How To Convert Double To String In C++

In C++, a double data type is used to store decimal values. However, sometimes it may be necessary to convert a double value to a string for various purposes like printing, storing or displaying the value.

In C++, there are multiple approaches to convert a double to a string. Each approach has its own advantages and disadvantages, such as performance, ease of use, and compatibility with different versions of C++.

It is important to consider the precision of the output, rounding rules, and desired output format when converting a ‘double’ to a ‘string’.

Why conversion of double to string in C++ is important

In C++, converting a double to a string is crucial for several reasons:

• Converting ‘double’ to ‘string’ in C++ is important for displaying numeric values in a human-readable format.
• It is commonly used in applications such as user interfaces, reports, and data visualization.
• In some cases, a ‘double’ value needs to be converted to a ‘string’ in order to be stored in a database or transmitted over a network.
• By converting ‘double’ to ‘string’, precision and rounding can be controlled, which is important for applications.
• Additionally, converting ‘double’ to ‘string’ allows for the manipulation of the numeric value as a string.

Approaches for converting double to string in C++

There are several approaches to converting a double to a string in C++. Some of the common approaches are:

1. Using stringstream
2. Using to_string()
3. Using sprintf()
4. Using std::ostringstream class

Approach 1: Using stringstream:

In C++, you can use the ‘stringstream’ class to convert a ‘double’ to a ‘string’. This class is available in the ‘<sstream>’ header and allows you to perform formatted input and output operations on strings. You can create a ‘stringstream’ object and insert the ‘double’ value into it using the << operator, just like you would with ‘cout’.

CODE:

#include <iostream>
#include <sstream>  // Include the header file for stringstream
using namespace std;

int main() {
   double d = 3.14159;
   stringstream ss;  // Create an instance of stringstream
   ss << d;  // Insert the value of d into stringstream object
   string s = ss.str();  // Get the string representation of stringstream object
   cout << "Output using stringstream: " << s << endl;  // Print the string representation of d
   return 0;
}

OUTPUT:

Output using stringstream: 3.14159

Explanation:

• We first declare a double variable ‘d’ and assign a value to it.
• We then create an instance of the ‘stringstream’ class called ‘ss’.
• We use the ‘<<’ operator to insert the value of d into ss.
• We then use the ‘str()’ function to get the string representation of the ‘stringstream’ object.
• Finally, we print the string representation of ‘d’ using the ‘cout’ statement.

Approach 2: Using to_string():

In C++, you can use the ‘to_string()’ function to convert a ‘double’ to a ‘string’. This function is available in the ‘<string>’ header and allows you to convert a numeric value to its equivalent ‘string’ representation.

CODE:

#include <iostream>
#include <string>  // Include the header file for string
using namespace std;

int main() {
   double d = 3.14159;
   string s = to_string(d);  // Convert the value of d to a string using to_string() function
   cout << "Output using to_string(): " << s << endl;  // Print the string representation of d
   return 0;
}

OUTPUT:

Output using to_string(): 3.141590

Explanation:

• We first declare a double variable ‘d’ and assign a value to it.
• We then use the ‘to_string()’ function to convert the value of ‘d’ to a string.
• Finally, we print the string representation of ‘d’ using the ‘cout’ statement.

Approach 3: Using sprintf():

In C++, you can use the ‘sprintf()’ function to convert a ‘double’ to a ‘string’. This function is available in the ‘<cstdio>’ header and allows you to format and store a series of characters in a buffer. You can use ‘sprintf()’ to format a ‘double’ value with a specific number of decimal places and store the result in a character buffer.

CODE:

#include <iostream>
#include <cstdio>  // Include the header file for sprintf()
using namespace std;

int main() {
   double d = 3.14159;
   char buffer[50];  // Declare a character array
   sprintf(buffer, "%.5f", d);  // Convert the value of d to a string using sprintf() function
   string s(buffer);  // Create a string object and initialize it with the contents of buffer
   cout << "Output using sprintf(): " << s << endl;  // Print the string representation of d
   return 0;
}

OUTPUT:

Output using sprintf(): 3.14159

Explanation:

• We first declare a double variable ‘d’ and assign a value to it.
• We then declare a character array called ‘buffer’ with a size of 50.
• We use the ‘sprintf()’ function to convert the value of ‘d’ to a string and store it in the ‘buffer’ using a format specifier.
• We then create a string object called ‘s’ and initialize it with the contents of ‘buffer’.
• Finally, we print the string representation of ‘d’ using the ‘cout’ statement.

