invalid input

This is a discussion on invalid input within the C Programming forums, part of the General Programming Boards category; I need my program to output a message "invalid input" when the input is not correct by the user. Currently ...

  1. #1
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    invalid input

    I need my program to output a message "invalid input" when the input is not correct by the user.

    Currently my programs use is to allow the user to enter a number and the program outputs that number in words.

    This is whats meant to happen:

    Input: 30
    Output: thirty
    Input: 30 eight
    Output: invalid input

    This is what happens:

    Input: 30
    Output: thirty
    Input 30 eight
    Output: thirty

    Can anyone help me with how to get the invalid input thing to work. Here is a snippet of my code:

    Code:
    int res;
          int num;
          res = sscanf( cmd, "%d", &num);
          switch(num){
          
          /*0 - 20*/
          
             case 0: printf("zero\n");
                break;
             case 1: printf("one\n");
                break;
             case 2: printf("two\n");
                break;
             case 3: printf("three\n");
                break;
             case 4: printf("four\n");
                break;

  2. #2
    and the hat of wrongness Salem's Avatar
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    > res = sscanf( cmd, "%d", &num);
    This only tells you that there was a valid number at the start of the line.

    res = sscanf( cmd, "%d%n", &num, &pos);
    if ( cmd[pos] != '\n' ) // there's trailing chars, probably its an invalid line
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    But theres a possibility that theres trailing chars, coz the numbers go up to 1000. I basically need to say that if a number inst entered its invalid input dont i? how do i go about doing that?

  4. #4
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    It wouldnt catch out all the invalid input by saying if theres any more chars after [3] its invalid input. How do i get it to catch it out if say the user entered: 56hj for example?

  5. #5
    and the hat of wrongness Salem's Avatar
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    I'm guessing my previous example would result in cmd[pos] == 'h' in that case....
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  6. #6
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    This catches the invalid input but wont print out the number now when it is valid input.

    Code:
          int res;
          int num;
          int pos;
         /* res = sscanf( cmd, "%d", &num); */
          res = sscanf( cmd, "%d%n", &num, &pos);
          if ( cmd[pos] != '\n' )
             switch(num){
             
             /*0 - 20*/
             
                case 0: printf("zero\n");
                   break;
                case 1: printf("one\n");
                   break;
                case 2: printf("two\n");
                   break;

  7. #7
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    and if i enter 7hello for example. it prints out seven.

  8. #8
    Been here, done that.
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    Originally posted by sworc66
    and if i enter 7hello for example. it prints out seven.
    try debugging by adding a printf() statement to ouput pos as an decimal and cmd[pos] as a hex value. This will tell you if you're getting the values you expect.
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  9. #9
    ATH0 quzah's Avatar
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    This catches the invalid input but wont print out the number now when it is valid input.
    Code:
    if ( cmd[pos] != '\n' )
        switch(num){
    Well of course it's not going to work. You're telling it "if this spot IS NOT a newline, then process that as a valid number".

    That's the opposite of what you want. You want it to process it if the next position is the newline, meaning, there is no invalid data.

    Quzah.
    Hope is the first step on the road to disappointment.

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