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The boolean data type in C

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• The Boolean data type in C (or the lack there of)
  • Before 1999, the C programming language does not have any boolean (logic) data type

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  • C99 (see Wikipedia page: click here) introduced the

    _Bool

    data type that is not available in other C derivates (like C++)

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  • The _Bool "data type" is the same as an (ordinary) int

    They also define some symbolic names to represent the Boolean constants true and false in a header file to make a C program "feels like" it is using boolean values

    Take a look at:

    /usr/lib/gcc/x86_64-linux-gnu/7/include/stdbool.h

    You will see these definitions that simulate Boolean constants:

    #define bool _Bool #define true 1 #define false 0

    You can include this header file in your C program using:

    #include <stdbool.h> // <...> means: search in C's include files

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  • Summary:

    C does not provide a boolean data type The C programming language uses the int integer type to represent the boolean data type

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• Boolean values (constants)
  • Boolean values (in general)

    Unlike Java, the C programming language does not define Boolean values BTW, the Boolean values in Java are defined using the keywords: true false Example: Java: boolean a; a = true;

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  • Boolean values in C:

    0 = false Any other value = true

    Example: here are 4 if-statements using variable numerical values in C:

    #include <stdio.h> #include <stdbool.h> int main( int argc, char* argv[] ) { printf("true = %d\n", true); printf("false = %d\n\n", false); if ( 1 ) printf("value 1 is true\n"); else printf("value 1 is false\n"); if ( 0 ) printf("value 0 is true\n"); else printf("value 0 is false\n"); if ( 0.1 ) printf("value 0.1 is true\n"); else printf("value 0.1 is false\n"); if ( 0.0 ) printf("value 0.0 is true\n"); else printf("value 0.0 is false\n"); }

    Output:

    true = 1 false = 0 value 1 is true value 0 is false value 0.1 is true value 0.0 is false

    Confirming the fact that: 0 ≡ false and any value ≠ 0 ≡ true

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/logic1.c

    How to run the program:

    To compile:   gcc logic1.c          To run: ./a.out