Thread: getchar() wierd Behaviour

I am starting to learn C and do it with the "c programming language" by
Kerningham and Ritchie. I am currently at page 31.

there is an example program which has to be refined:

Code:

#include <stdio.h>
#define MAXLINE 1000

int getLine(char line[], int maxline);
void copy(char to[], char from[]);

int main(){
  int len,max,m;
  char line[MAXLINE];
  char longest[MAXLINE];

  m=0;
  max =0;
  while((len=getLine(line, MAXLINE)) > 0)
    if(len > max) {
      max = len;
      copy(longest, line);
    }
  }
  if(max > 0)
    printf("%s\n", longest);
  return 0;
}

in an followup excercise i should handle an error, coming to life, after a
line is fed in longer than MAXSIZE. for this i should display the actual length of the line and display the maximum amount of chars i can (which is MAXSIZE -1).

I use the function getLine, which looks like this:

Code:

int getLine(char s[], int lim){
  int c,i;
  for(i=0; i<lim-1 && (g=getchar()) != EOF && c != '\n'; ++i)
    s[i] = c;
  if(c == '\n'){
    s[i] = c;
    ++i;
  }
  s[i] = '\0';
  return i;
}

when i implement my solution inside the while loop where i call the function "getLine"(using the function getchar()) like this:

Code:

while((len=getLine(line, MAXLINE)) > 0)
  if(len > max) {
    max = len;
    copy(longest, line);
    if(max == MAXLINE -1 && longest[MAXLINE-1] == '\0' 
            && longest[MAXLINE-2] != '\n'){
      int c;
      while((c=getchar()) != EOF && c != '\n'){
        ++m;
      }
      printf("input line too long.\n chars: %d\n", m+MAXLINE);
    }
  }
  ..code refer to the main program at the top
}

i get the proper output and count of chars from input.
BUT when i call getchar() outside the while loop. where I call the function using getchar to get the initial input. like this:

Code:

...code refer to the whole program at the top
if(max > 0){
  if(max == MAXLINE -1 && longest[MAXLINE-1] == '\0' 
            && longest[MAXLINE-2] != '\n'){
    int c;
    while((c=getchar()) != EOF && c != '\n'){
      ++m;
    }
    printf("input line too long.\n chars: %d\n", m+MAXLINE);
  }
}

then c == -1 which is equal to EOF.

My Question is the following:
why does it seem that the input buffer is getting emptied as soon as i leave the loop where i first read it with getchar()?

I am a beginner and just starting out with c. as this is a relative hard problem to search for i thought ill ask here. thanks for your time. i am open to any suggestions.

yay first post

Thanks very much for your response.

your right i dont quite understand the behaviour of the while loop there.

Code:

while ((len = getline(line, MAXLINE)) > 0)

the first time executed (assuming the line exceeds MAXLINE), len would be equal to 999. the while would be therefor true.
the code inside gets executed. then it gets evaluated again. with above mentioned function:

Code:

int getLine(char s[], int lim){

  int c,i;

  for(i=0; i<lim-1 && (g=getchar()) != EOF && c != '\n'; ++i)

    s[i] = c;

  if(c == '\n'){

    s[i] = c;

    ++i;

  }

  s[i] = '\0';

  return i;

}

if it gets called again, will i be initialized a second time? and therefor be 0?
or will it store its previous value somehow, know i == 999 which is != lim-1
which will prevent the for loop from executing.
but if i were initialized again and be equal to 0, it would store a null terminator
at position 0 of the array s; which it doesnt do

therefor i have to assume i is stored somehow, but this would create an infinite loop
as return i never would drop to 0 or below, which would make len always above 0
and the while loop would always be true.

you see i have some issues interpreting the execution of the code correctly
any help is appreciated.

pps:
i corrected the error at:

Code:

if(max == MAXLINE -1 && longest[MAXLINE-1] == '\0'
            && longest[MAXLINE-2] != '\n')

pointed out by you and corrected it with:

Code:

if(len == MAXLINE -1 && longest[MAXLINE-1] == '\0'
            && longest[MAXLINE-2] != '\n')

i have also put the error checking (if line too long for MAXLINE) at the top. like this:

Code:

while((len=getLine(line, MAXLINE)) > 0)
  if(max == MAXLINE -1 && longest[MAXLINE-1] == '\0'
            && longest[MAXLINE-2] != '\n'){

    int c;

    while((c=getchar()) != EOF && c != '\n'){

      ++m;

    }

    printf("input line too long.\n chars: %d\n", m+MAXLINE);

  }


  if(len > max) {

    max = len;

    copy(longest, line);

  }

}