How To Convert String To Integer In C++

Different data types are used in programming for various purposes, and sometimes it is necessary to convert one type of data to another. One such conversion that programmers often encounter is converting a string to an integer. Strings are a sequence of characters, whereas integers are whole numbers.

This article explains how to convert string to integer in C++ and provides different methods to do so.

Why Is There A Need for Converting From String To Integer In C++

Converting a string to an integer is a crucial operation, especially when dealing with user input, reading data from files, or performing arithmetic operations on numerical data. Here are some reasons listed:

• User input: When taking input from the user, it is typically in the form of a string. Converting it to an integer allows us to perform numerical operations on the input.

• Reading data from files: When reading data from files, the data is usually in string format. Converting it to an integer enables us to process the data more effectively.

• Data validation: Converting a string to an integer allows us to validate the input and ensure that it meets our requirements.

• Manipulating data: Converting a string to an integer allows us to manipulate the data in various ways.

Approaches on how to convert string to integer in c++

Numerous approaches on how to change string to integer in c++ are possible. There are some approaches listed and discussed below.

• Approach -1: Using stoi() function
• Approach -2: Using stringstream class
• Approach -3: Using atoi() function

Approach -1: Using stoi() function

How to convert string to int in C++? The stoi() function is a built-in function in C++ that converts a string to an integer. It takes the string as an argument and returns the corresponding integer value.

Algorithm:

Step 1: Include the header file <string>.

Step 2: Declare a string variable and initialize it with the string to be converted.

Step 3: Use the stoi() function and pass the string variable as an argument.

Step 4: Store the result in an integer variable.

Step 5: Print the integer variable.

Code:

#include <iostream>
#include <string>

using namespace std;

int main() {
    string str = "123";
    int num = stoi(str);
    cout << "Integer value of string is: " << num << endl;
    return 0;
}

Output:

Integer value of string is: 123

Code Explanation:

• First, include the necessary header files.
• declare a string variable “str” and initialize it with the string “123”.
• use the stoi() function to convert the string “123” to an integer and store it in the integer variable “num”.
• print the result.
• return 0 to indicate successful program execution.
• The output displays the integer value of the given string.

Approach-2: Using stringstream class

How to convert string to int in C++? The stringstream class in C++ allows us to read data from a string and perform various operations on it. We can use the >> operator to extract the integer value from the stringstream object.

Algorithm:

Step 1: Include the header files <iostream>, <string>, and <sstream>.

Step 2: Declare a string variable and initialize it with the string to be converted.

Step 3: Declare a stringstream object and initialize it with the string variable.

Step 4: Declare an integer variable and initialize it with 0.

Step 5: Use the >> operator to extract the integer value from the stringstream object and store it in the integer variable.

Step 6: Print the integer variable.

Code:

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main() {
    string str = "456";
    stringstream ss(str);
    int num = 0;
    ss >> num;
    cout << "Integer value of string is: " << num << endl;
    return 0;
}

Output:

Integer value of string is: 456

Code Explanation:

• Includes the necessary header files.
• Declare a string variable “str” and initialize it with the string “456”.
• Declare a stringstream object “ss” and initialize it with the string variable “str”.
• Declare an integer variable “num” and initialize it with 0.
• Use the >> operator to extract the integer value from the stringstream object “ss” and store it in the integer variable “num”.
• Print the result.
• Return 0 to indicate successful program execution.
• The output displays the integer value of the given string.

Approach-3: Using atoi() function

How to convert string to int in C++? The atoi() function is a standard library function in C++ that converts a string to an integer. It takes the string as an argument and returns the corresponding integer value.

Algorithm:

Step 1: Include the header file <cstdlib>.

Step 2: Declare a string variable and initialize it with the string to be converted.

Step 3: Use the atoi() function and pass the string variable as an argument.

Step 4: Store the result in an integer variable.

Step 5: Print the integer variable.

Code:

#include <iostream>
#include <cstdlib>

using namespace std;

int main() {
    string str = "789";
    int num = atoi(str.c_str());
    cout << "Integer value of string is: " << num << endl;
    return 0;
}

Output:

Integer value of string is: 789

Code Explanation:

• Include the necessary header file.
• Declare a string variable “str” and initialize it with the string “789”.
• Use the atoi() function to convert the string “789” to an integer and store it in the integer variable “num”.
• Print the result.
• Return 0 to indicate successful program execution.
• The output displays the integer value of the given string.

Note: Since the atoi() function accepts a character array as an argument, we need to convert the string to a character array using the c_str() method before passing it to the atoi() function.

Best Approach: Using stoi() function

Of the three approaches for converting a string to an integer in C++, the best approach is typically considered to be the first one, which uses the stoi() function. Here are some reasons why:

• stoi() is a built-in function in C++, so it’s already included in the standard library and doesn’t require any additional code or libraries to be imported.
• The stoi() function is designed specifically for converting strings to integers, so it’s generally more efficient and reliable than other methods.
• stoi() has built-in error handling to detect invalid input and throw an exception, which can be helpful in preventing program crashes or errors.
• Using stoi() is also generally more readable and concise than other methods, which can make code easier to understand and maintain.

