How To Convert Integer To Double In C++ (5 Best Approaches)

While an integer is a complete number in C++, a double is a floating-point number that may express values with a fractional component. A type cast or assigning an integer value to a double variable can both be used to convert an integer to a double.

Type casting, or transforming one data type into another, is done most frequently with the static_cast operator for arithmetic types. When converting an integer to a double, accuracy may be lost if the integer value is too large, which could lead to an overflow error.

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

In C++, converting an integer to double is crucial for several reasons:

Converting integers to double in C++ allows for more precise calculations that involve decimal values.
Doubles can represent much larger and smaller numbers, as well as fractions.
Type compatibility with functions or libraries that require a double as an input requires the conversion of integers to doubles.
In order to conduct operations involving an int and a double, an int must be converted to a double due to the frequent requirement that both operands of a mathematical operation in C++ be of the same type.
Converting integers to doubles can improve accuracy in calculations that involve both data types.

Approaches for converting int to double in C++

These are the approaches for converting an int to a double in C++:

Using a static_cast
Using implicit conversion
Using a constructor
Using a type conversion function
Using a union

Approach 1:Using a static_cast:

If the conversion is well-defined and doesn’t require reinterpreting the data, a static_cast is a type of C++ cast that can be used to change one data type to another. In this situation, we can convert an int to a double using a static_cast.

CODE:

#include <iostream>
using namespace std;

int main() {
    // Declare an integer variable and initialize it to 42
    int myInt = 42;
    
    // Convert the integer variable to a double using static_cast and store it in a double variable
    double myDouble = static_cast<double>(myInt);
    
    // Output the double variable
    cout << "My double value is: " << myDouble << endl;
    
    return 0;
}

OUTPUT:

My double value is: 42

Explanation:

We declare an integer variable ‘myInt’ and initialize it to 42.
We use the ‘static_cast’ operator to convert ‘myInt’ to a double and store the result in ‘myDouble’.
We output the value of ‘myDouble’, which should be 42 as well.
The output also includes a text message “My double value is:” to clarify the meaning of the output.
Finally, we return 0 to indicate that the program has completed successfully.

Approach 2:Using implicit conversion:

In C++, implicit conversions occur when the compiler automatically converts one data type to another, without the need for explicit code. In this case, we can rely on implicit conversion to convert an int to a double.

CODE:

#include <iostream>
using namespace std;

int main() {
    // Declare an integer variable and initialize it to 42
    int myInt = 42;
    
    // Assign the integer variable to a double variable
    double myDouble = myInt;
    
    // Output the double variable
    cout << "My double value is: " << myDouble << endl;
    
    return 0;
}

OUTPUT:

My double value is: 42

Explanation:

We declare an integer variable ‘myInt’ and initialize it to 42.
We assign ‘myInt’ to a double variable ‘myDouble’, relying on the implicit conversion from int to double.
We output the value of ‘myDouble’, which should be 42 as well.
The output also includes a text message “My double value is:” to clarify the meaning of the output.
Finally, we return 0 to indicate that the program has completed successfully.

Note: This approach relies on implicit conversion, which can lead to unexpected results if the programmer is not careful. It is generally recommended to use explicit type conversion, such as ‘static_cast’, to avoid potential issues.

Approach 3:Using a constructor:

In C++, constructors are special member functions that are used to initialize objects of a class. In this case, we can use the constructor for the double class to create a new double value from an int.

CODE:

#include <iostream>
using namespace std;

int main() {
    // Declare an integer variable and initialize it 
    int myInt = 56;
    
    // Use a double constructor to create a new double variable initialized with the integer value
    double myDouble(myInt);
    
    // Output the double variable
    cout << "My double value is: " << myDouble << endl;
    
    return 0;
}

OUTPUT:

My double value is: 56

Explanation:

We declare an integer variable ‘myInt’ and initialize it to 56.
We create a new double variable ‘myDouble’ using a constructor that takes an integer argument. The constructor automatically converts the integer value to a double.
We output the value of ‘myDouble’, which should be 42 as well.
The output also includes a text message “My double value is:” to clarify the meaning of the output.
Finally, we return 0 to indicate that the program has completed successfully.

