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

submitted 1 year ago* (last edited 1 year 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] 16 points 1 year ago* (last edited 1 year ago) (3 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 1 year 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 1 year ago

Good point!

[–] [email protected] 4 points 1 year ago* (last edited 1 year ago)

Ah ha! Yes, I did check the docs but I think I just glanced over that portion. Be more careful next time. Now that I took another look at the other ctype.h functions, they all return 1 or 0. I think I confused equivalent python built-in functions as those evaluated to true/false. The < is a less than sign but it seems it doesn't render correctly on Lemmy.

[–] [email protected] 3 points 1 year ago* (last edited 1 year ago)

Never, ever write "== true" or "== false" unless it's absolutely necessary. In C that's pretty close to never.

[–] [email protected] 4 points 1 year ago* (last edited 1 year ago) (1 children)

what language? needs more context

portion of main and function are just titles for me, if that is supposed to contain code

[–] [email protected] 3 points 1 year ago

Sorry. It's in C. Updated post. Yes those are titles. I just included the relevant portions rather than the entire code.

[–] [email protected] 3 points 1 year ago* (last edited 1 year ago)

Please consider posting language specific questions to language specific communities in the future. For example, [email protected]