Approach 4: Using std::ostringstream

In C++, you can use the ‘std::ostringstream’ class to convert a ‘double’ to a ‘string’. This class provides an output stream that can be used to construct a ‘string’ object. You can insert a ‘double’ value into a ‘ostringstream’ object using the ‘<<’ operator, just like you would with ‘cout’.

CODE:

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

int main() {
   double d = 3.14159;
   ostringstream ss;
   ss << d;  // Insert the value of d into the ostringstream object
   string s = ss.str();  // Get the string representation of d from the ostringstream object
   cout << "Output using std::ostringstream: " << s << endl;  // Print the string representation of d
   return 0;
}

OUTPUT:

Output using std::ostringstream: 3.14159

Explanation:

• We first declare a double variable ‘d’ and assign a value to it.
• We then create an ‘ostringstream’ object ‘ss’.
• We use the ‘<<’ operator to insert the value of ‘d’ into the ‘ostringstream’ object ‘ss’.
• Finally, we get the string representation of ‘d’ from the ‘ostringstream’ object ‘ss’ using the ‘str()’ member function and print it using the ‘cout’ statement.

Best Approach for converting double to string in c++

The best approach to convert a ‘double’ to a ‘string’ in modern C++ code is using the ‘to_string()’ function. This approach offers the following advantages:

• Simple and straightforward: The ‘to_string()’ function is easy to use and requires minimal code to convert a ‘double’ to a ‘string’.
• Safe and reliable: The to_string() function performs bounds checking to ensure that the output string fits into the memory allocated for it, avoiding potential buffer overflow vulnerabilities.
• Portable: The ‘to_string()’ function is part of the C++11 standard and is widely supported by modern compilers and platforms, making it a portable solution for converting ‘double’ values to ‘string’ representations.

Sample Problem For Converting double to string in C++

Sample problem 1

To display numeric values with a specific format or precision. For example, you may need to display a stock price or a currency exchange rate with a certain number of decimal places or with a specific symbol or unit.

(“using std::ostringstream”)

Solution:

• The ‘format_currency()’ function takes three arguments: the ‘double’ value to be formatted, the desired precision, and the currency symbol to be used.
• Inside the function, a ‘ostringstream’ object is created to store the formatted output.
• The precision of the output stream is set using the ‘fixed’ and ‘setprecision()’ manipulators.
• The ‘value’ parameter is written to the output stream, along with the currency symbol, using the ‘<<’ operator.
• The formatted string is obtained from the ‘ostringstream’ object using the ‘str()’ member function.
• In the ‘main()’ function, a ‘double’ value, a precision value, and a currency symbol are initialized.
• The ‘format_currency()’ function is called with these values to obtain a formatted string.
• The formatted string is printed to the console using ‘cout’.

CODE:

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

string format_currency(double value, int precision, string symbol) {
   // Create a string stream object
   ostringstream oss;

   // Set the precision of the output stream
   oss << fixed << setprecision(precision);

   // Write the value to the output stream with the currency symbol
   oss << symbol << value;

   // Return the formatted string
   return oss.str();
}

int main() {
   double price = 123.4567;
   int precision = 2;
   string symbol = "$";

   // Convert the double value to a formatted string
   string formatted_price = format_currency(price, precision, symbol);

   // Print the formatted string
   cout << "The price is: " << formatted_price << endl;

   return 0;
}

OUTPUT:

The price is: $123.46

Sample problem 2

Suppose you are developing an application that requires converting a double data type to a string in C++. The double represents a monetary value and you need to display it as a currency string in a specific format.

