Should I use char or int for input validation in C?

isdigit(c) - Utilizing char vs. int for Input Validation

When examining your code, which aims to validate user input as a digit, the question arises: what should the input variable type be - char or int?

Understanding this dilemma necessitates delving into the intricacies of the isdigit function in C. Contrary to its name, isdigit expects an integer argument, raising concerns about compatibility with char input.

The explanation lies in the history of C, particularly getchar(), which retrieves characters as integers to accommodate both the character code and error codes (negative values representing EOF). This design creates compatibility issues with char, which can be either signed or unsigned depending on the compiler.

Signed char presents a challenge because negative values overlap with EOF, potentially leading to errors. Thus, isdigit and related functions require an input treated as an unsigned char to ensure positivity.

To ensure compatibility, modify your code as follows:

<code class="c">if(isdigit((unsigned char)c))</code>

Additionally, it's crucial to verify that the input stream is valid before using c since a closed stream could leave it uninitialized.

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