C++ || How To Filter & Select Items In An Array/Vector/Container & Get The Results Using C++

December 08, 2020 admin No comments

The following is a module with functions which demonstrates how to filter and select items in an array/vector/container, and get the results using C++.

The function demonstrated on this page is a template, so it should work on containers of any type. It also uses a predicate to determine the items to select.

1. Filter – Integer Array

The example below demonstrates the use of ‘Utils::filter‘ to filter an integer array based on a predicate and return its results.

The predicate determines the items to filter and select.





		
		
			1. Filter - Integer Array
			
C++
			
			// Filter - Integer Array

// Declare numbers
int numbers[] = { 1987, 19, 22, 2009, 2019, 1991, 28, 31 };

// Get array size
int size = sizeof(numbers) / sizeof(numbers[0]);

// Declare predicate
auto predicate = [](const auto& x) { return x == 1987 || x == 1991; };

// Get results
auto results = Utils::filter(numbers, numbers + size, predicate);

// Display the results
for (const auto& number : results) {
    std::cout << "Number: " << number << std::endl;
}

// expected output:
/*
    Number: 1987
    Number: 1991
*/
			
				
					
				
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						// Filter - Integer Array
 
// Declare numbers
int numbers[] = { 1987, 19, 22, 2009, 2019, 1991, 28, 31 };
 
// Get array size
int size = sizeof(numbers) / sizeof(numbers[0]);
 
// Declare predicate
auto predicate = [](const auto& x) { return x == 1987 || x == 1991; };
 
// Get results
auto results = Utils::filter(numbers, numbers + size, predicate);
 
// Display the results
for (const auto& number : results) {
    std::cout << "Number: " << number << std::endl;
}
 
// expected output:
/*
    Number: 1987
    Number: 1991
*/

2. Filter – String Vector

The example below demonstrates the use of ‘Utils::filter‘ to filter a sequence of values based on a predicate and return its results.

The predicate determines the items to filter and select.





		
		
			2. Filter - String Vector
			
C++
			
			// Filter - String Vector

// Declare vector
std::vector<std::string> names = { "Kenneth", "Jennifer", "Lynn", "Sole" };

// Declare predicate
auto predicate = [](const auto& x) { return x == "Kenneth" || x == "Jennifer"; };

// Get the results
auto results = Utils::filter(names.begin(), names.end(), predicate);

// Display the results
for (const auto& name : results) {
    std::cout << "Name: " << name << std::endl;
}

// expected output:
/*
    Name: Kenneth
    Name: Jennifer
*/
			
				
					
				
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						// Filter - String Vector
 
// Declare vector
std::vector<std::string> names = { "Kenneth", "Jennifer", "Lynn", "Sole" };
 
// Declare predicate
auto predicate = [](const auto& x) { return x == "Kenneth" || x == "Jennifer"; };
 
// Get the results
auto results = Utils::filter(names.begin(), names.end(), predicate);
 
// Display the results
for (const auto& name : results) {
    std::cout << "Name: " << name << std::endl;
}
 
// expected output:
/*
    Name: Kenneth
    Name: Jennifer
*/

3. Utils Namespace

The following is the Utils Namespace. Include this in your project to start using!





		
		
			Utils Namespace
			
C++
			
			// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Dec 8, 2020
//    Taken From: http://programmingnotes.org/
//    File:  Utils.h
//    Description: Handles general utility functions
// ============================================================================
#pragma once
#include <vector>
#include <algorithm>
#include <iterator>

namespace Utils {
    /**
    * FUNCTION: filter 
    * USE: Filters a sequence of values in the given range [first, last) 
    *   based on a predicate and returns its results
    * @param first: The first position of the sequence
    * @param last: The last position of the sequence
    * @param predicate: A function to test each element for a condition
    * @return: A collection that contains elements from the input
    *   sequence that satisfies the condition
    */
    template<typename InputIt, typename T = typename std::iterator_traits<InputIt>::value_type, typename Pred>
    std::vector<T> filter(InputIt first, InputIt last, const Pred& predicate) {
        std::vector<T> result;
        std::copy_if(first, last, std::back_inserter(result), predicate);
        return result;
    }
}// http://programmingnotes.org/
			
				
					
