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Why does isalpha() fail to identify an alphabetic character? (lemm.ee)

submitted 10 months ago* (last edited 10 months ago) by [email protected] to c/[email protected]

8 comments fedilink hide all child comments

This is in C language. When I call rotate() in main, the function returns false for isalpha() even though the string entered for plaintext uses alphabetic characters. Perhaps it's identifying an alphabetic character by its ASCII value ('A' = 65)? I tried to test that out and used (char) with the letter variable in rotate() but it didn't change anything.

PORTION OF MAIN

string plaintext = get_string("plaintext:  ");

    int length = strlen(plaintext);
    char ciphertext[length];

    for (int i = 0; i &lt; length; i++)
    {
        ciphertext[i] = rotate(plaintext[i], key);
    }

ROTATE FUNCTION

char rotate(char letter, int key)
{
    if (isalpha(letter) == true)
    { ...

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[–] [email protected] 15 points 10 months ago* (last edited 10 months ago) (4 children)

isalpha documentation:

Return value

Non-zero value if the character is an alphabetic character, zero otherwise.

You should be either checking for not equal to 0 instead of true, as its not necessarily guaranteed to be 1 ~= true, or removing the comparison entirely

Also make sure that your loop condition is < and not "& lt" without the space unless that's a weird formatting issue

For more information, make sure to check the documentation for the standard library functions

[–] [email protected] 16 points 10 months ago (1 children)

I would drop the "== true" entirely. C will evaluate any nonzero int as true in an "if" statement.

[–] [email protected] 2 points 10 months ago

Good point!

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