Converting Number to String in C++

In C++, converting numbers to strings is a very common task, especially while working with user input, file handling, and competitive programming problems. Sometimes we need to convert an integer into a string for concatenation, formatting, or digit manipulation.

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

int main(){

    int num = 123;

    string str = to_string(num);

    cout << str;

    return 0;
}

123

Explanation: The to_string() function converts the integer 123 into the string "123".

Common Method to Converting Number to String

C++ provides multiple methods to convert a numeric value into a string. Some methods are modern and simple, while others are useful for older compilers or special formatting needs.

Method 1: Using to_string()

The to_string() function can be used to convert an integer, floating point values, or any number to a string. This function accepts a number(which can be any data type) and returns the number as the desired string.

Syntax:

string to_string(data_type value);

• Parameters: value -> Any numeric value such as int, float, double, or long.
• Return Type: Returns a string object representing the numeric value.

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

// Driver Code
int main()
{
    // Declaring integer
    int i_val = 20;

    // Declaring float
    float f_val = 30.50;

    // Conversion of int into string using
    // to_string()
    string stri = to_string(i_val);

    // Conversion of float into string using
    // to_string()
    string strf = to_string(f_val);

    // Displaying the converted strings
    cout << "The integer in string is : ";
    cout << stri << endl;
    cout << "The float in string is : ";
    cout << strf << endl;

    return 0;
}

The integer in string is : 20
The float in string is : 30.500000

• Time Complexity: O(n)
• Auxiliary Space: O(n)

Method 2: Using string streams

In this method, a string stream declares a stream object which first inserts a number, as a stream into an object and then uses "str()" to follow the internal conversion of a number to a string.

Syntax:

stringstream_object << value;
string_variable = stringstream_object.str();

Parameters

• value -> Numeric value inserted into the stream.

Return Type

• The str() function returns the converted string.

#include<iostream>
#include <sstream>  
#include <string> 
using namespace std;
int main()
{
    int num = 2016;

    // declaring output string stream
    ostringstream str1;

    // Sending a number as a stream into output
    // string
    str1 << num;

    // the str() converts number into string
    string geek = str1.str();

    // Displaying the string
    cout << "The newly formed string from number is : ";
    cout << geek << endl;

    return 0;
}

The newly formed string from number is : 2016

• Time Complexity: O(n)
• Auxiliary Space: O(n)

Method 3: Using the sprintf() function

sprintf() function stores the output on the char buffer specified in the function, instead of printing the output on the console.

Syntax:

sprintf(char_array, "format_specifier", value);

Parameters

• char_array -> Character buffer where output will be stored.
• "format_specifier" -> Specifies the data type format such as %d, %f.
• value -> Numeric value to convert.

Return Type

• Returns the number of characters written into the buffer.

#include <iostream>
using namespace std;

int main()
{
    // any num
    int n = 12234;
    // string buffer
    char str[1000];

    // sprintf() to print num to str buffer
    sprintf(str, "%d", n);

    cout << "the string is : " << str;

    return 0;
}

// this code is contributed by shivanisingh

the string is : 12234

• Time Complexity: O(n)
• Auxiliary Space: O(n)

Method 4: Using boost lexical cast

Similar to string conversion, the " lexical_cast() " function remains the same, but in the 'boost lexical cast' time argument list modifies to "lexical_cast(numeric_var). 

 Syntax:

lexical_cast<string>(value);

Parameters

• value -> Numeric value to be converted into a string.

Return Type

• Returns the converted string value.

#include <boost/lexical_cast.hpp> 
#include <iostream>
#include <string> 
using namespace std;

int main()
{
    // Declaring float
    float f_val = 10.5;

    // Declaring int
    int i_val = 17;

    // lexical_cast() converts a float into string
    string strf = boost::lexical_cast<string>(f_val);

    // lexical_cast() converts a int into string
    string stri = boost::lexical_cast<string>(i_val);

    // Displaying string converted numbers
    cout << "The float value in string is : ";
    cout << strf << endl;
    cout << "The int value in string is : ";
    cout << stri << endl;

    return 0;
}

The float value in string is : 10.5
The int value in string is : 17

• Time Complexity: O(n)
• Auxiliary Space: O(n)

Comparison of Methods