How to assign true to a boolean variable in c language

Question

I want to assign true to the boolean variable leapYear if the integer variable year is a leap year (A leap year is a multiple of 4, and if it is a multiple of 100, it must also be a multiple of 400.)in a c program

Here's the code I tried.

bool leapYear;
int year;

printf("Enter a year ");
scanf("%d", &year);
if (year %4 = 0 || year %100 = 0 || year %400 = 0)
    printf("true");

I tried to compile the code I wrote but it gives an error saying unknown type name 'bool'.

Also, your leap year logic is wrong, it returns true for all multiples of 4, it doesn't do the century exceptions correctly. See https://stackoverflow.com/questions/3029417/correct-expression-for-checking-leap-years — Barmar, Jul 16 '19 at 17:25
Even if you use `_Bool b = true;` you still need to `#include ` for the `true` to be accepted. — Weather Vane, Jul 16 '19 at 17:30

Vlad from Moscow · Accepted Answer · 2019-07-16T17:49:48.230

You have to include the header file <stdbool.h>

In this file bool is defined as a macro and expands to the C standard unsigned integer type _Bool.

Any value that is not equal to 0 is converted to 1 and assigned to a variable of the type _Bool.

To set for example a variable to true You could just write

bool leapYear = 1;

The header file also contains macros for true and false.

Here is a demonstrative program

#include <stdio.h>
#include <stdbool.h>

int main(void) 
{
    bool leapYear = true;

    printf( "leapYear = %u\n", leapYear );

    return 0;
}

Its output is

leapYear = 1

If you do not want to include the header then you could just use the standard integer type _Bool.

For example

#include <stdio.h>

int main(void) 
{
    _Bool leapYear = 1;

    printf( "leapYear = %u\n", leapYear );

    return 0;
}

Or even the type int like

int leapYear = 1;

In C logical operations return integer 1 if an expression is true or 0 otherwise.

Zachary Oldham · Answer 2 · 2019-07-16T17:54:50.233

1

You would need to #include <stdbool.h> to use booleans, but in C we usually use an integer. The way it works is 0 is false, anything else is true.

In your case,

int leapYear = 0;    // default to false
... 
leapYear = 1;    // set to true. Any non-zero value works.

edited Jul 16 '19 at 17:54

answered Jul 16 '19 at 17:24

Zachary Oldham

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`bool` *is* a standard type since C99 – Eugene Sh. Jul 16 '19 at 17:27
@EugeneSh. sorry, wrong terminology. What I meant was it is not included by default, you have to include it yourself. – Zachary Oldham Jul 16 '19 at 17:28
3

`_Bool` is a standard type that is available without inclusion of any headers. So still, yes, booleans are "included by default". They need to be, because `_Bool` has special rules that don't apply to any other type. `bool` is just an alias for `_Bool`, provided for convenience by stdbool.h. – John Bollinger Jul 16 '19 at 17:42
@JohnBollinger It is my understanding that using `_Bool` is considered bad practice – Zachary Oldham Jul 16 '19 at 17:54
3

Dunno where you got that idea, @ZacharyOldham. If anything, I'd be inclined to put it the other way around -- using `bool` in C is potentially problematic because it risks colliding with code that provides its own definition for that symbol, and such code isn't particularly uncommon. But that's entirely beside the point. `_Bool`s are booleans. The `bool` of stdbool.h is the same type, but you don't need the header to have booleans. (And `_Bool` *is* an integer.) – John Bollinger Jul 16 '19 at 18:21

Lee Daniel Crocker · Answer 3 · 2019-07-17T16:56:03.513

1

As everyone has pointed out, your main problem was failure to #include <stdbool.h>. Also, your use of = instead of == in the conditional. I, personally, would not try to use a single conditional because the rules for leap year are complex and hard to translate into a single statement. I'd use a function for clarity:

#include <stdbool.h>

bool is_leap(int year) {
    if (0 == (year % 400)) return true;
    if (0 == (year % 100) && year > 1582) return false;
    if (0 == (year % 4)) return true;
    return false;
}

int main(int argc, char *argv[]) {
    . . .
    bool leap = is_leap(2019);
    . . .
}

edited Jul 17 '19 at 16:56

answered Jul 16 '19 at 18:00

Lee Daniel Crocker

12,927
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Off by 1. 1700 is often considered a non-leap year [ref](https://www.quora.com/Why-are-years-such-as-1700-1800-1900-not-leap-years-even-though-they-are-divisible-by-4-but-1600-and-2000-are). Suggest `0 == (year % 100) && year >= 1700` or `0 == (year % 100) && year > 1582` – chux - Reinstate Monica Jul 17 '19 at 02:39

How to assign true to a boolean variable in c language

3 Answers3