Double while loop for flushing data before a switch statement - c

Question

I need to write a script for the user to enter a score between 0 and 10, flush the bad input out, if user inputs it and then using a switch statement, tell a user what grade did he/she got.
Here is my script:
...
int main()
{
int input; // input from user
printf("Enter the number between 0 and 10 and I will tell you your grade!");
while ((input=scanf("Your input:", &input) != EOF))
{
if (input < 0 || input > 10) //input is invalid
{
printf("Sorry, invalid character data.");
while (getchar() !='\n')
{
printf("Your input must be from 0 to 10.", input);
scanf("%d", &input); //This part looks very bad for me
}
}
else
switch (input)
{
case 0:
case 1:
case 2:
case 3:
case 4:
case 5:
printf("Your grade is F. \n");
break;
case 6:
printf("Your grade is D. \n");
break;
...
I got this far with my homework, and here are some "leftover" problems I can't fight with.
1) Whenever user submits anything after enter, it goes into infinite loop and prints Your grade is F., even when case = 6 for example.
2) I used break; at the end of each case. It looks like they don't work(?)
3) It looks like the problem in the second line in the second loop
scanf("%d", &input); //This part looks very bad for me
but then I guess the scripts accepts it as true since the else statements that includes switch begins to work, because otherwise it wouldn't print Your grade is F.

Answer 1

Try the following code. Have a look at what flush_stream is doing when we have invalid data...
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
void flush_stream();
void flush_stream() {
char c;
do {
c = getchar();
}
while (!isdigit(c) && c != '\n');
ungetc(c, stdin);
}
int main(void) {
const char *prompt = "Input please: ";
int input; // input from user
printf("Enter the number between 0 and 10 and I will tell you your grade!\n");
while(1) {
printf("%s", prompt);
int ret = scanf("%d", &input);
if(ret == 0) {
printf("Sorry, invalid character data, your input must be from 0 to 10.\n");
flush_stream();
continue;
}
if(ret > 0) {
if (input < 0 || input > 10) {
printf("Sorry, invalid character data, your input must be from 0 to 10.\n");
flush_stream();
continue;
}
switch (input) {
case 0:
case 1:
case 2:
case 3:
case 4:
case 5:
printf("Your grade is F. \n");
break;
case 6:
printf("Your grade is D. \n");
break;
}
}
}
}

Answer 2

It seems you are using scanf and printf wrong.
To input a number:
scanf("%d", &input);
To output a number:
printf("%d\n", input);
And in you while loop, when the input is illegal, why not just continue to the next loop?
while (true) {
printf("input your grade here: ");
if (scanf("%d", &input) == EOF) {
break;
}
if (input < 0 || input > 10) {
printf("your input is illegal.\n");
continue;
}
switch (input) {
...
}
}