Comparing user input to a predefined array of characters in C programming language

Question

I have the following code which is supposed to get user input as commands then check whether the command is predefined. However, for any command entered the output is "You asked for help". I figured that the problem might be related to the way I'm comparing the user input string with the set string(s) but I still need help solving the problem.

char command[10];
char set[10];
char set1[10];
strcpy(set, "help");
strcpy(set1, "thanks");

int a = 0;
while (a != 1)//the program should not terminate.
{
   printf("Type command: ")
   scanf("%s", command);
   if (strcmp(set, command))
   {
       printf("You asked for help");
   }
    else if (strcmp(set1, command))
   {
       printf("You said thanks!");
   }
   else
   {
       printf("use either help or thanks command");
   }
}

Answer 1

if (strcmp(set, command))

should be

if (strcmp(set, command) == 0)

The reason is that strcmp returns a nonzero value if the LHS or RHS is larger, and zero if they are equal. Since zero evaluates to "false" in a conditional, you'll have to explicitly add the == 0 test to make it true in the sense that you're expecting, which is equality.

Answer 2

First of all, always length-bound your input while using scanf(), like

 scanf("%9s", command);

for a 10-element char array, to avoid buffer overflow by overly-long inputs.

That said, the if...else block logic works in below way:

if (expression is true) {         // produce a non-zero value, truthy 
   execute the if block
   }
else {                             // expression is falsy
   execute the else block
   }

In your case, the controlling expression is strcmp(set, command).

Now, point to note, in case of a match, strcmp() returns a 0, in case of mismatch, it returns a non-zero value.

Thus,

• If your input matches the expected pre-selected string, you'll get a 0 - which will be evaluated as falsy and against your expectation, it'll go to the else part.
• If your input does not match the expected pre-selected string, you'll get a non-zero value which will be evaluated as truthy and once again, the if block will be executed erroneously.

So, in your case, you need to change the condition to negate the return value, like

   if (!strcmp(set, command))
   {
       printf("You asked for help");
   }
    else if (!strcmp(set1, command))
   {
       printf("You said thanks!");
   }
   else
   {
       printf("use either help or thanks command");
   }