Fortran77 User Input Validation

Question

I need the user to enter only real numbers when prompted. An error message should be displayed and program should stop in case the user enters a character or any other symbol. How can i do this in fortran 77.

Answer 1

I assumed, you probably want to have more tries, because what you have in your question does the compiler does automatically.

You can use the IOSTAT= or ERR= specifiers.

   INTEGER IE
   DO I = 1, MAX
     READ(*,*,IOSTAT=IE) X
     IF (IE.eq.0) GOTO 10
     PRINT *,"Wrong input, try again."
   END DO
10 CONTINUE

or

   GOTO 10
20 PRINT *,"Wrong input, try again."
10 READ(*,*,ERR=20) X

Answer 2

improving the user interface a little, i like to read a string, then do the error check on an internal read:

    character insting*80
    ie=1
    do while(ie.ne.0)
    read(*,'(a)')instring
    if(instring(1:1).eq.'q')stop
    read(instring,*,iostat=ie)x
    if(ie.ne.0)then
        write(*,*)'invalid input ',trim(instring),' is not a real number'
    endif
    enddo

(note trim is not f77 btw, but its easy enough to replicate if needed)

Note, with some effort you can handle the pathalogic cases. A leading '/' or ',' throws no error but leaves the value of x unchanged (likely system/compiler dependent however!), so you can do something like this:

     x=-999999.   
     read(instring,*,iostat=ie)x
     if(ie.eq.0.and.x.eq.-999999.)then
            .. code to handle error..
     endif

You could alternately use index to look for the bad characters. On my system a decimal . by itself is taken as zero. That gets a bit cumbersome to handle, but it can be done by parsing instring