name). The underlying concept is the same, though: an if statement evaluates its parameter, which is considered false if it evaluates to 0 (and thus will not execute the following expression), true if it evaluates to anything else than 0.

Jongware
  • 22,200
  • 8
  • 54
  • 100