How do I get this program to stop execution after I run it for the first time?

Question

#include<stdio.h>

/*input: 3 4 -5 2 6 1 -2 0
output:12 3 -10 1 6 0*/
int main(){
    int a,b;
    int flag=0;
    while(scanf("%d %d",&a,&b)!=EOF){
        if(b!=0){
            if(flag==0)printf("%d",a*b);
            else printf(" %d",a*b);
            printf(" %d",b-1);
            flag=1;//if b==0,this loop performs nothing.
        }
    }
    if(flag==0)printf("0 0");
    return 0;
}

This is a program that takes a polynomial(the first number of a pair is the coefficient,the second is the power) and compute its derivative in the same fashion.However,this program doesn't give me the intended result when I input 6 0 or 0 0,the result should be 0 0 in both cases. Additionally,I intend for this program to stop after one execution(one press of "Enter".How should I do that without changing entirely the code?

Does this answer your question? [how use EOF stdin in C](https://stackoverflow.com/questions/8094702/how-use-eof-stdin-in-c) — gkhaos, Mar 19 '21 at 13:44
@gkhaos isn't CTRL-C `SIGINT`? Because I can handle a CTRL-C signal, while I believe `SIGKILL` is just a forceful kill. — mediocrevegetable1, Mar 19 '21 at 13:53
when your conditions are met, I would simply `break`, or you could test `flag` _first_ in the `while` condition: `while (!flag && scanf(...` — yano, Mar 19 '21 at 13:55
@mediocrevegetable1 indeed, anyway it does not send an `EOF` — gkhaos, Mar 20 '21 at 17:10

score 0 · Answer 1 · answered Mar 19 '21 at 13:46

0

Change while(scanf("%d %d",&a,&b)!=EOF) to while(scanf("%d %d",&a,&b)==2).

answered Mar 19 '21 at 13:46

i486

6,491
4
24
41

that's actually my original approach ,still doesn't work. – ansley77777723 Mar 19 '21 at 13:53

score 0 · Answer 2 · answered Mar 19 '21 at 14:20

You can

Read full line via fgets()
Parse the line via sscanf()
During the parsing, use %n to read the number of bytes consumed and proceed to next token

#include<stdio.h>

/*input: 3 4 -5 2 6 1 -2 0
output:12 3 -10 1 6 0*/
int main(){
    char line[8192];
    while(fgets(line,sizeof(line),stdin)!=NULL){
        int pos=0, delta;
        int a,b;
        int flag=0;
        while(sscanf(line+pos,"%d %d%n",&a,&b,&delta)==2){
            pos+=delta;
            if(b!=0){
                if(flag==0)printf("%d",a*b);
                else printf(" %d",a*b);
                printf(" %d",b-1);
                flag=1;//if b==0,this loop performs nothing.
            }
        }
        if(flag==0)printf("0 0");
        putchar('\n');
    }
    return 0;
}

How do I get this program to stop execution after I run it for the first time?

2 Answers2