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C++ <cmath>

C++ Vector (Vector)c++ Keywords and Identifiers

C++ String (string) and float/Interconversion between double (floating-point) numbers

In this tutorial, we will learn how to convert strings to floating-point numbers and vice versa with examples.

C ++String conversion to floating-point and double floating-point numbers

The simplest way to convert a string to a floating-point number is to use the followingC ++ 11Function:

std :: stof() - Convert string to float
std :: stod() - Convert string to double
std :: stold() - Convert string to long double.

These functions are defined in the string header file.

Example1: C ++Convert string to floating-point and double floating-point numbers

Example

#include<iostream>
#include<string>
int main() {
　　　　std::string str = "123.4567";
　　　　//　Convert a string to a floating-point number
　　　　float num_float = std::stof(str);
　　　　//　Convert string to double floating-point number double
　　　　double num_double = std::stod(str);
　　　std::cout << "num_float = " << num_float << std::endl;
　　　std::cout << "num_double = " << num_double << std::endl;
　　　　return 0;
}

Output Result

num_float =　123.457
num_double =　123.457

Example2：Convert C ++ Convert a char array to double

We can use the std::atof() function to convert a char array to double.

Example

#include<iostream>
//　atoi() needs cstdlib
#include<cstdlib>
int main() {
　　　　//　Declare and initialize a character array
　　　　char str[] = ""123.4567";
　　　　double num_double = std::atof(str);
　　　　std::cout << "num_double = " << num_double << std::endl;
　　　　
　　　　return 0;
}

Output Result

num_double =　123.457

C ++Floating-point and double floating-point numbers to strings

We can use C ++ 11 std::to_string() function converts float and double to strings. For older C ++Compiler, we can use the std::stringstream object.

Example3：Use to_string() to convert floating-point numbers and double floating-point numbers to strings

Example

#include<iostream>
#include<string>
int main() {
　　　　float num_float =　123.4567F;
　　　　double num_double =　123.4567;
　　　　std::string str1　= std::to_string(num_float);
　　　　std::string str2　= std::to_string(num_double);
　　　std::cout << "Float to String = " << str1　<< std::endl;
　　　std::cout << "Double to String = " << str2　<< std::endl;
　　　　return 0;
}

Output Result

Float to String =　123.456703
Double to String =　123.456700

Example4：Use stringstream to convert floating-point numbers and double floating-point numbers to strings

Example

#include<iostream>
#include<string>
#include<sstream>　//　Use stringstream
int main() {
　　　　float num_float =　123.4567F;
　　　　double num_double =　123.4567;
　　
　　　　//　Create a stringstream object
　　　　std::stringstream ss1;
　　　　std::stringstream ss2;
　　
　　　　//　Assign the value of num_float to ss1
　　　　ss1　<< num_float;
　　
　　　　//　Assign the value of num_float to ss2
　　　　ss2　<< num_double;
　　　　　//Use ss1and ss2Initialize two string variables
　　　　　//Then use the str() function to convert it to a string format
　　　　std::string str1　= ss1.str();
　　　　std::string str2　= ss2.str();
　　
　　　　std::cout << "Float to String = " << str1　<< std::endl;
　　　　std::cout << "Double to String = " << str2　<< std::endl;
　　　　return 0;
}

Output Result

Float to String =　123.457
Double to String =　123.457

Related conversions: Convert C ++String to int Conversion.

C++ Vector (Vector)c++ Keywords and Identifiers