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Advanced Programming Techniques in D

Function Pointer as a Parameter & a Member

Explore how to define and use function pointers as parameters and members in D programming. Understand how to parameterize filtering and conversion behaviors, create reusable algorithms, and employ concise anonymous functions for clearer code.

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Function pointer as a parameter

Let’s design a function that takes an array and returns another array. This function will filter out elements with values less than or equal to zero and multiply the others by ten:

D
int[] filterAndConvert(const int[] numbers) {

    int[] result;

    

    foreach (e; numbers) {

        if (e > 0) { // filtering,

            immutable newNumber = e * 10; // and conversion

            result ~= newNumber; 

        }

    }

    

    return result;

}

The following program demonstrates its behavior with randomly generated values:

D
import std.stdio; 

import std.random;



int[] filterAndConvert(const int[] numbers) {

    int[] result;

    

    foreach (e; numbers) {

        if (e > 0) { // filtering,

            immutable newNumber = e * 10; // and conversion

            result ~= newNumber; 

        }

    }

    

    return result;

}



void main() {

    int[] numbers;

    // Random numbers

    foreach (i; 0 .. 10) {

        numbers ~= uniform(0, 10) - 5;

    }

    

    writeln("input : ", numbers);

    writeln("output: ", filterAndConvert(numbers)); 

}

The output contains numbers that are ten times the original numbers, which were greater than zero to begin with. The original numbers that have been selected are highlighted.

filterAndConvert() is for a very specific task: It always selects numbers that are greater than zero ...