Remove all characters from string except alphabets in C++

Understanding String Filtering

Filtering a string means removing all non-alphabetic characters from it.

We will explore three different methods to achieve this in C++.

Method 1: Using a Loop

This method iterates through the string and keeps only alphabetic characters.

#include <iostream>
#include <cctype>
#include <algorithm>

using namespace std;

void remove_non_alpha(string &str) {
    int j = 0;
    for (int i = 0; i < str.length(); i++) {
        if (isalpha(str[i])) {
            str[j++] = str[i];
        }
    }
    str.resize(j);
}

int main() {
    string str = "H3ll0 W@rld!";
    remove_non_alpha(str);
    cout << "Filtered String: " << str << endl;
    return 0;
}
            

Method 2: Using Recursion

This method filters the string recursively.

#include <iostream>
#include <cctype>
#include <algorithm>

using namespace std;

void filter_recursive(string &str, int index, int new_index) {
    if (index == str.length()) {
        str.resize(new_index);
        return;
    }
    if (isalpha(str[index])) {
        str[new_index++] = str[index];
    }
    filter_recursive(str, index + 1, new_index);
}

int main() {
    string str = "C0d!ng123";
    filter_recursive(str, 0, 0);
    cout << "Filtered String: " << str << endl;
    return 0;
}
            

Method 3: Using an Extra String

This method uses an auxiliary string to store only alphabetic characters.

 
#include <iostream>
#include <cctype>
#include <algorithm>

using namespace std;

void filter_with_aux(const string &str, string &result) {
    for (char ch : str) {
        if (isalpha(ch)) {
            result += ch;
        }
    }
}

int main() {
    string str = "P@r0gr@mm1ng";
    string filtered;
    filter_with_aux(str, filtered);
    cout << "Filtered String: " << filtered << endl;
    return 0;
}