help with error checking

[mfitzp]

Hi i'm doing a project for college thats an cash register style program that lets you choose ither euro or sterling to make ur transaction in anyways i had no problem writing the program at all but we have to have the program to error check its self and i came across a very weird error that i can't think of a way to fix i'd aperiate any hints on how to fix it thank you

while ( num != 0 )
{
  printf( "Please enter product price: " ) ;
  scanf( "%f" , &num ) ;
  total += num ;
  printf( "The total price so far is %5.2f\n\n\n" , total ) ;
}

total and num are float vars but the error is when i type in a letter it will continuiously add 1 pound on every second how would i stop this from happening like displaying an error message is someone types in an letter? thanks for the help.

[dwks]

scanf() returns

The number of items succesfully read. This count doesn't include any ignored fields with asterisks (*).
If EOF is returned an error has occurred before the first assignation could be done.

So, test scanf() like so:

if(scanf("%f" , &num) != 1) {
    clearerr(stdin);
    /* do something about error */
}

[mfitzp]

Oh Alright thanx alot i shall give it a try cheers