How can I convert string to double in C/C++?

Given a string like "-27.89", we can convert it into a double value such as -27.890000. In this article, we use various approaches from C/C++ library functions to perform this string into double conversion:

• Using atof() Function
• Using strtod() Function
• Using stod() Function
• Using stringstream() Function

String to Double Conversion Using atof() Function

You can use atof() function of C library that accepts the parameter of string value to convert the string into a double.

Example

In this example, we illustrate atof() to convert string into double.

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

int main() {
   char str[] = "-27.89";
   double num = atof(str);
   printf("Converted number: %.5f", num);
   return 0;
}

The above program produces the following output:

Converted number: -27.89000

String to Double Conversion Using strtod() Function

strtod() is a C library function that pointed to the string to convert it into a double-precision floating-point number. The use the strtod() function shows greater precision and handles a larger range of values as compared to the atof() function.

Example

Following is the example of strtod() function that shows its usage to C program.

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

int main() {
   char str[] = "123.45abc";
   char* endptr;
   double num = strtod(str, &endptr);
   
   if(*endptr != '\0')
       printf("Extra characters after number: %s", endptr);
   else
       printf("Converted number: %.2f", num);
   return 0;
}

The above program produces the following output:

Extra characters after number: abc

String to Double Conversion Using stod() Function

The stod() function of C++ accepts the input parameter as string that convert the string into double return the desired result.

Example

The basic illustration of stod() function in C++ program to convert string into double.

#include <iostream>
#include <string>

using namespace std;

int main() {
   string str = "98.76";
   double num = stod(str);
   cout << "Converted number: " << num;
   return 0;
}

The above program produces the following output:

Converted number: 98.76

String to Double Conversion Using stringstream() Function

The stringstream() is used to manage a string object as a stream that follows the header, like sstream and string. Here, we take a string str that holds a number in text form. Then we use stringstream() to convert this string into a double type. Finally, the converted number is printed as a double value.

Example

Following is the C++ program to show the usage of stringstream() function for the conversion of string into double.

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

using namespace std;

int main() {
   string str = "45.67";
   double num;
   stringstream(str) >> num;
   cout << "Converted number: " << num;
   return 0;
}

The above program produces the following output:

Converted number: 45.67

Comparison of C Vs. C++ Functions

Below are the comparisons of C and C++ functions based on error handling and headers as follows: