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Converting String to Number and Vice-Versa in C++

  • Difficulty Level : Medium
  • Last Updated : 08 Jan, 2022

In general or more specifically in competitive programming there are many instances where we need to convert a number to a string or string to a number. But lack of knowledge of certain essential tools binds us to do so. Some methods to achieve this task are mentioned in this article. 

Converting String to Number

Method 1: Using stringstream class or sscanf(): stringstream is a convenient way to manipulate strings. sscanf() is a C style function similar to scanf(). It reads input from a string rather than standard input. 

Method 2: String conversion using stoi() or atoi().

  • stoi() : The stoi() function takes a string as an argument and returns its value.
  • atoi() : The atoi() function takes a character array or string literal as an argument and returns its value.

Both these methods have been discussed in detail in this article.

Method 3: Using boost lexical cast: Boost library offers an inbuild function “lexical_cast(“string”)”, which directly converts a string to a number. It returns an exception “bad_lexical_cast” in case of invalid input. 
 

CPP




// C++ code to demonstrate working of lexical_cast()
#include <boost/lexical_cast.hpp> // for lexical_cast()
#include <iostream>
#include <string> // for string
using namespace std;
int main()
{
    string str = "5";
    string str1 = "6.5";
 
    // Initializing f_value with casted float
    // f_value is 6.5
    float f_value = boost::lexical_cast<float>(str1);
 
    // Initializing i_value with casted int
    // i_value is 5
    int i_value = boost::lexical_cast<int>(str);
 
    // Displaying casted values
    cout << "The float value after casting is : ";
    cout << f_value << endl;
    cout << "The int value after casting is : ";
    cout << i_value << endl;
 
    return 0;
}
Output
The float value after casting is : 6.5
The int value after casting is : 5

Converting Number to String

Method 1: 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 number to string. 

Implementation:   

CPP




// C++ code to demonstrate string stream method
// to convert number to string.
#include<iostream>
#include <sstream>  // for string streams
#include <string>  // for 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;
}
Output
The newly formed string from number is : 2016

Method 2: Using to_string() : This function accepts a number(can be any data type) and returns the number in the desired string. 

CPP




// C++ code to demonstrate "to_string()" method
// to convert number to string.
#include <iostream>
#include <string> // for string and to_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;
}
Output
The integer in string is : 20
The float in string is : 30.500000

Method 3: Using boost lexical cast: Similar to string conversion, the ” lexical_cast() ” function remains the same, but this time argument list modifies to “lexical_cast(numeric_var).  

CPP




// C++ code to demonstrate "lexical_cast()" method
// to convert number to string.
#include <boost/lexical_cast.hpp> // for lexical_cast()
#include <iostream>
#include <string> // for string
using namespace std;
 
// Driver Code
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;
}
Output
The float value in string is : 10.5
The int value in string is : 17

This article is contributed by Manjeet Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 


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