How Can I Efficiently Convert a Character Digit to an Integer?

Converting a Character to an Integer

Question:

How can I efficiently convert a single character into an integer value? Specifically, given a string of digits, I need to extract each one for use in calculations. Is there a better approach than using atoi()?

Answer:

Rather than converting the character to a string before calling atoi(), you can leverage the numerical representation of digits in character encoding. In most encodings, including ASCII, UTF-x, and others, the character encodings for digits are in sequential order, with '0' mapped to 48 and '9' mapped to 57.

This knowledge allows you to convert a digit character to an integer by simply subtracting its ASCII value from '0' (or 48). For example:

char c = '1';
int i = c - '0'; // i is now equal to 1, not '1'

This is equivalent to:

char c = '1';
int i = c - 48; // i is now equal to 1, not '1'

While both methods achieve the same result, the former (c - '0') is generally considered more readable due to its simplicity.

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