How to read character until a char character?

Question

I'm trying to do a loop that read from a file a single character until it finds '\n' or '*'.

This is the loop that I wrote:

i=0;
do { 
      fscanf(fin,"%c",&word[i]);
      i++;
    } while(word[i]!='*'&&word[i]!='\n');

Now I tried to see why it doesn't work with a debugger. When I print word[i] immediately after the fscanf it shows me the correct character but then, if I try to print word[i] while it's doing the comparison with '*' or '\n' it shows me that word[i] is '\000' and the loop never ends.

I also tried with fgetc but I have the same error.

It looks to me like the `i++` means that the `i` in `fscanf(fin,"%c",&word[i])` does not have the same value as the `i` in `while(word[i]!='*'&&word[i]!='\n')`. — Andrew Morton, Nov 23 '19 at 19:45
I didn't realise that, I put an if clause inside the loop and now it works thank you — pingu19, Nov 23 '19 at 19:53
You should also add a condition for checking that you do not exceed the length of word array. — Dan, Nov 23 '19 at 20:04

Roberto Caboni · Accepted Answer · 2019-11-23T20:39:26.423

1

You have to make sure that the character you are processing is the same you just read.

Actually you increment counter i before testing word [i], that's why your check fails.

Try instead

i=0;
do { 
      fscanf(fin,"%c",&word[i]);
    }while(word[i]!='*'&&word[i++]!='\n');

I would rather move the check in the loop (break if the condition is satisfied) leaving in the while check the test on word array length.

edited Nov 23 '19 at 20:39

answered Nov 23 '19 at 19:52

Roberto Caboni

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Should there be an `&& (i < word.length)` before the `&&word[i++]!='\n'`, as [suggested by Dan](https://stackoverflow.com/questions/59011511/how-to-read-character-until-a-char-character#comment104271167_59011511)? (I don't know C for the length of an array.) – Andrew Morton Nov 23 '19 at 20:26
@Andrew Morton. I agree and I've written it. But I just provided the code according to the known information (word.lenght sounds like a c++ feature, and thi is C) – Roberto Caboni Nov 23 '19 at 20:42
1

Apparently it would be best if the OP used the length they had set the array to. (For the OP's reference: [How do I determine the size of my array in C?](https://stackoverflow.com/q/37538/1115360).) – Andrew Morton Nov 23 '19 at 20:51

score 1 · Answer 2 · answered Nov 23 '19 at 20:54

1

Another way:

for(;;) {
   int c = fgetc(fin);
   if ( c == EOF ) {
      break;
   word[i] = c;
   if( c == '*' || c == '\n' ) {
      break;
   }
}

answered Nov 23 '19 at 20:54

FredK

4,094
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+1 for a way that doesn't involve any spurious assignments; one might need `i` to do more stuff, (like terminating the string.) – Neil Nov 23 '19 at 21:07
1

My example is only part of what is needed. It also needs to ensure that word[] is not over-indexed. It should add the terminating NUL only if it is really supposed to be a String, and not just an array of chars. – FredK Nov 23 '19 at 21:10

score 0 · Answer 3 · answered Nov 23 '19 at 19:49

Your while condition is not testing the same element of word that you just read, because i++ incremented the variable before the test.

Change the test to use word[i-1] instead of word[i] to adjust for this.

BTW, word[i] = fgetc(fin); is a simpler way to read one character.

How to read character until a char character?

3 Answers3