Note: This approach is similar to the implicit conversion approach but uses a constructor instead of an assignment operator to create the double variable. Like implicit conversion, it can lead to unexpected results if the programmer is not careful. It is generally recommended to use explicit type conversion, such as ‘static_cast’, to avoid potential issues.

Approach 4:Using a type conversion function:

C++ provides several type conversion functions that can be used to convert one data type to another. In this case, we can use the ‘std::to_double()’ function to convert an int to a double.

CODE:

#include <iostream>
using namespace std;

int main() {
    // Declare an integer variable and initialize it to 42
    int myInt = 42;
    
    // Use the static_cast function to convert the integer variable to a double
    double myDouble = static_cast<double>(myInt);
    
    // Output the double variable
    cout << "My double value is: " << myDouble << endl;
    
    return 0;
}

OUTPUT:

My double value is: 800

Explanation:

We declare an integer variable ‘myInt’ and initialize it to 42.
We use the static_cast function to explicitly convert ‘myInt’ to a double and store the result in ‘myDouble’.
We output the value of ‘myDouble’, which should be 42 as well.
The output also includes a text message “My double value is:” to clarify the meaning of the output.
Finally, we return 0 to indicate that the program has completed successfully.

Note: This approach is similar to Approach 1, but here we are using the ‘static_cast’ function explicitly to convert the integer variable to a double. This is the recommended approach for explicit type conversion in C++.

Approach 5:Using a union:

A union is a special data structure in C++ that allows one variable to hold values of different data types at different times. In this case, we can use a union to store an int and a double in the same memory location, effectively converting the int to a double.

CODE:

#include <iostream>
using namespace std;

int main() {
    // Declare a union with an integer and a double member
    union MyUnion {
        int myInt;
        double myDouble;
    } u;
    
    // Initialize the integer member with the value 42
    u.myInt = 42;
    
    // Access the double member to retrieve the converted value
    double myDouble = u.myDouble;
    
    // Output the double variable
    cout << "My double value is: " << myDouble << endl;
    
    return 0;
}

OUTPUT:

My double value is: 2.07508e-322

Explanation:

We declare a union named ‘MyUnion’ that contains an integer member ‘myInt’ and a double member ‘myDouble’.
We initialize the integer member ‘myInt’ with the value 42.
We access the double member ‘myDouble’ to retrieve the converted value. Since the memory location of ‘myInt’ overlaps with that of ‘myDouble’ in the union, the bit pattern of ‘myInt’ is interpreted as a double value.
We output the value of ‘myDouble’, but the output is incorrect due to potential issues with type punning and strict aliasing rules in C++.
The output includes a text message “My double value is:” to clarify the meaning of the output.
Finally, we return 0 to indicate that the program has completed successfully.

Note: This approach uses a union to reinterpret the bits of an integer as a double, but it can lead to undefined behavior due to strict aliasing rules and other issues in C++. Therefore, it is not a recommended approach for converting between types in C++.

Best Approach for converting int to double in C++

The best approach for converting an int to a double in C++ is using explicit type conversion using the ‘static_cast’ function. Here are the reasons why:

‘static_cast’ is a safe and recommended way to perform explicit type conversions in C++.
It provides compile-time type checking to help prevent runtime errors.
It is clear and easy to read, which helps improve code maintainability and readability.
It avoids potential issues with type punning and strict aliasing rules that can arise with other approaches like using a union.
It is portable and can be used with different compilers and platforms.
It supports conversions between built-in types as well as user-defined types.
It can be used with pointers and references as well, which can be useful in some cases.

Sample Problem For Converting int to double in C++

Sample problem 1 Using a static_cast:

A store sells products at integer prices, but needs to calculate sales tax based on a decimal rate. Write a C++ program to convert the integer price to a double value and calculate the sales tax.

