C: Scanf in while loop executes only once - c

Question

I am trying something simple in C, a program to get the exchange and do some conversions.
To make sure scanf gets the right type I placed it into a while loop, which continues to ask for input until a number is inserted.
If I enter a character instead of a number it does not ask again for an input.
exRate = 0;
scanfRes = 0;
while(exRate <= 0){
printf("Enter the exchange rate:");
while(scanfRes != 1){
scanfRes = scanf(" %f", &exRate);
}
if(scanfRes == 1 && exRate > 0){
break;
}
printf("Exchange rate must be positive.\n");
}
UPDATE: As this is a course assignment, I was not supposed to use anything outside of the taught material. When I asked the academic staff about handling unexpected input, I got an answer that this is a scenario I am not supposed to take into consideration.
The answers and help in the comments is all useful and I added 1 to all useful suggestions. The staff answer makes this question no longer needed.

Answer 1

Change handling of scanf() result.
If the input is not as expected, either the offending input data needs to be read or EOF should be handled.
for (;;) {
printf("Enter the exchange rate:");
scanfRes = scanf("%f", &exRate);
if (scanfRes == 0) {
printf("Exchange rate must be numeric.\n");
// somehow deal with non-numeric input, here just 1 char read & tossed
// or maybe read until end-of-line
fgetc(stdin);
} else if (scanfRes == EOF) {
// Handle EOF somehow
return;
} exRate > 0){
break;
}
printf("Exchange rate must be positive.\n");
}
Note: the " " in " %f" is not needed. "%f" will consume leading white-space.