C program to check for presence of ternary operator

Question

I am unable to find out if the ternary operator is present or not through following code.. Plz help.

#include <stdio.h>
#include <ctype.h>
#include <string.h>
void one();
int main()
{
    FILE *fp;
    fp=fopen("C:/Users/HP/Documents/NetBeansProjects/CD2/1.txt","r");
    char c;
    void one()
    {
        char c;
        while((c=fgetc(fp)!=EOF))
            {
                if(c==':')
                {
                    printf("\nThe ternary operator present");
                    return;
                }
            }
    }
    while((c=fgetc(fp))!=EOF)
    {
        printf("\n-->%c",c);
        if(c=='?')
        {
            one();
        }
     }

    return 0;
}

I want to know why this code doesn't work and say if the ternary operator is present or not in file 1.txt

The output shows all characters till '?' if we print them, but why it's not finding for a colon ':' ?

I suspect you're reading the file twice, with the second time which does nothing because the file has reached EOF. — Jean-François Fabre, Jul 14 '18 at 13:43
@Jean-FrançoisFabre : let's suppose that they are not nested,then too the presence of ?: is not detected.. — Raj Chauhan, Jul 14 '18 at 13:43
you're using the same file handle the second time. Why not checking for both ? and : in the first loop? (because the second time the file handle is at the end of the file and nothing is detected) — Jean-François Fabre, Jul 14 '18 at 13:46
make a variable to look for "?" then ":" instead. That's not going to detect the ternary (lots of false alarms, though) — Jean-François Fabre, Jul 14 '18 at 13:52
Note that getchar() returns an int, not char. That's one more bug in your program. — Jens, Jul 14 '18 at 14:03

J...S · Accepted Answer · 2018-07-14T14:07:03.677

0

The exit condition of the while loop may be the problem. = has lesser precedence than != operator. So

(c=fgetc(fp)!=EOF)

gets evaluated like

(c= (fgetc(fp)!=EOF) )

See this for the precedence of various operators in C.

You could do

while((c=fgetc(fp))!=EOF)

instead. First assign the return value of fgetc() to c and then do the comparison.

ie, the variable c gets the result of the comparison which means the value is either 0 or 1.

Your program should be checking for the ? and the other operands as well. See this.

A simple check may not be enough for cases like the 'operator' being part of a string like

char str[]="the ternary operator is ?:";

Checking for the occurrence in such cases is a bit complex.

Edit: As Jens pointed out, fgetc() returns an int not a char. So make c an int instead of a char.

See this post.

edited Jul 14 '18 at 14:07

answered Jul 14 '18 at 13:47

J...S

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while((c=fgetc(fp))!=EOF) -- this worked! Thnx. a little yet powerful concept of operator precedence. – Raj Chauhan Jul 14 '18 at 13:58
@RajChauhan No it doesn't work. It fails (terminates prematurely before the end-of-file) when there's a character with code 255 in the input which gets sign-extended to -1, the value of the EOF macro. – Jens Jul 14 '18 at 14:05
@RajChauhan You must use `int c;` instead of `char c;` since an int can fit all char values plus another for EOF. – Jens Jul 14 '18 at 14:11

C program to check for presence of ternary operator

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