Question

I have a read statement that expects a number, very simple example code:

program test
integer var
read(*,*) var
end

The thing is that I usually input a string of characters (ie: yes) on account of being distracted. How can I prevent my code from halting entirely and instead display an error message of the type You’ve entered an incorrect value. Try again?

I’m thinking something like:

    program test
    integer var
10  read(*,*) var
    if (var.not.a.number) then
      write(*,*)'You've entered an incorrect value. Try again'
      goto 10
    endif
    end

What would that var.not.a.number condition look like?
I use gfortran to compile under Ubuntu.

---

Edit: Thank you all! I ended up implementing HPM’s 3rd option since it was the simplest one:

    program test
    integer var,iostat,ios
10  read(*,*,iostat=ios) var
    if (ios.ne.0) then
      write(*,*)'You've entered an incorrect value. Try again'
      goto 10
    endif
    end

Special thanks to User7391 who took the effort to write all that code!

Answer 1

You’re using list-directed input. The second * in the statement read(*,*) essentially tells the compiler/run-time system that you will provide it with something at run time that can be interpreted as an integer. If you want to give yourself the freedom to make mistakes on input you have (at least) 3 choices.

1. You can, as @User7391’s answer already says, read the input into a character variable and parse it yourself. That kind user has even offered to write the code for you !
2. You could modify the read command to something like read(*,*,err=1234) where 1234 is the label of your error-handling statement(s). This approach is now considered rather old-fashioned and might be frowned upon.
3. You could modify the read command to something like read(*,*,iostat=ios) where ios is an integer variable which catches the iostat (i/o status flag) reported by the read statement. You might then write the line if (iostat/=0) ... for error handling. This is considered to be the more up to date approach.