Can I use || instead of else if in C? - c

Question

I'm new to C, and I'm trying to improve a program's source code that I found in a book. I noticed that the source code contains a lot of "else if" conditions but has the same results, so I tried to compress them into a smaller code using || operator in the if function.
1st code:
#include <stdlib.h>
#include <stdio.h>
int main(){
char card_name[3];
puts("Enter the card_name: ");
scanf("%2s", card_name);
int val = 0;
if (card_name[0] == 'K'){
val = 10;
} else if (card_name[0] == 'Q'){
val = 10;
} else if (card_name[0] == 'J'){
val = 10;
} else if (card_name[0] == 'A'){
val = 11;
} else {
val = atoi(card_name);
}
printf("The card name value is %i\n", val);
return 0;
}
My improvement:
#include <stdlib.h>
#include <stdio.h>
int main(){
char card_name[3];
puts("Enter the card_name: ");
scanf("%2s", card_name);
int val = 0;
if (card_name[0] == 'K' || 'Q' || 'J' ){
val = 10;
} else if (card_name[0] == 'A'){
val = 11;
} else {
val = atoi(card_name);
}
printf("The card name value is %i\n", val);
return 0;
}
The problem with mine: When I write any another value it always prompt 10. What should I do ?

Answer 1

The issue is in C anything that is not 0 is a truthy value so in your code you have if (card_name[0] == 'K' || 'Q' || 'J'). This checks if card_name[0] == 'K' but this will always evaluate to true because you have || 'Q' and C treats 'Q' as truth. Here is the proper fix
#include <stdlib.h>
#include <stdio.h>
int main(){
char card_name[3];
puts("Enter the card_name: ");
scanf("%2s", card_name);
int val = 0;
if (card_name[0] == 'K' || card_name[0] == 'Q' || card_name[0] == 'J' ){
val = 10;
} else if (card_name[0] == 'A'){
val = 11;
} else {
val = atoi(card_name);
}
printf("The card name value is %i\n", val);
return 0;
}

Answer 2

This is not the correct expression:
(card_name[0] == 'K' || 'Q' || 'J' )
This will be always evaluate to true because 'Q' and 'J' are not 0.
You should write:
(card_name[0] == 'K') || (card_name == 'Q') || (card_name == 'J')

Answer 3

if (card_name[0] == 'K' || 'Q' || 'J' ){
val = 10;
}
'Q', etc is evaluated as a truth statement and since the value of the char is not 0 will always be true
should be
if (card_name[0] == 'K' || card_name[0] == 'Q' || card_name[0] == 'J' ){
val = 10;
}

Answer 4

if (card_name[0] == 'K' || 'Q' || 'J' )
should be
if (card_name[0] == 'K' || card_name[0] == 'Q' || card_name[0] == 'J' )
'Q' will always evaluate to true, you need the full comparison in each or.

Answer 5

Check the operator precedence in you if statement. If you look here http://en.cppreference.com/w/c/language/operator_precedence, you can see, that == has higher priority than ||.
The correct if statement should be like this:
if (card_name[0] == 'K' || card_name[0] == 'Q' || card_name[0] == 'J' ){
If you are not sure, what is the operator precedence, you can use brackets like this:
if ((card_name[0] == 'K') || (card_name[0] == 'Q') || (card_name[0] == 'J') ){
Note:
Your code is not correct, because if(card_name[0] == 'K' || 'Q' || 'J' ) is understood as if(card_name[0] == 'K' || true || true) which is always true

Answer 6

No, as many others have pointed out, using if (card_name[0] == 'K' || 'Q' || 'J' ) is wrong, and they have also described why and how to correct it. However, if you really just want a shorter and snappier way of writing the test, you could go for a switch-statement:
#include <stdio.h>
#include <stdlib.h>
int main()
{
char card_name[3];
puts("Enter the card_name: ");
scanf("%2s", card_name);
int val = 0;
switch (card_name[0]) {
case 'K': /* fallthrough */
case 'Q': /* fallthrough */
case 'J': val = 10; break;
case 'A': val = 11; break;
default: val = atoi(card_name);
}
printf("The card name value is %i\n", val);
return 0;
}
Note that a switch statement is not a generic replacement for if-else if in C as each case needs to be constant integer expression (expressing it sloppily, a C book will tell you the restrictions correctly).

Answer 7

Always keep in mind ||means one of the condition must be true to proceed . if && is used , it checks all the condition and proceeds only if all the conditions satisfy. Pretty basic info for noobies...