C: Scanf in while loop executes only once

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.

Think about your loop logic a bit more. scanf returns 1 if it read successfully; or 0 if you entered a non-number, or EOF if the input was closed. The innermost while loop doesnt make any sense — M.M
– M.M, Commented Nov 13, 2014 at 22:07
When scanf() finds invalid input, it pushes back and returns prematurely. You have to consume the characters that are not numbers, otherwise scanf will always be failing with the same erroneous input. — Filipe Gonçalves
– Filipe Gonçalves, Commented Nov 13, 2014 at 22:09
@AlexD: I thought of that, but this is a C course assignment, and they supply the input in files. If I clear the buffer it will flush all the input that I got from the input file. — Dean
– Dean, Commented Nov 13, 2014 at 22:27
@Dean Did you check this answer: stackoverflow.com/a/1716066/3246555? — AlexD
– AlexD, Commented Nov 13, 2014 at 22:30

chux · Accepted Answer · 2014-11-13 23:09:09Z

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.

answered Nov 13, 2014 at 23:09

chux

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2 Comments

Dean Over a year ago

There's nothing I would love more than to go for a better C solution here. But I am supposed to follow the course material and allowed to use only the functions and libraries they taught in the university. I am checking it with the academic staff... maybe they didn't take it into account.

chux Over a year ago

@dean There are no new standard library functions here, aside from fgetc(), that is not all ready in your post. Thus I do not see how "use only the functions and libraries" applies. If using fgetc() is a problem, then replace it with scanf("%*c") for the same effect.

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