Extract all integers from string in C++

In this article, we will discuss how to extract all the integers from a string using C++ program. We will explore all the possible approaches to do so. First of all, let's understand the problem statement.

We have a string that contains a mix of digits and non-digits. We need to extract all the integers from the string and store them in a vector. The integers can be positive or negative. For example,

// Input String
string str = "ab24wj-123fow"

// Output Vector
vector<int> vec = {24, -123}

Approaches to Extract Integers from String

Here is the list of approaches to extract integers from a string in C++ that we will be discussing in this article with stepwise explanation and complete example codes.

• Using Regular Expressions
• Using Stringstream
• Using String Tokenizer
• Using Manual loop

Using Regular Expressions to Extract Integers

The regular expressions are used to match a pattern in a string. In this approach, we will use the c++ regex library to extract all the integers from the string and store them in a vector. This method is only supported in C++ 11 and above. versions. Let's see an example of how to use this approach.

Example

In the code below, we defined a input string "abc123x4y56" and a regular expression "\d+" to extract all the integers from the string. We then used the sregex_iterator class to iterate through the matches and stored them in a vector. Finally, we printed the vector to see the output.

#include <iostream>
#include <regex>
#include <string>
#include <vector>
using namespace std;

int main() {
    string input = "23abc123x4y56";
    regex numberRegex(R"(\d+)");
    sregex_iterator it(input.begin(), input.end(), numberRegex);
    sregex_iterator end;
    vector<int> numbers;

    while (it != end) {
        numbers.push_back(stoi(it->str()));
        ++it;
    }

    for (int n : numbers)
        cout << n << " ";
}

The output of the above code will be:

23 123 4 56

Using Stringstream to Extract Integers

The stringstream is an object of string class defined in the <sstream> header file. This approach involves using a stringstream object to remove all the non-digits from the string and then converting the remaining string to an integer. Let's see an example of how to use this approach.

Example

In the code below, we defined a input string "abc123x4y56" and a stringstream object ss. We then used the isdigit function to check if the character is a digit or not in the string. If it is a digit, we added it to the stringstream object. Otherwise, we added a blank space to the stringstream object. After that, we iterated through the stringstream object and converted the remaining string to an integer and stored it in a vector.

#include <iostream>
#include <sstream>
#include <vector>
using namespace std;

int main() {
    string input = "abc123x4y56";
    stringstream ss;
    vector<int> numbers;

    // Remove all non-digits from the string
    for (char c : input) {
        if (isdigit(c)) ss << c;
        else ss << ' ';
    }

    // Convert the remaining string to an integer and store it
    int num;
    while (ss >> num)
        numbers.push_back(num);

    // Print the vector
    for (int n : numbers)
        cout << n << " ";
}

The output of the above code will be:

123 4 56

Using String Tokenizer to Extract Integers

The string tokenizer is a class defined in the <string> header file. This approach involves using a string tokenizer object to extract all the integers from the string and store them in a vector. Let's see an example of how to use this approach.

Example

In the code below, we defined a input string "abc123x4y56" and a string tokenizer object tok. We then used the getToken() function to extract all the integers from the string and stored them in a vector. Finally, we printed the vector to see the output.

#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main() {
    string input = "Tutorials23Point3224";
    vector<int> numbers;
    string token;

    for (char c : input) {
        if (isdigit(c)) {
            token += c;
        } else {
            if (!token.empty()) {
                numbers.push_back(stoi(token));
                token.clear();
            }
        }
    }

    if (!token.empty()) {
        numbers.push_back(stoi(token)); // Handle trailing number
    }

    for (int n : numbers)
        cout << n << " ";
}

The output of the above code will be:

23 2324

Using Manual Loop to Extract Integers

In this approach, we will run a loop through the string and check if the character is a digit or not. If it is a digit, we add it to a vector.

Example

In the code below, we run a loop through the string and check if the character is a digit or not. If it is a digit, we add it to a vector.

#include <iostream>
#include <vector>
using namespace std;

int main() {
    string input = "This is a string with numbers 123 and 456";
    vector<int> numbers;
    int current = 0;
    bool inNumber = false;

    for (char c : input) {
        if (isdigit(c)) {
            current = current * 10 + (c - '0');
            inNumber = true;
        } else if (inNumber) {
            numbers.push_back(current);
            current = 0;
            inNumber = false;
        }
    }
    if (inNumber)
        numbers.push_back(current);  // for trailing numbers

    for (int n : numbers)
        cout << n << " ";
}

The output of the above code will be:

123 456