Scanf doesn't work as expected in VS Code [duplicate]

Question

I practice a simple example to input operator in C: The code as here:

#include <stdio.h>
int main() {
    int a,b;
    char opera;
    printf("input 2 integer number ");
    scanf("%d %d",&a,&b);

    printf("\n input the operator: ");
    scanf("%c", &opera);

    switch (opera)
    {
    case '+':
        printf("result is %d \n", a+b);
        break; 
    default:
        break;
    }
}

Problem: Terminal will pass the input operator

input 2 integer number
4 5
input the operator:
PS D:\Quang\3. Study\C\Bai 2\.vscode>

But if I input operate first, it work:

#include <stdio.h>
int main() {
    int a,b;
    char opera;
    printf("\n input the operator: ");
    scanf("%c", &opera);
    printf("input 2 integer number");
    scanf("%d %d",&a,&b);
    switch (opera)
    {
    case '+':
        printf("result is %d \n",a+b);
        break; 
    default:
        break;
    }
}

Result:

input the operator: +
input 2 integer number 4 5
result is 9

Anyone has the same issue with VS Code?

Answer 1

Well after scanning the two integers, the stdin buffer is not empty a '\n' new line char is left there, so after reading one char to be the operator, you actually read that new line char, so you can fix that by making a custom flush function, that just reads the chars left in the stdin like this:

#include <stdio.h>

// make stdin buffer empty
void flush() {
  int c;
  while(1) {
    c = fgetc(stdin);
    if(c == EOF || c == '\n') break;
  }
}

int main() {
  int a, b;
  char opera;
  printf("input 2 integer number ");
  scanf("%d %d",&a,&b);
  flush();
  printf("input the operator: ");
  scanf("%c", &opera);

  // I have added other operators
  switch (opera) {
    case '+': printf("result is %d", a + b); break;
    case '-': printf("result is %d", a - b); break;
    case '/': printf("result is %d", a / b); break;
    case '*': printf("result is %d", a * b); break;
    case '%': printf("result is %d", a % b); break;
    default: printf("unknown operation");
  }
}

or simply just read the new line char with the scanf before reading the actual operator like this:

#include <stdio.h>

int main() {
  int a, b;
  char opera;
  printf("input 2 integer number ");
  scanf("%d %d",&a,&b);
  printf("input the operator: ");
  // read the new line char before reading the operator
  scanf(" %c", &opera);

  // I have added other operators
  switch(opera) {
    case '+': printf("result is %d", a + b); break;
    case '-': printf("result is %d", a - b); break;
    case '/': printf("result is %d", a / b); break;
    case '*': printf("result is %d", a * b); break;
    case '%': printf("result is %d", a % b); break;
    default: printf("unknown operation");
  }
}

Result:

input 2 integer number 4 5
input the operator: *
result is 20

Answer 2

Well that is because you enter 2 numbers at first...

input 2 integer number 4 5

at the end, you press enter key. So this '\n' character is stored in input buffer... When your next statement executes :

scanf("%c", &opera);

this input is fulfilled by '\n' already present in buffer. This causes skipping of input.

SOLUTION :-

Use the below statement.

scanf(" %c",&opera);        // Any extra spaces or newline will be discarded...

You wanna read this :-

C - FAQs

Answer 3

There are two methods to solve the issue:

1. Use fflush(stdin) just before the scanf(..., &opera) statement.

2. If you don't want to follow the aforementioned step, just leave a whitespace before %c character of scanf(..., &opera), something like:

   scanf(" %c", &opera);