(“Using stringstream”)

Solution:

• We first declare a double variable called ‘money’ and initialize it with a value of ‘1234.56789’.
• We then create a stringstream object called ‘stream’ using the ‘std::stringstream’ class.
• We use the ‘<<’ operator to insert the value of ‘money’ into the stringstream object.
• We use the ‘std::fixed’ and ‘std::setprecision(2)’ manipulators to format the double value to 2 decimal places.
• We then call the ‘str()’ method of the stringstream object to convert the stream into a string and store it in the ‘moneyString’ variable.
• Finally, we output the value of ‘moneyString’ to the console.

CODE:

#include <iostream>
#include <sstream>
#include <iomanip>

int main() {
    // Declare and initialize a double variable called money
    double money = 1234.56789;

    // Create a stringstream object called stream
    std::stringstream stream;

    // Insert the value of money into the stringstream object and format it to 2 decimal places using the fixed and setprecision manipulators
    stream << std::fixed << std::setprecision(2) << money;

    // Convert the stringstream object to a string and store it in the moneyString variable
    std::string moneyString = stream.str();

    // Output the value of moneyString to the console
    std::cout << "Money: " << moneyString << std::endl;

    // Return 0 to indicate successful program execution
    return 0;
}

OUTPUT:

Money: 1234.57

Sample problem 3

Assume that you are writing a program that calculates the average temperature of a city over a week and stores the value in a double variable. You want to display this value as a string in the format “The average temperature for this week is XX.XX degrees.”

(“Using sprintf()”)

Solution:

• ‘sprintf()’ function is used to format and store a series of characters and values into a string.
• In the above code, ‘sprintf()’ function is used to format the string str with the value of the ‘avg_temp’ variable.
• ‘%.2f’ is used to format the ‘avg_temp’ value as a floating-point number with two decimal places.
• The formatted string is stored in the character array ‘str’.
• Finally, the ‘str’ string is printed using the ‘cout’ statement.

CODE:

#include <iostream> // includes input/output stream functionality
#include <cstdio> // includes C-style input/output functionality

int main() {
    double avg_temp = 21.68; // declares and initializes a double variable named avg_temp with the value 21.68
    char str[50]; // declares a character array named str with size 50
    sprintf(str, "The average temperature for this week is %.2f degrees.", avg_temp); 
    // formats the string "The average temperature for this week is XX.XX degrees." with the value of avg_temp and stores it in the str array
    std::cout << str << std::endl; // prints the str array to the console
    return 0; // returns 0 to the operating system indicating successful termination of the program
}

OUTPUT:

The average temperature for this week is 21.68 degrees.

Sample problem 4

Scenario:

Suppose you are working on a program that involves calculating the total cost of an online shopping order. You need to display the total cost as a string so that it can be easily displayed to the user.

Problem:

Convert a double variable representing the total cost to a string (“using the to_string()”) function in C++.

Solution:

• We start by declaring a double variable ‘total_cost’ and initializing it with the value 45.99.
• We then declare a string variable ‘str_total_cost’ and use the ‘to_string()’ function to convert the double variable ‘total_cost’ to a string.
• Finally, we display the result using ‘std::cout’ and ‘<<’ operator.
• The output shows the total cost as a string with six decimal places, because the default precision of ‘to_string()’ is 6.

CODE:

#include <iostream>
#include <string>

int main() {
  // Declare a double variable named total_cost and initialize it with the value 45.99
  double total_cost = 45.99;

  // Declare a string variable named str_total_cost and use the to_string() function to convert the double variable total_cost to a string
  std::string str_total_cost = std::to_string(total_cost);

  // Display the result using cout and the insertion operator
  std::cout << "Total cost: " << str_total_cost << std::endl;

  // Indicate successful program execution by returning 0
  return 0;
}

OUTPUT:

Total cost: 45.990000

Conclusion

In conclusion, there are multiple approaches available to convert a ‘double’ value to a ‘string’ in C++. Each approach has its own advantages and disadvantages in terms of performance, ease of use, and compatibility with different versions of C++.

When converting a ‘double’ value to a ‘string’, it is important to consider the precision of the output, rounding rules, and the desired format of the output string.

Overall, understanding how to convert a ‘double’ value to a ‘string’ is a useful skill in C++, especially in applications that require numeric input or output in a human-readable format.