How To Convert String To Double In C++

A string is an object of the std::string class in the C++ programming language. It is a collection of characters. Contrarily, a double data type is used to represent decimal numbers with a wider range of values and more accuracy. In some C++ applications, you might need to convert a string to a double.

For instance, you might need to convert a string of user input into a double in order to do calculations. In this post, we’ll go over numerous C++ conversion methods for strings to doubles.

Why is there a need to know how to convert string to double in c++

For a number of reasons, converting a string to a double in C++ is a crucial process. The following are some of the key causes for which we would need to learn how to do this:

• Input validation: It’s frequently required to validate user input when it comes in the form of a string to make sure it satisfies specific requirements.
• Mathematical calculations: Many mathematical operations need the use of numerical values, which are commonly represented in C++ as doubles.
• Data processing: Processing vast amounts of data that is stored in text format may occasionally be necessary.
• Formatting the output: When printing out numerical numbers in a computer, it may be essential to style the output specifically.

Approaches on how to convert string to double in c++

Numerous approaches are present on how to change double to string in c++. There are some approaches listed and discussed below.

• Approach-1: Using atof() function

• Approach-2: Using stringstream class

• Approach-3: Using stod() function

Lets dive in details for each approach.

Approach-1: Using atof() function

Using the atof() method is the simplest and most direct approach to convert a string to a double in C++. The matching double value is returned by this function after receiving a string as input.

Algorithm:

1. The input string should be declared as a string variable and initialised.
2. Create a double variable and declare it to hold the transformed value.
3. As an argument, pass the input string when using the atof() function.
4. In the double variable, save the double value that was returned.
5. Show the converted value on paper.

Code:

#include <iostream>
#include <cstdlib>
using namespace std;

int main()
{
   string str = "3.14";
   double num = atof(str.c_str());
   //using the inbuilt atof() function to convert string str into double num
   cout << "The converted value is: " << num << endl;
   return 0;
}

Output:

The converted value is: 3.14

Explanation:

1. The string variable str is given the input string “3.14”.
2. The str.c_str() function is used to call the atof() function. The atof() function can accept a C string that has been null-terminated as a parameter from the c_str() function.
3. The double variable num holds the double value that the atof() function returns.
4. The console displays the converted value.

Approach-2: Using stringstream class

Using a stringstream object is an additional C++ method for converting a string to a double. In this method, the string is transformed into a stringstream object, the double value is extracted from the stringstream, and the double value is then assigned to a double variable.

Algorithm:

1. The input string should be declared as a string variable and initialised.
2. Provide a stringstream object and supply it with the input string.
3. Create a double variable and declare it to hold the transformed value.
4. Use the >> operator to remove the double value from the stringstream.
5. Put the double value you extracted in the double variable.
6. Show the converted value on paper.

Code:

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

int main()
{
   string str = "3.14";
   double num;
   stringstream ss(str);
   //creating an object ss using the stringstream class
   ss >> num;
   //using the object and the >> operator to get the converted double value
   cout << "The converted value is: " << num << endl;
   return 0;
}

Output:

The converted value is: 3.14

Explanation:

1. The string variable str is given the input string “3.14”.
2. str is given as a parameter to a stringstream object called ss that is already declared.
3. Declaring the double variable num.
4. The double value is taken out of the stringstream and given to num using the >> operator.
5. The console displays the converted value.

Approach-3: Using stod() function

A new function named stod() that transforms a string into a double was added in C++. The stod() function is more reliable and capable of processing a greater variety of input types.

Algorithm:

1. The input string should be declared as a string variable and initialised.
2. Create a double variable and declare it to hold the transformed value.
3. By giving the input string as a parameter, call the stod() function.
4. In the double variable, save the double value that was returned.
5. Show the converted value on paper.

Code:

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

int main()
{
   string str = "3.14";
   double num = stod(str);
   //the stod() builtin function is used to convert the string into a double value
   cout << "The converted value is: " << num << endl;
   return 0;
}

Output:

The converted value is: 3.14

Explanation:

1. The string variable str is given the input string “3.14”.
2. The parameter str is used to call the stod() method.
3. The double variable num holds the double value that the stod() method returned.
4. The console displays the converted value.

Best Approach-Using stod() function

The stod() function in C++ is the best method for converting a string to a double. Compared to the other two techniques, this one has the following advantages:

1. Simpleness: Converting a string to a double only requires one line of code when using the stod() function.
2. Adaptability: The stod() function is capable of processing a wide variety of input formats and unique values like infinity and NaN.
3. Robustness: The stod() function, which conducts input validation and throws an exception if the input string cannot be transformed to a double, is more resilient than the other two methods.
4. Performance: While it’s possible that small programmes won’t give much thought to performance, using the stod() function is typically quicker than using a stringstream or the atof() function.
5. Standardisation: The stod() function is widely supported and may be used with various systems and compilers.

Sample Problems on converting string to double in c++

Sample Problem-1: Using Approach-1

Problem Definition: Priya is given a string representing a matrix of integers, and she wants to write a program in C++ that computes its transpose. Help Priya to write the program.

