C++ || How To Filter & Select Items In A Vector & 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 a vector, and get the results using C++.

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

1. Filter

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.



		
		
			1. Filter
			
C++
			
			// Filter 

// 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, 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 
 
// 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, predicate);
 
// Display the results
for (const auto& name : results) {
    std::cout << "Name: " + name << std::endl;
}
 
// expected output:
/*
    Name: Kenneth
    Name: Jennifer
*/

2. 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;
    }
    
    /**
    * FUNCTION: filter 
    * USE: Filters a sequence of values based on a predicate and returns 
    *   its results
    * @param source: The collection to filter
    * @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 T, typename Pred>
    std::vector<T> filter(const std::vector<T>& source, const Pred& predicate) {
        return filter(source.begin(), source.end(), predicate);
    }
}// 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;
    }
    
    /**
    * FUNCTION: filter 
    * USE: Filters a sequence of values based on a predicate and returns 
    *   its results
    * @param source: The collection to filter
    * @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 T, typename Pred>
    std::vector<T> filter(const std::vector<T>& source, const Pred& predicate) {
        return filter(source.begin(), source.end(), predicate);
    }
}// http://programmingnotes.org/

3. 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 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, predicate);

        // Display the results
        for (const auto& name : results) {
            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 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, predicate);
 
        // Display the results
        for (const auto& name : results) {
            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/