Reading three characters per loop iteration in C?

Question

This is making no sense to me, but hopefully one of you understand why it's doing this.

I have an assignment that requires three characters be read with getchar() as the three integers next to one another are relevant to each other, so I set up a loop structured as such:

int c1, c2, c3 = 0;

while(c3 != EOF) {
    c1 = getchar();
    c2 = getchar();
    c3 = getchar();
    ... do something with them...
}

The problem is that if the amount of characters is not divisible by three, the last iteration is not executed. So, if the characters "Abcd" were entered, it would do the first iteration on Abc, but the second iteration wouldn't do anything with D. Same for "Abcde", but Abcdef would work.

This is a homework assignment so don't solve the problem for me, but is this something weird about getchar that it just terminates the loop if that many characters aren't found?

c2 and c3 should be zero and c1 should contain the value for D — user1661781, Sep 18 '12 at 00:40
How are you providing the input? If you're entering the text at the command-line, are you pressing `Ctrl+D` (Linux) or `Ctrl+Z` (Windows) afterwards? — nneonneo, Sep 18 '12 at 00:41
I realize that the newline is counted as a char. I'm counting that as one of the entered chars. — user1661781, Sep 18 '12 at 00:42
It's looping through sequentially, so your work won't be done until after `c3` is entered. So if you put in "abcd" that's 4 characters, so you are on the `c2 = getchar();` line of code... you still have 2 more chars to go before your work gets done again. — Scott Lemmon, Sep 18 '12 at 00:44

score 1 · Accepted Answer · answered Sep 18 '12 at 00:47

getchar is a blocking call, so it won't return until it reads a character or EOF is encountered.

So, either check to see if any of c1, c2 or c3 is a newline after the corresponding getchar call (to see if the line of input has ended), or press Ctrl+D (*nix, OS X) or Ctrl+Z (Windows) to signal the end-of-file (EOF). After EOF, getchar will always return immediately with EOF.

score 0 · Answer 2 · answered Sep 18 '12 at 14:43

Break down your problem into states. Define what should be done on different input in different states. Translate to code.

In your case, you need to figure out when input of an unwanted length will affect you and how to handle it. Reading in triplets works fine, but for every character you get, you need to see if you should raise an error or treat it in a special way.

Try to run the strings "", "a", "ab", "abc" and "abcd" through your described states and see if you can handle them all in a sane way. When this works, writing the code should not be a problem.

score 0 · Answer 3 · answered Sep 18 '12 at 15:35

An easy way to simulate a short file is to use echo and pipe to your program:

echo ABC | ./program_name

This way, you can test with different strings on the command line. The output of echo will be piped to the stdin of your program. The stdin in your program will be closed at end of string. This avoids the EOF problem that causes getchar to block when you test using the keyboard directly (besides the terminal line/char input method).

Another gotcha is that EOF is an int constant. So check if your code don't convert c1, c2, c3 to char later on. It is ok, but if you convert them to char, you can't compare anymore with EOF.

Reading three characters per loop iteration in C?

3 Answers3