Solution: The code to resolve this issue using the atof() function is as follows:

Algorithm:

• The input character array is given the value “1 2 3\n4 5 6\n7 8 9”.
• Using ‘ ‘ and ‘n’ as delimiters, the strtok() method is used to extract each token from the input array.
• Each token is transformed into an integer using the atof() method.
• Each row serves as a row in the matrix, a two-dimensional array in which the numbers are stored.
• By switching rows and columns, the transpose of the matrix is calculated and stored in a separate transpose two-dimensional array.
• The transpose array is iterated through and its values are printed to the console using two nested for loops.

Code:

#include <iostream>
#include <cstring>
using namespace std;

const int MAX_ROWS = 10, MAX_COLS = 10;

int main()
{
   char input[] = "1 2 3\n4 5 6\n7 8 9";
   char* token = strtok(input, " \n");
   //extracting tokens using the strtok() function

   int matrix[MAX_ROWS][MAX_COLS];
   //initialising the matrix

   int row = 0, col = 0;
   while (token != NULL) {
      int num = atof(token);
      //transforming into int using the atof() function

      matrix[row][col] = num;
      col++;
      if (col == 3) {
         row++;
         col = 0;
      }
      token = strtok(NULL, " \n");
   }
   int transpose[MAX_COLS][MAX_ROWS];
   for (int i = 0; i < 3; i++) {
      for (int j = 0; j < 3; j++) {
         transpose[i][j] = matrix[j][i];
      }
   }
   for (int i = 0; i < 3; i++) {
      for (int j = 0; j < 3; j++) {
         cout << transpose[i][j] << " ";
      }
      cout << endl;
   }
   return 0;
}

Output:

1 4 7 
2 5 8 
3 6 9 

Sample Problem-2: Using Approach-2

Problem Definition: Sita is given a string of integers separated by commas, and she wants to compute their sum and average by writing a program in C++. Help Sita write the program.

Solution: The code to resolve this issue using the stringstream class method is as follows:

Algorithm:

• The input string variable is given the value “1,2,3,4,5”.
• Input serves as the basis for the creation of a stringstream object.
• Each integer number, separated by commas, is read from the stringstream using a while loop.
• Each token is taken out of the stringstream using the getline() function, which uses the character “,” as the delimiter.
• Each token is transformed into an integer using the stoi() function.
• Each integer value updates the sum and count variables.
• The sum divided by the count is used to calculate the average, which is then converted to a double for floating point results.
• On the console, the total and average are printed.

Code:

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

int main()
{
   string input = "1,2,3,4,5";
   stringstream ss(input);
   //using the stringstream class and creating the ss object

   string token;
   int sum = 0, count = 0;
   while (getline(ss, token, ',')) {
      int num = stoi(token);
      //using the stoi() function to convert token into int value

      sum += num;
      count++;
   }
   double average = static_cast<double>(sum) / count;
   //typecasting into double

   cout << "The sum is: " << sum << endl;
   cout << "The average is: " << average << endl;
   return 0;
}

Output:

The sum is: 15
The average is: 3

Sample Problem-3: Using Approach-3

Problem Definition: You are given a string representing a complex number in Cartesian form, write a program in C++ to compute its modulus and argument.

Solution: The code to resolve this issue using the stod() function is as follows:

Algorithm:

• The input string variable is given the value “3.5+2.5i.”
• The real component of the complex number is transformed into a double using the stod() function and put in the real variable.
• The imaginary portion of the complex number is converted to a double using the stod() function once more, this time with a substring that begins after the plus sign (+). This double is saved in the variable imag.
• The formula sqrt(real 2 + imag 2) is used to get the complex number’s modulus.
• Using the atan2() function, which accepts the imaginary component as the first argument and the real part as the second parameter, the complex number’s argument is calculated.
• Both the argument and the modulus are printed to the console.

Code:

#include <iostream>
#include <cmath>
using namespace std;

int main()
{
   string input = "3.5+2.5i";
   double real = stod(input);
   //calculating the real part of the complex number
   //using the builtin stod() function to convert input into double

   double imag = stod(input.substr(input.find('+') + 1));
   //calculating the imaginary part of the complex number
   //using stod() function along with substr() and find() functions

   double modulus = sqrt(pow(real, 2) + pow(imag, 2));
   //calculating modulus using the square root and power functions

   double argument = atan2(imag, real);
    //calculating the angle using atan2() function

   cout << "The modulus is: " << modulus << endl;
   cout << "The argument is: " << argument << endl;
   //executing the output to the console

   return 0;
}

Output:

The modulus is: 4.30116
The argument is: 0.620249

Conclusion

In summary, converting a string to a double in C++ is a frequent activity that can be carried out in a variety of ways. Despite the fact that each of the three ways has pros and cons of its own, the stod() function is the ideal strategy because of its ease of use, adaptability, robustness, performance, and standardisation.

stod() is a dependable function that is frequently used. It can handle a variety of input formats and assures that the application won’t crash if an invalid input string is supplied.

C++ programmers can select the optimal option for their unique needs by studying these various methods.

24×7 Live C++ Homework Help From Expert Tutors! Get Help Now

Author:

Anushka Gupta