How to Convert a Char to an Int in C and C++

Converting a character (char) to an integer (int) is a common task in programming, especially when working with ASCII values or numeric characters. In both C and C++, the process is straightforward due to the relationship between characters and their ASCII values.

This article looks at the various methods to convert a char to an int in C and C++.

Understanding Char to Int Conversion

In C and C++, characters are internally represented by their ASCII values. For example:

• The character 'A' has an ASCII value of 65.
• The character '0' has an ASCII value of 48.

Converting a char to an int typically involves:

1. Directly obtaining the ASCII value of the character.
2. Converting a numeric character (e.g., '5') to its integer representation (5).

1. Direct Conversion Using ASCII Values

To convert a character to its ASCII integer representation:

In C++:

The cast is optional because the type char is implicitly convertible to int.

2. Converting Numeric Characters to Integers

If the character represents a numeric digit (e.g., '0' to '9'), you can convert it to its integer value by subtracting '0':

In C++:

This works because the numeric characters '0' to '9' are sequentially represented in ASCII.

3. Using the isdigit Function

Before converting, you can check if a character is a valid numeric digit using the isdigit function from <ctype.h> (C) or <cctype> (C++):

In C++:

4. Using atoi for String Conversion

If you have a single-character string (e.g., "5"), you can use atoi from <stdlib.h> (C) to convert it:

In C++:

However, this approach is more commonly used for multi-character strings.

5. Using std::stoi in Modern C++

For C++11 and later, you can use the std::stoi function from the <string> header to convert a single-character string:

This is ideal when working with strings, as it automatically handles conversion.

6. Handling Edge Cases

Non-Numeric Characters

• Subtracting '0' from a non-numeric character results in unintended values.
• Always validate the input using isdigit or similar methods.

Multi-Character Strings

• Direct subtraction or casting only works for single characters. For strings like "123", use functions like atoi or std::stoi.

Signed Characters

• If char is signed (compiler-specific), casting it directly to int might yield unexpected negative values for extended ASCII characters. Use unsigned char if needed.

Performance Comparison

• Direct subtraction (char - '0'): Most efficient for single numeric characters.
• atoi/std::stoi: Useful for strings but introduces overhead.
• isdigit check: Adds validation, useful in robust applications.

Converting a char to an int in C and C++ is straightforward, thanks to the ASCII representation of characters. For simple cases, subtracting '0' is efficient and reliable. For more complex scenarios, such as handling strings, use functions like atoi or std::stoi. Always validate input to ensure proper handling of edge cases.