if i want to take two numbers as input at first, why It's not work?

Question

#include<stdio.h>
int main ()
{
double num1, num2;
char oper;
printf("Enter two numbers: ");
scanf("%lf %lf", &num1, &num2);
printf("Enter an opearator (+, -, *,/) : ");
scanf("%c", &oper);
switch(oper)
{
case '+':
{
printf("%lf + %lf = %lf\n", num1, num2, num1+num2);
break;
}
case '-':
{
printf("%lf - %lf = %lf\n", num1, num2, num1-num2);
break;
}
case '*':
{
printf("%lf * %lf = %lf\n", num1, num2, num1*num2);
break;
}
case '/':
{
printf("%lf / %lf = %lf\n", num1, num2, num1/num2);
break;
}
default:
printf("Not a valid operator\n");
}
return 0;
}

Answer 1

scanf("%c", &oper);
Is where your problem lies.
This is due to the remaining newline character left over from entering the numbers. You can resolve it by simply adding a space prior to the %c so that it consumes the leading whitespace characters.
scanf(" %c", &oper);

https://stackoverflow.com/questions/13542055/how-to-do-scanf-for-single-char-in-c