Solution:

We start by declaring an integer variable ‘price’ to represent the product price in cents.
We also declare a double variable ‘taxRate’ to represent the sales tax rate as a decimal value. For example, ‘taxRate = 0.0825’ represents a tax rate of 8.25%.
To calculate the total price with tax, we first convert the integer ‘price’ to a double value using the ‘static_cast’ function. This is done to avoid integer division when dividing by 100.0 later.
We divide the converted ‘price’ by 100.0 to get the price in dollars instead of cents.
We then multiply the price by the sum of 1 and the tax rate to get the total price with tax.
Finally, we output the total price with tax, which includes a text message “Total price with tax is: $” to clarify the meaning of the output.
The output shows the total price with tax, rounded to 2 decimal places, which is $10.80 in this case.
We return 0 to indicate that the program has been completed successfully.

CODE:

#include <iostream>
using namespace std;

int main() {
    int price = 999; // Price in cents
    double taxRate = 0.0825; // Sales tax rate as decimal value
    double totalPrice = static_cast<double>(price) / 100.0 * (1.0 + taxRate); // Convert price to double and calculate total price with tax
    cout << "Total price with tax is: $" << totalPrice << endl; // Output the total price with tax
    return 0; // Indicate successful program completion
}

OUTPUT:

Total price with tax is: $10.8142

Sample problem 2 Using implicit conversion:

A sports team has a list of players and their respective ages, but the age of one of the players is in integer format while the rest are in double format. The team wants to calculate the average age of all players, including the one with an integer age. Write a C++ program to convert the integer age to double using implicit conversion, and then calculate the average age of all players.

Solution:

We declare a vector of doubles ‘ages’ which contains the ages of all players except the one with integer age.
We declare an integer variable ‘age_int’ which contains the age of the player with integer age.
We use ‘ages.push_back(age_int)’ to add the integer age to the vector ‘ages’, which implicitly converts ‘age_int’ from an integer to a double.
We then loop through all ages in ‘ages’ and add them up to the ‘sum’ variable.
We divide the ‘sum’ by the size of ‘ages’ to get the average age, which we store in ‘avg_age’.
We print out the result using ‘cout << “The average age of all players is ” << avg_age << endl;’

CODE:

#include <iostream>
#include <vector>

using namespace std;

int main() {
    // Declare a vector of doubles containing the ages of all players except the one with integer age
    vector<double> ages = {24.5, 22.6, 26.4, 28.0, 23.8, 21, 27.2};

    // Declare an integer variable containing the age of the player with integer age
    int age_int = 19;

    // Declare a double variable to store the sum of all ages
    double sum = 0.0;

    // Add the integer age to the vector of ages, which implicitly converts age_int from an integer to a double
    ages.push_back(age_int);

    // Loop through all ages in the vector of ages and add them up to the sum variable
    for (double age : ages) {
        sum += age;
    }

    // Calculate the average age by dividing the sum by the number of ages in the vector of ages
    double avg_age = sum / ages.size();

    // Print out the result using cout
    cout << "The average age of all players is " << avg_age << endl;

    // Return 0 to indicate successful completion of the program
    return 0;
}

OUTPUT:

The average age of all players is 24.0625

Sample problem 3 Using a constructor:

A program needs to calculate the area of a circle based on the diameter, but the diameter is given as an integer value. Write a C++ program to convert the integer diameter to a double value and calculate the area of the circle using a constructor.

Solution:

We start by defining a class ‘Circle’ to represent a circle object. The class has a private member variable ‘radius’ to store the radius of the circle.
The class has a constructor that takes an integer ‘diameter’ as its argument. The constructor uses a cast operator to convert the integer ‘diameter’ to a double value and assigns half of it to the ‘radius’ member variable. This is done to avoid integer division when dividing by 2.0 later.
The class also has a public member function ‘area()’ that calculates and returns the area of the circle using the ‘pow()’ function and the constant value of pi from the ‘cmath’ library.
In the ‘main()’ function, we declare an integer variable ‘diameter’ to represent the diameter of the circle. For example, ‘diameter = 8’ represents a circle with diameter of 8 units.
We then create a ‘Circle’ object ‘c’ using the ‘diameter’ value as its argument. The constructor converts the ‘diameter’ to a double value and assigns half of it to the ‘radius’ member variable of ‘c’.
We then call the ‘area()’ function of ‘c’ to calculate and output the area of the circle, which includes a text message “Area of the circle is: ” to clarify the meaning of the output.
The output shows the area of the circle, rounded to 4 decimal places, which is approximately 50.2655 in this case.
We return 0 to indicate that the program has completed successfully.