				
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						// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Dec 8, 2020
//    Taken From: http://programmingnotes.org/
//    File:  Utils.h
//    Description: Handles general utility functions
// ============================================================================
#pragma once
#include <vector>
#include <algorithm>
#include <iterator>
 
namespace Utils {
    /**
    * FUNCTION: filter 
    * USE: Filters a sequence of values in the given range [first, last) 
    *   based on a predicate and returns its results
    * @param first: The first position of the sequence
    * @param last: The last position of the sequence
    * @param predicate: A function to test each element for a condition
    * @return: A collection that contains elements from the input
    *   sequence that satisfies the condition
    */
    template<typename InputIt, typename T = typename std::iterator_traits<InputIt>::value_type, typename Pred>
    std::vector<T> filter(InputIt first, InputIt last, const Pred& predicate) {
        std::vector<T> result;
        std::copy_if(first, last, std::back_inserter(result), predicate);
        return result;
    }
}// http://programmingnotes.org/

4. More Examples

Below are more examples demonstrating the use of the ‘Utils‘ Namespace. Don’t forget to include the module when running the examples!





		
		
			More Examples
			
C++
			
			// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Dec 8, 2020
//    Taken From: http://programmingnotes.org/
//    File:  program.cpp
//    Description: The following demonstrates the use of the Utils Namespace
// ============================================================================
#include <iostream>
#include <string>
#include <exception>
#include <vector>
#include "Utils.h"

void display(const std::string& message);

int main() {
    try {
        // Declare numbers
        int numbers[] = { 1987, 19, 22, 2009, 2019, 1991, 28, 31 };

        // Get array size
        int size = sizeof(numbers) / sizeof(numbers[0]);

        // Declare predicate
        auto predicate = [](const auto& x) { return x == 1987 || x == 1991; };

        // Get results
        auto results = Utils::filter(numbers, numbers + size, predicate);

        // Display the results
        for (const auto& number : results) {
            display("Number: " + std::to_string(number));
        }

        display("");

        // Declare vector
        std::vector<std::string> names = { "Kenneth", "Jennifer", "Lynn", "Sole" };

        // Declare predicate
        auto predicate2 = [](const auto& x) { return x == "Kenneth" || x == "Jennifer"; };

        // Get the results
        auto results2 = Utils::filter(names.begin(), names.end(), predicate2);

        // Display the results
        for (const auto& name : results2) {
            display("Name: " + name);
        }
    } catch (std::exception& e) {
        display("\nAn error occurred: " + std::string(e.what()));
    }

    std::cin.get();
    return 0;
}

void display(const std::string& message) {
    std::cout << message << std::endl;
}// http://programmingnotes.org/
			
				
					
				
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						// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Dec 8, 2020
//    Taken From: http://programmingnotes.org/
//    File:  program.cpp
//    Description: The following demonstrates the use of the Utils Namespace
// ============================================================================
#include <iostream>
#include <string>
#include <exception>
#include <vector>
#include "Utils.h"
 
void display(const std::string& message);
 
int main() {
    try {
        // Declare numbers
        int numbers[] = { 1987, 19, 22, 2009, 2019, 1991, 28, 31 };
 
        // Get array size
        int size = sizeof(numbers) / sizeof(numbers[0]);
 
        // Declare predicate
        auto predicate = [](const auto& x) { return x == 1987 || x == 1991; };
 
        // Get results
        auto results = Utils::filter(numbers, numbers + size, predicate);
 
        // Display the results
        for (const auto& number : results) {
            display("Number: " + std::to_string(number));
        }
 
        display("");
 
        // Declare vector
        std::vector<std::string> names = { "Kenneth", "Jennifer", "Lynn", "Sole" };
 
        // Declare predicate
        auto predicate2 = [](const auto& x) { return x == "Kenneth" || x == "Jennifer"; };
 
        // Get the results
        auto results2 = Utils::filter(names.begin(), names.end(), predicate2);
 
        // Display the results
        for (const auto& name : results2) {
            display("Name: " + name);
        }
    } catch (std::exception& e) {
        display("\nAn error occurred: " + std::string(e.what()));
    }
 
    std::cin.get();
    return 0;
}
 
void display(const std::string& message) {
    std::cout << message << std::endl;
}// http://programmingnotes.org/

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

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Posted in: Advanced, C++ Tags: c++, filter

C++ || How To Filter & Select Items In An Array/Vector/Container & Get The Results Using C++

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