Convert String to int in C++

In C++, converting a string into an integer is a common operation used while handling user input, file data, and numeric processing. Since string and int are different data types, C++ provides several built-in methods and functions to perform this conversion safely and efficiently

• C++ provides multiple ways to convert a string into an integer such as stoi(), atoi(), stringstream, sscanf(), and strtol().
• Different methods are useful in different situations depending on safety, compatibility, and input format.

Methods to Convert String to int in C++

String to int Conversion Using stoi()

The stoi() function in C++ takes a string as an argument and returns its value in integer form. This approach is popular in current versions of C++, as it was first introduced in C++11.

Syntax:

stoi(str, position, base);

Parameters:

• str: string to be converted. (compulsory)
• position: starting position. (optional with default value = 0)
• base: base of the number system. (optional with default value = 10)

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

int main() {
  string str1 = "45";
  string str2 = "3.14159";
  char str3[] = "31337 geek";

  // Store the integer value which
  // by stoi()
  int myint1 = stoi(str1);
  int myint2 = stoi(str2);
  int myint3 = stoi(str3);

  cout << "stoi(\"" << str1 << 
          "\") is " << myint1 << '\n';
    cout << "stoi(\"" << str2 << 
            "\") is " << myint2 << '\n';
    cout << "stoi(\"" << str3 << 
            "\") is " << myint3;
    return 0;
}

stoi("45") is 45
stoi("3.14159") is 3
stoi("31337 geek") is 31337

Explanation:

• "45" is converted directly into integer 45.
• "3.14159" stops conversion at . and returns 3.
• "31337 geek" stops at the first non-digit character and returns 31337.

String to int Conversion Using atoi()

The atoi() function in C++ takes a character array or string literal as an argument and returns its value in an integer. It is defined in the <stdlib.h> header file. This function is inherited by C++ from C language, so it only works on C style strings i.e. array of characters.

Syntax:

atoi(str);

#include <iostream>
#include <stdlib.h>
using namespace std;

int main() {
    char str1[] = "141";
    char str2[] = "3.14";
    
    // Convert string into integer
    // using atoi()
    int res1 = atoi(str1);
    int res2 = atoi(str2);

    cout << "atoi(" << str1 << ") is " << res1 << "\n";
    cout << "atoi(" << str2 << ") is " << res2;
    return 0;
}

atoi(141) is 141
atoi(3.14) is 3

Explanation:

• atoi() reads numeric characters from the beginning of the string.
• Conversion stops when a non-numeric character is found.

String to int Conversion Using stringstream

The stringstream class allows strings to be treated like input/output streams. It is flexible and works with both integers and floating-point values.

Syntax:

stringstream ss;
ss << str;
ss >> num;

The below C++ program demonstrates how to convert a string to int using a stringstream object:

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

int main() {
    string s = "12345";

    // object from the class stringstream
    stringstream geek;

    // inserting string s in geek stream
    geek << s;

    // The object has the value 12345
    // and stream it to the integer x
    int x = 0;
    geek >> x;

    // Now the variable x holds the
    // value 12345
    cout << "Value of x + 1 : " << x + 1;
    return 0;
}

Value of x + 1 : 12346

To summarize, stringstream is a convenient way to manipulate strings. This method also works for both C style strings or C++ style strings.

Converting String to int Using sscanf()

 'sscanf()' is a C-style function similar to scanf(). It reads input from a string rather than standard input. 

Syntax:

sscanf(source, formatted_string, ...);

Parameters:

• source:  source string.
• formatted_string: a string that contains the format specifiers.
• ... :  variable arguments list that contains the address of the variables in which we want to store input data.

There should be at least as many of these arguments as the number of values stored by the format specifiers.

Return Value:

• On success, the function returns the number of variables filled.
• In the case of an input failure, before any data could be successfully read, the end of the function (EOF) is returned.

#include <iostream>
using namespace std;

int main() {
    const char* str = "12345";
    int x;
    sscanf(str, "%d", &x);
    cout << "The value of x: " << x;
    return 0;
}

The value of x: 12345

Explanation: "%d" tells sscanf() to read an integer. The converted value is stored in variable x.

Using For Loop Convert Strings into int

It is the naive method of string to int conversion where we compare each character to its ASCII values and then get the value of that numeric character.

#include <bits/stdc++.h>
using namespace std;

int main() {
    string number = "13";
    int i = 0;

    // Traversing string
    for (char c : number) {
        
        // Checking if the element is number
        if (c >= '0' && c <= '9') {
            i = i * 10 + (c - '0');
        }
        
        // Otherwise print bad output
        else {
            cout << "Bad Input";
            return 1;
        }
    }
    cout << i;
}

13

Explanation: Above program converts a string into an integer by iterating through each character, checking if it's a digit, and then calculating the integer value by updating the variable i as i = i * 10 + (c - '0') for each valid digit. Non-digit characters result in a "Bad Input" message and terminate the program.

String to int Conversion Using strtol()

The strtol function is used to converts a string to a long integer value, respectively.

Syntax:

strtol(str, endptr, base);

Parameters:

• st: Pointer to the null-terminated string to be converted
• endptr: Reference to a pointer to character. The function stores the address of the first invalid character in *endptr. If endptr is a null pointer, the function does not store this information.
• base: The number base to interpret the string. Must be between 2 and 36, inclusive, or 0. If base is 0, the function determines the base from the string itself. If the base is 16, the string may begin with the prefix "0x" or "0X", in which case the number is parsed as a hexadecimal value.

Return Value:

• If the string is empty or does not contain any digits, 0 is returned.
• If the conversion is successful, the function returns the converted long integer value.
• If the conversion fails, the function returns 0 and sets *endptr to point to the beginning of the string.   

#include <stdio.h>
#include <stdlib.h>

int main() {
  char str[] = "12345";
  char* endptr;
  long int value = strtol(str, &endptr, 10);
  if (endptr == str) {
    printf("No digits were found\n");
  } else if (*endptr != '\0') {
    printf("Invalid input: %s\n", str);
  } else {
    printf("The value is %ld\n", value);
  }
  return 0;
}