C++ Program For String to Double Conversion

In this article, we will learn how to convert strings to double in C++. For this conversion, there are 3 ways as follows:

1. Using stod().
2. Using stold().
3. Using atof().

Let’s start discussing each of these methods in detail.

Example: 

Input: s1 = “14.25” 
           s2 = “34.87” 

Output: s1 + s2
              long: 49.12 

1. Using stod()

In C++, the stod() function performs a string to double conversion. 

Syntax: 

double stod(const std::string& str, std::size_t* pos = 0);

Parameters:

• str: the string to convert.
• pos: address of an integer to store the number of characters processed. This parameter can also be a null pointer, in which case it is not used.

Below is the C++ program to convert string to double using stod():

C++

// C++ program to convert string
// to double using stod()
#include <iostream>
using namespace std;

// Driver code
int main()
{
    char s1[] = "14.25";
    char s2[] = "34.87";
    double n1 = stod(s1);
    double n2 = stod(s2);
    double res = n1 + n2;
    cout << res;
    return 0;
}

Output

49.12

Time complexity: O(1).

Auxiliary Space: O(1).

2. Using stold()

In C++, the stold() function performs a string to long double conversion.   

Syntax: 

long double stold(const string& str, size_t *pos = 0);

Parameters: 

• str: the string to convert.
• pos: address of integer to store the index of the first unconverted character. This parameter can also be a null pointer, in which case it is not used.

Below is the C++ program to convert string to long double using stold():

C++

// C++ program to convert string
// to long double using stold()
#include <iostream>
using namespace std;

// Driver code
int main()
{
    char s1[] = "14.25";
    char s2[] = "34.87";
    long double n1 = stold(s1);
    long double n2 = stold(s2);
    long double res = n1 + n2;
    cout << res;
    return 0;
}

Output

49.12

Time complexity: O(1).

Auxiliary Space: O(1).

3. Using atof() 

In C++, the atof() method translates a string’s contents as a floating-point integer and returns its value as a double. 

Syntax:

double atof (const char * str)

Parameters: The function accepts single mandatory parameter str which is the representation of a floating-point number.

Below is the C++ program to convert string to double using atof():

C++

// C++ program to convert string
// to double using atof()
#include <iostream>
using namespace std;

// Driver code
int main()
{
    char s[] = "14.25";
    double num = atof(s);
    cout << num;
    return 0;
}

Output

14.25

Time complexity: O(1).

Auxiliary Space: O(1).