CODEWITHSUNDEEP
c
Swarnagowri
Swarnagowri

Reputation: 295

C Program to read number character by character giving incorrect result

#include<stdio.h>
#include<ctype.h>

int peekchar() {
    int c;
    c = getchar();
    if (c != EOF) {
        ungetc(c, stdin);
    }   
    return c;
}

int readNumber(void) {
    int c;
    int accumulator = 0;
    while ((c = peekchar() != EOF) && isdigit(c)) {
       c = getchar();
       accumulator *= 10; 
       accumulator += c - '0';
    }   
    return accumulator;
}

int main() {
    int result = readNumber();
    printf("%d\n", result);
    return 0;
}

I am trying to read an integer written in decimal notation from stdin until the first non-digit. But its not giving the correct result:

M1508444:CProg sb054043$ gcc -g3 readNumber.c -o readNumber
M1508444:CProg sb054043$ ./readNumber 
123
0

Can someone please help me identify the problem?

Upvotes: 1

Views: 127

Answers (2)

Bathsheba
Bathsheba

Reputation: 234875

The issue is with operator precedence. c = peekchar() != EOF is grouped as c = (peekchar() != EOF), and so c is either 0 or 1, which accounts for the result.

Fix with (c = peekchar()) != EOF.

Or, given that isdigit is defined to be 0 for EOF, your loop conditional can be simplified to

while (isdigit(c = peekchar())){

Upvotes: 5

Abhijit Pritam Dutta
Abhijit Pritam Dutta

Reputation: 5601

Hi you need to modify your while loop like below:-

 while ( (c = peekchar()) != EOF && isdigit(c)) {
   c = getchar();
   accumulator *= 10; 
   accumulator += c - '0';
}

First of all you need to read the value and store it in variable c and that you can achieve by doing (c = peekchar()). Once the value stored in c now your while loop will first check whether it is EOF if not then only it will check whether it is a digit or not.

Upvotes: 0

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