CODEWITHSUNDEEP
c
Kevin
Kevin

Reputation: 27

Reading Only the First Character of a Line

I wrote a program to read the first character in a line by using the C here is my code:

#include <stdio.h>

int main(void) {
    char ch;

    printf("Please type text for test(# for terminate)\n");

    while((ch = getchar()) != '#') {
        printf("\n %c", ch);
        while (getchar() != '\n')
            continue;
        printf("\nPlease type text for test(# for terminate)\n");
    }
    return 0;
}

My question is when I changed the second while into if, the program would print all the character in odd position(the first,the third,the fifth....) instead of only the first character. I don't know why

Upvotes: 0

Views: 1963

Answers (5)

David Frye
David Frye

Reputation: 498

Every time getchar() is called, it's eating a single character from stdin. You have two calls to getchar() in your above code, and they're alternating (the call in your while() is made, then the call in your if() is made). However, only the first call ever stores the returned character and prints it, the second call just eats the character and does nothing with it. An example:

Input: "Example"

  1. ch=getchar() is called, grabbing "E", and ch is printed, printing "E".
  2. getchar() is called, grabbing "x". Nothing is printed.
  3. ch=getchar() is called, grabbing "a", and ch is printed, printing "a".
  4. getchar() is called, grabbing "m". Nothing is printed.
  5. ch=getchar() is called, grabbing "p", and ch is printed, printing "p".
  6. getchar() is called, grabbing "l". Nothing is printed.
  7. ch=getchar() is called, grabbing "e", and ch is printed, printing "e".

See how that works?

Upvotes: 3

rohit
rohit

Reputation: 228

The second WHILE loop will execute till '\n' character is seen. Beacuse when CONTINUE is called, control goes to second WHILE loop itself. So it prints only first character.

But if second WHILE loop is replaced with IF loop, then when CONTINUE is called, the control goes to the first WHILE loop. Here also a getchar() is called. So it keeps printing characters in odd positions.

Upvotes: 0

EducatedIdiot
EducatedIdiot

Reputation: 1

  1. The call to getchar(): Everytime you call getchar(), the program reads a character from the input.
  2. While vs. if: as others have mentioned.
  3. Continue: Continue transfers to a do, for or while loop.

When you have a second while, that's where your continue takes you. But when you have an if, the continue takes you to the first while. You then have 2 calls to getchar(), resulting in odd positions being read.

You should try debugging this step-by-step to understand better.

Upvotes: 0

Vallabh Patade
Vallabh Patade

Reputation: 5110

It will ignore just every other letter. Now, with second while, you are printing first letter and ignoring rest of the letters from the line. Changing it to the if you reading every odd positioned letter.

Keep id mind while is looping construct while if is conditional construct. Anything which is inside if will be executed only once.

Upvotes: 0

Rudy Velthuis
Rudy Velthuis

Reputation: 28806

The second while loop reads all characters after the first character until a newline is pressed. In other words, it reads the rest of the line and ignores it.

If you change the while to if, it only reads one character and then goes on with the outer loop. That explains why you get all these characters on different positions.

Upvotes: 0

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