scanf different position in the code in c [duplicate] - c

Question

This question already has answers here:
problem in scanning character in C
(8 answers)
Closed 1 year ago.
int main()
{
char Operator;
int num1, num2, result;
/* printf("Enter operator: ");
scanf("%c", &Operator);*/
printf("Enter first and second value: ");
scanf("%d %d", &num1, &num2);
printf("Enter operator: ");
scanf("%c", &Operator);
if(Operator == '+')
{
result = num1 + num2;
printf("%d\n", result);
}
else if(Operator == '-')
{
result = num1 - num2;
printf("%d\n", result);
}
return 0;
}
I tried making a simple calculator, and I put scanf(the one that requests an Operator) where it is now, and it does not work. But if I put it where the comment is, above the "Enter first and second value" it then works. I would just like to know why does it work on one place and not the other.
Thanks in advance.

Answer 1

As pointed out in comments, Your first scanf is reading the integer values but it's not reading the newline in buffer. When you do scanf("%c", &Operator) again for taking character input, it reads that newline character still present in input buffer. To avoid this, you can use scanf(" %c", %Operator) instead which will discard all whitespace characters before matching character:
printf("Enter operator: ");
scanf(" %c", &Operator);
You can also use fgets() to read input into a string and then use sscanf() to read integers from string:
char input[100];
printf("Enter first and second value: ");
if (fgets(input, 100, stdin) == NULL) {
printf("No input provided!\n");
return -1;
}
if (strchr(input, '\n') == NULL) {
printf("Input Buffer Overflow\n");
return -1;
}
if (sscanf(input, "%d %d", &num1, &num2) != 2) {
printf("Error in reading characters\n");
return -1;
}
printf("Enter operator: ");
if (fgets(input, 100, stdin) == NULL) {
printf("No input provided!\n");
return -1;
}
if (strchr(input, '\n') == NULL) {
printf("Input Buffer Overflow\n");
return -1;
}
if (sscanf(input, "%c", &Operator) != 1) {
printf("Error in reading characters\n");
return -1;
}

Answer 2

From the Open Group Base Specifications Issue,
Input white-space characters (as specified by isspace()) shall be
skipped, unless the conversion specification includes a [, c, C, or n
conversion specifier.
%d format specifier will skip preceding input newline character (\n). That's why it works in the first case (case, where operator was read first) .
scanf leaves an newline character \n in the input buffer, after the first scanf.
The second scanf will read the extra \n from the input buffer to variable Operator. That's why it will not work.
Since the format specifier is %c, the newline character will not be skipped.
Solution : Use scanf(" %c", &Operator);. Add an extra space at the beginning.