Question

Look at the following statements. char str[] = "237.89"; double value; Write a statement that converts the string in str to a double and stores the result in value.

Short answer

Answer: We can use the atof() function from the "stdlib.h" library to convert a string containing a floating-point number into a double value. Include the "stdlib.h" library at the beginning of the code and then use the atof() function to convert the string.

Step-by-step solution

Include necessary library

To use the atof() function, include the "stdlib.h" library at the start of the program by adding the following line: ```c #include ```

Convert the string to a double value using atof() function

Use the atof() function to convert the string "str" into a double value and store the result in the variable "value": ```c value = atof(str); ``` So, the complete code would look like this: ```c #include #include int main() { char str[] = "237.89"; double value; value = atof(str); printf("String: %s\n", str); printf("Double: %.2lf\n", value); return 0; } ``` This code will convert the string "237.89" into a double and store it in the variable "value." The program then prints out both the original string and the converted double value.

Key concepts

atof() function

The `atof()` function is a convenient way to convert strings to floating-point numbers in C++. It stands for "ASCII to float". When you have a string that contains a numerical value and you need to use it in mathematical calculations, `atof()` becomes extremely useful.

While using the `atof()` function, you provide a string argument, and it returns its conversion into a `double`. This is particularly beneficial when dealing with user inputs or data file reads, where numbers are often represented as strings. Here's an example of using `atof()`:

• Declare a string with a number, e.g., `char number[] = "123.45";`
• Use the function: `double num = atof(number);`
• The variable `num` now holds the value `123.45` as a `double`.

However, one should be cautious, as `atof()` does not perform error checking. It will return zero if the string is not a valid number, without any indication of the failure, which could lead to misleading results if not handled properly.

stdlib.h library

The `stdlib.h` library is a standard C library important for many general functions, including memory allocation, process control, conversions, and others. When working in C++ and you need to include functionalities like string to number conversions, `stdlib.h` is often included at the beginning of your program.

Some of the functionalities provided by `stdlib.h` include:

• Memory allocation/deallocation functions such as `malloc()` and `free()`
• Controlled termination of the program using `exit()` and `abort()`
• Mathematical computations and random number generation with `rand()`
• String manipulation functions like `atof()` for string to double conversion

Including `stdlib.h` is straightforward: simply add `#include ` at the top of your C++ code file.

This inclusion ensures that your compiler knows about all the functions defined in the library, allowing you to leverage its robust set of utilities for efficient and versatile programming solutions.

data type conversion

Data type conversion is an essential concept in programming that involves converting data from one type to another. In the context of C++, converting a string to a numeric type like a `double` is one of the common tasks, especially when dealing with data input from varied sources.

There are two kinds of data type conversions:

• Implicit Conversion: Automatically performed by the compiler when an expression involves more than one data type, for example, using an `int` and a `double` together may convert the `int` to `double` for the expression.
• Explicit Conversion: Also known as type casting, where the programmer specifies the type conversion explicitly.

When using the `atof()` function, you are explicitly converting a string to a `double`, which is a form of explicit conversion.

Sometimes, C++ also provides more powerful and versatile conversion methods, like `std::stod()` in the `` header, which throws exceptions when conversions fail. Understanding and choosing the right kind of conversion depending on your needs is crucial to ensuring your code is robust and error-free.