Using 0 and 1 as false and true in C

Question

I am learning C from a book and the author is explaining using 1 for true and 0 for false with while loops. He explained with the following the sample code but I couldn't understand it.

INPUT

#include<stdio.h>
int main(void)
{
    int n = 3;

    while (n)
        printf("%2d is true\n", n--);
    printf("%2d is false\n", n);


    n = -3;
    while (n)
        printf("%2d is true\n", n++);
    printf("%2d is false\n", n);


    getchar();
    return 0;
}

OUTPUT

3 is true
2 is true
1 is true
0 is false
-3 is true
-2 is true
-1 is true
0 is false

Answer 1

May be if you add braces you can understand the code properly.

#include<stdio.h>
int main(void)
{
    int n = 3;

    while (n)
    {
      printf("%2d is true\n", n--);
    }
    printf("%2d is false\n", n);

    n = -3;
    while (n)
    {
      printf("%2d is true\n", n++);
    }
    printf("%2d is false\n", n);


    getchar();
    return 0;
}

The code that you posted and the above code are same. If there are no braces the next statement after the while will be executed.

This:

printf("%2d is true\n", n--);

in the first while will continue to execute until while(n) is false. while(n) is equivalent to while(n!=0). while(something) is false only when something is 0.

So, when n=3 the condition in the first loop is true and then n--(n=n-1) makes n 2 and for 2 the condition is true and so on. When n is 0 the condition becomes false and exits the loop. The program then prints false and similarly the second while loop is executed.

Answer 2

There are several places in C where any expression are used a Boolean values deciding the program's flow:

• Conditional statement if
• Loop constructs for, while, and do-while
• Conditional expression ?-:

In all these places where a Boolean value is consumed, an implicit != 0 is added, so

while (n)

means

while (n != 0)

Hence, all values other than zero are considered true.

There are other places in the language where a Boolean value is produced by a logical expression, e.g. the logical NOT ! operator. In this case, the values produced are confined to the 0, 1 set. Specifically, a logical NOT of a zero is 1.

Answer 3

In fact C treats 0 as FALSE and non-0 as TRUE.

Answer 4

The first time it reaches while(n) the value of n is 3. Because only 0 counts a false, 3 counts are true and that statement amounts to while(3) that is "while true" so the following line gets executed:

printf("%2d is true\n", n--);

That line outputs the value of n. If you don't understand why that is going to output that value go back a chapter or two and read up on printf(.). I don't know what book you're reading but I assume it follows a logical order and introduced output pretty early on.

That line also subtracts one from n (that's what n-- means). If you don't understand that, again go back a bit. That expression reduces n by one but returns its value before the subtraction.

That results in the first line of output:

3 is true

Then the program loops back to the while condition. Now n is 2. But 2 still counts as true so the in-loop code (the first printf) gets executed again giving:

2 is true

And ends with n holding the value 1. That leads to one further execution of the in-loop code giving

1 is true

Now that execution leaves with n having the value 0. That counts as false so the loop condition while(n) is while(0) which means "while false".

So execution bypasses the in-loop code and executes:

printf("%2d is false\n", n);

The leads to the line

0 is false

The rest of the program does a similar thing in reverse starting with n having the value -3 and then incrementing it (by n++) till again it hits false (n being 0).

That gives the lines

-3 is true
-2 is true
-1 is true
0 is false