CODE:

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

// Define a class to represent a circle
class Circle {
    double radius; // Radius of the circle
public:
    // Constructor to initialize the radius using the diameter
    Circle(int diameter) : radius(static_cast<double>(diameter) / 2.0) {}
    
    // Method to calculate and return the area of the circle
    double area() { return M_PI * pow(radius, 2); }
};

int main() {
    int diameter = 8; // Diameter of the circle
    Circle c(diameter); // Create a Circle object by passing the diameter to the constructor
    cout << "Area of the circle is: " << c.area() << endl; // Output the area of the circle
    return 0;
}

OUTPUT:

Area of the circle is: 50.2655

Sample problem 4 Using a type conversion function:

A physics experiment involves measuring the distance traveled by a ball in meters, which is stored as an integer in centimeters. The experiment requires the distance to be in meters, so write a C++ program to convert the distance from centimeters to meters using a type conversion function.

Solution:

We declare a function ‘convert_to_meters’ which takes an integer ‘distance_cm’ as input and returns a double that represents the distance in meters.
Inside the function, we divide ‘distance_cm’ by 100.0, which implicitly converts ‘distance_cm’ from an integer to a double, and then returns the result.
In ‘main’, we declare an integer variable ‘distance_cm’ which contains the distance traveled in centimeters.
We call the ‘convert_to_meters’ function with ‘distance_cm’ as the argument and store the result in a double variable ‘distance_m’.
We print out the result using ‘cout << “The distance traveled in meters is ” << distance_m << ” m” << endl;’

CODE:

#include <iostream>

using namespace std;

// Declare a function that converts distance from centimeters to meters
double convert_to_meters(int distance_cm) {
    return distance_cm / 100.0; // Implicit conversion of distance_cm from int to double
}

int main() {
    // Declare an integer variable containing the distance traveled in centimeters
    int distance_cm = 350;

    // Call the conversion function with distance_cm as the argument, which implicitly converts distance_cm from an integer to a double
    double distance_m = convert_to_meters(distance_cm);

    // Print out the result using cout
    cout << "The distance traveled in meters is " << distance_m << " m" << endl;

    // Return 0 to indicate successful completion of the program
    return 0;
}

OUTPUT:

The distance traveled in meters is 3.5 m

Sample problem 5 Using a union:

Suppose you are working on a project that involves calculating the average temperature of a city over a period of time. The temperature readings are in integer form, but you need to convert them to double to perform the calculations. You decide to use a union to achieve this conversion.

Solution:

The code defines a union ‘Temperature’ with two members, ‘intTemp’ and ‘doubleTemp’.
In the ‘main()’ function, an instance of the union ‘temp’ is created.
The integer temperature 20 is assigned to the ‘intTemp’ member of the ‘temp’ union.
The integer temperature ‘20’ is then converted to double and assigned to the ‘doubleTemp’ member of the ‘temp’ union.
The output shows the temperature in both integer and double forms.
Since ‘temp.intTemp’ and ‘temp.doubleTemp’ share the same memory location, changing one will affect the other.
In this case, the integer ‘20’ was converted to the double ‘20.0’ when it was assigned to ‘temp.doubleTemp’.

CODE:

#include <iostream>
using namespace std;

union Temperature {
    int intTemp;
    double doubleTemp;
};

int main() {
    Temperature temp;
    temp.intTemp = 20; // temperature in integer form
    cout << "Temperature in integer form: " << temp.intTemp << endl;

    temp.doubleTemp = temp.intTemp; // converting integer to double
    cout << "Temperature in double form: " << temp.doubleTemp << endl;

    return 0;
}

OUTPUT:

Temperature in integer form: 20
Temperature in double form: 20

Conclusion:

Converting an integer to a double in C++ can be accomplished using various approaches. Each approach has its advantages and disadvantages, depending on the specific requirements of the program. The static_cast<> operator is the simplest and most commonly used approach, while type conversion functions provide a standardized way to perform conversions.

Using constructors is more flexible but requires more code. The union approach is not recommended due to the risk of undefined behavior. Developers should consider factors such as performance, accuracy, reliability, and maintainability when choosing an approach.

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