Sample Problems On How To Change String To Int In C++

Sample Problem-1:

Scenario:

A program needs to read user input as a string and convert it to an integer. The program must ensure that the user enters a valid integer, and should prompt the user to enter a new value if the input is not valid.

Solution steps:

1. Declare a string variable to store the user input.
2. Use a loop to repeatedly prompt the user to enter a value and read the input using getline().
3. Convert the input string to an integer using stoi(), and store the result in an integer variable.
4. Check if the conversion was successful by checking if the input string contains only digits and/or a negative sign.
5. If the conversion was successful, exit the loop and use the integer value in the rest of the program. If not, prompt the user to enter a new value.

Code:

#include <iostream>
#include <string>

using namespace std;

int main() {
    string input;
    int num;
    bool valid = false;
    
    while (!valid) {
        cout << "Enter an integer: ";
        getline(cin, input);
        
        try {
            num = stoi(input);
            valid = true;
        } catch (invalid_argument& e) {
            cout << "Invalid input! Please enter a valid integer." << endl;
        }
    }
    
    cout << "You entered: " << num << endl;
    
    return 0;
}

Output:

Enter an integer: abc
Invalid input! Please enter a valid integer.
Enter an integer: 123
You entered: 123

Code Explanation:

• Declare the necessary variables.
• Use a while loop to repeatedly prompt the user for input and attempt to convert the input string to an integer using stoi().
• Prompt the user to enter an integer and read the input using getline().
• Attempt to convert the input string to an integer using stoi().
• If the conversion fails, an invalid_argument exception is thrown, and the catch block on lines prints an error message and prompts the user to enter a new value.
• If the conversion is successful, the valid flag is set to true and the loop exits.
• print the integer value entered by the user.
• Return 0 to indicate successful program execution.
• The output shows an example of the program rejecting an invalid input and accepting a valid one.

Sample Problem-2:

Scenario:

Suppose you are working on a project that requires you to read multiple lines of input from the user. Each line contains a string followed by an integer value separated by a space. You need to extract the integer value from each line and perform some calculations based on the values. You can use the stringstream class in C++ to extract the integer values from the input string.

Solution steps:

1. Read the input string from the user.
2. Create a stringstream object and initialize it with the input string.
3. Use the >> operator to extract the integer value from the stringstream object.
4. Perform the required calculations based on the integer value.

Code:

#include <iostream>
#include <sstream>

using namespace std;

int main() {
    string input;
    cout << "Enter input string: ";
    getline(cin, input);

    stringstream ss(input);
    string str;
    int value;
    ss >> str >> value;

    cout << "String: " << str << endl;
    cout << "Integer value: " << value << endl;
    // perform calculations using value

    return 0;
}

Output:

Enter input string: hello 123
String: hello
Integer value: 123

Code Explanation:

• We start by including the required header files for input/output operations and the stringstream class.
• In the main function, we declare a string variable ‘input’ to read the input string from the user using the getline() function.
• Then, we create a stringstream object ‘ss’ and initialize it with the input string.
• We declare two variables of type string and integer to store the string and integer values extracted from the stringstream object.
• Using the >> operator, we extract the values from the stringstream object and store them in the corresponding variables.
• Finally, we output the string and integer values extracted from the input string and perform the required calculations using the integer value.

Sample Problem-3:

Scenario:

A user enters their age as a string while filling out a form on a website. The website backend requires the age to be stored as an integer value. The website’s code is written in C++.

Solution steps:

1. Receive the age entered by the user as a string.
2. Use the atoi() function to convert the string to an integer value.
3. Store the integer value of age in a variable.
4. Use the stored age value for further calculations or store it in a database.

Code:

#include <iostream>
#include <cstdlib>
#include <string>

using namespace std;

int main() {
    string ageStr;
    int age;

    // Receive the age entered by the user as a string
    cout << "Enter your age: ";
    getline(cin, ageStr);

    // Use the atoi() function to convert the string to an integer value
    age = atoi(ageStr.c_str());

    // Output the converted age value
    cout << "Your age in integer form is: " << age << endl;

    return 0;
}

Output:

Enter your age: 25
Your age in integer form is: 25

Code Explanation:

• We first include the necessary header files for using the required functions and data types.
• We declare two variables – ageStr of type string to store the age entered by the user and age of type integer to store the converted age value.
• We prompt the user to enter their age using cout and getline() function to store the input in ageStr.
• We use the atoi() function to convert the ageStr string to an integer value and store it in the age variable.
• We output the converted age value using cout.

Conclusion

In conclusion, we’ve explored different approaches on how to convert string into int in C++. Each method has its pros and cons, and we identified the best approach based on the scenario.

The stoi() function is the most efficient and preferred method for converting strings to integers in C++.

However, if you need more control over the conversion process or need to handle specific edge cases, you can use the stringstream class or atoi() function.

By understanding these different methods, you can choose the best one for your needs and efficiently convert strings to integers in your C++ programs.