Question

How to check whether a number is positive/zero/negative without using conditional statements in C++

Answer 1

#include <iostream>

int main() {
    int n = 89;

    // create text labels for each possible case
    const char* result[3] = {
        "negative",  // index 0
        "zero",      // index 1
        "positive"   // index 2
    };

    // compute an index without using any conditional statements
    // (n > 0) becomes 1 if true, 0 if false
    // (n < 0) becomes 1 if true, 0 if false
    // index = 1 + (n > 0) - (n < 0)
    int index = 1 + (n > 0) - (n < 0);

    // print the classification
    std::cout << result[index] << std::endl;
    
     /*
       Checks:

       n = -3
       (n > 0) = 0
       (n < 0) = 1
       index = 1 + 0 - 1 = 0
       result = negative

       n = 0
       (n > 0) = 0
       (n < 0) = 0
       index = 1 + 0 - 0 = 1
       result = zero

       n = 89
       (n > 0) = 1
       (n < 0) = 0
       index = 1 + 1 - 0 = 2
       result = positive
    */
}


/*
run:

positive

*/

 

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Answer 2

#include <iostream>

int main() {
    int number = 88;

    // boolean checks
    bool is_positive = number > 0;
    bool is_negative = number < 0;
    bool is_zero     = number == 0;

    // lookup table for the three possible results
    const char* result[3] = {
        "negative",  // index 0
        "zero",      // index 1
        "positive"   // index 2
    };

    // convert booleans to an index:
    // is_positive → 1 if true
    // is_negative → 1 if true
    // index = 1 + is_positive - is_negative
    int index = 1 + is_positive - is_negative;

    std::cout << result[index] << std::endl;

    /*
       Checks:

       number = -3
       is_positive = 0
       is_negative = 1
       is_zero     = 0
       index = 1 + 0 - 1 = 0
       result = negative

       number = 0
       is_positive = 0
       is_negative = 0
       is_zero     = 1
       index = 1 + 0 - 0 = 1
       result = zero

       number = 89
       is_positive = 1
       is_negative = 0
       is_zero     = 0
       index = 1 + 1 - 0 = 2
       result = positive
    */
}



/*
run:

positive

*/

 

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Answer 3

#include <iostream>

int main() {
    int number = 87;

    // bitwise masks:
    // negative → bit 0 (value 1)
    // zero     → bit 1 (value 2)
    // positive → bit 2 (value 4)
    unsigned mask =
          (number < 0)        // becomes 1 if true → negative mask
        | ((number == 0) << 1) // becomes 2 if true → zero mask
        | ((number > 0) << 2); // becomes 4 if true → positive mask

    // lookup table indexed by mask value
    const char* result[8] = {
        "undefined", // 0 (should never occur)
        "negative",  // 1
        "zero",      // 2
        "undefined", // 3
        "positive",  // 4
        "undefined", // 5
        "undefined", // 6
        "undefined"  // 7
    };

    std::cout << result[mask] << std::endl;

    /*
       Checks:

       number = -3
       (number < 0)  = 1
       (number == 0) = 0
       (number > 0)  = 0
       mask = 1 | 0 | 0 = 1
       result = negative

       number = 0
       (number < 0)  = 0
       (number == 0) = 1 << 1 = 2
       (number > 0)  = 0
       mask = 0 | 2 | 0 = 2
       result = zero

       number = 89
       (number < 0)  = 0
       (number == 0) = 0
       (number > 0)  = 1 << 2 = 4
       mask = 0 | 0 | 4 = 4
       result = positive
    */
}



/*
run:

positive

*/

 

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