C program: How do I validate the length of user input?

[ekay1]

For the following program, I'd like to make sure the user input is between 1 and 40 characters. To do this, how would I complete the if statement below?

Thank you.

 char p_input[41];
   
   printf("Enter a name (1-40 characters):");
   fgets(p_input, 41, stdin);
   
   if (p_input ....??

[tabstop]

I don't believe you can.

This smells like a test question, and I'm sure that (as far as the instructor is concerned) the key fact is that fgets includes the \n (ie Enter-key) as part of the input; and so if the \n exists in the input you can be sure that the user typed less than 40 characters. If the user types 40 or more characters, there's no room for the \n (or the extra characters) and so it is not included. However, you cannot distinguish between 40 (which is okay) and "or more" (which is not).

[jimblumberg]

Since you're using fgets() you've already insured that at most 41 characters will be accepted into your string. That is the purpose of the second parameter to limit the number of characters this function will retrieve. If you want to see if the user actually entered more than the 41 characters you need to check for the end of line character in your string. If it is not present then the user entered more than the desired characters. If it's present then the user entered 41 or fewer characters and you can check the length with strlen().


Jim

[SirPrattlepod]

I don't believe you can.

This smells like a test question, and I'm sure that (as far as the instructor is concerned) the key fact is that fgets includes the \n (ie Enter-key) as part of the input; and so if the \n exists in the input you can be sure that the user typed less than 40 characters. If the user types 40 or more characters, there's no room for the \n (or the extra characters) and so it is not included. However, you cannot distinguish between 40 (which is okay) and "or more" (which is not).

I like your explanation, but I disagree with the statement "I don't believe you can". I'll PM you some source code because I don't really want to put it here

[grumpy]

Read the documentation for fgets(). It explains the behaviour and return values of fgets() if the user enters more than the specified number of characters.

[c99tutorial]

1. If you're on Linux

man 3 fgets

2. If you're on Windows, install Cygwin and then go to 1

3. If you later need help with a function other than fgets, go to 1 but replace fgets with the function you need help with

[tabstop]

The "I don't believe you can" was more of "finishing the if-statement". Although I imagine there is certainly a way in C to cram a check for 40 characters, doing another read, and then checking that new info all inside a single if-statement, I would not want to have to write it.