scanf from the buffer is not working properly - c

Question

I will use this small part of code in my assignment but there are some problems. When I don't use fflush(stdin) it doesn't scan the basetype. But when I use fflush(stdin) it scans the basetype but the scanf inside the while loop doesn't continue to take the characters from the buffer. For example when I enter 2A... Please help.
#include <stdio.h>
#include <math.h>
int main() {
int operation, basetype, number1;
char num1;
printf("Please enter the number of the operation you would like to perform:");
printf("\n1. Bitwise OR\n2. Bitwise NOT\n3. Bitwise COMPARE\n4. Exit");
scanf("%d", &operation);
if (operation == 1) {
printf("You chose Bitwise OR operation.");
printf("\nPlease enter the first number:");
scanf(" %c", &num1);
fflush(stdin);
printf("\nPlease specify the base(10/16):");
scanf("%d", &basetype);
if (basetype == 10) {
while (num1 != 10) {
number1 = num1 - '0';
if (number1 >= 1 && number1 <= 9) {
printf("ok");
} else
printf("not ok");
scanf("%c", &num1);
}
}
}
return 0;
}

Answer 1

fflush(stdin); has undefined behavior, do not use it to attempt to flush the standard input buffer. You could instead write:
{ int c; while ((c = getchar()) != EOF && c != '\n') continue; }
You should also decide whether you read a number in num1 or a digit. "%c" is for reading a single character, "%d" to read a number in decimal format.
There is a scanf format tailored for your needs: %i converts a number encoded in a user specified base, 8, 10 or 16 depending on the initial characters, just like the C source code syntax for integer literals.
Here is a simplified program that can deal with hexadecimal input entered with a 0x prefix, such as 0x40 for 64:
#include <stdio.h>
#include <math.h>
int main(void) {
int operation, number1, number2, result;
printf("Please enter the number of the operation you would like to perform:\n"
"1. Bitwise OR\n"
"2. Bitwise NOT\n"
"3. Bitwise COMPARE\n"
"4. Exit\n");
if (scanf("%i", &operation) != 1)
return 1;
if (operation == 1) {
printf("You chose Bitwise OR operation.\n");
printf("Please enter the first number: ");
if (scanf("%i", &number1) != 1)
return 1;
printf("Please enter the second number: ");
if (scanf("%i", &number2) != 1)
return 1;
result = number1 | number2;
printf("%d (0x%x) | %d (0x%x) = %d (0x%x)\n",
number1, number1, number2, number2, result, result);
}
return 0;
}

Answer 2

the following proposed code:
cleanly compiles
contains embedded comments about problems with the code
is organized to clearly show what each part of the code is doing
for Bitwise_OR operation clearly shows the code does nothing, especially not a bitwise OR operation
and now the proposed code:
#include <stdio.h> // scanf(), printf(), perror()
#include <stdlib.h> // exit(), EXIT_FAILURE
//#include<math.h>
enum
{
Bitwise_OR = 1,
Bitwise_NOT,
Bitwise_COMPARE,
Exit
};
int main( void )
{
char operation;
int basetype;
int number1;
char num1;
puts("Please enter the number of the operation you would like to perform:");
printf( "%s",
"1. Bitwise OR\n"
"2. Bitwise NOT\n"
"3. Bitwise COMPARE\n"
"4. Exit");
if( 1 != scanf(" %c",&operation) )
{
perror( "scanf for operation failed" );
exit( EXIT_FAILURE );
}
// implied else, scanf successful
switch( operation )
{
case Bitwise_OR: //<-- bitwise OR never performed
printf("You chose Bitwise OR operation.");
printf("\nPlease enter the first number:");
if( 1 != scanf(" %c",&num1) )
{
perror( "scanf for first number failed" );
exit( EXIT_FAILURE );
}
// implied else, scanf successful
//fflush(stdin);
printf("\nPlease specify the base(10/16):");
if( 1 != scanf("%d", &basetype) )
{
perror( "scanf for base failed" );
exit( EXIT_FAILURE );
}
// implied else, scanf successful
switch(basetype)
{
case 10:
while(num1 != 10) //<-- user entering 0x0A is extremely unlikely
{
number1=num1-'0'; //<-- what if num1 not '0'...'9'
if(number1>=1 && number1<=9)
{
printf("ok");
}
else
printf("not ok");
// either retry first digit or get second digit
scanf(" %c",&num1); //<-- leading space to consume 'white space'
}
break;
case 16:
break;
default:
puts( "base must be 10 or 16" );
break;
} // end switch()
break;
case Bitwise_NOT:
break;
case Bitwise_COMPARE:
break;
case Exit:
break;
default:
break;
} // end switch()
return 0;
} // end function: main