Trouble with comparing character strings in Fortran

[BruceJohnJennerLawso]

Hello all

So I am working on some basic Fortran (FORTRAN) programs in order to learn the basics. I have some experience coding in C++ before this

Im trying to write some basic input control structures, ie

      PROGRAM MENU
      implicit none
      character (LEN=5):: input
      write(*,*)'==Age Database=='
      do while(input/='Q'.or.input/='q')
        write(*,*)'==Main Menu=='
        write(*,*)'> '      
        read(*,*)input
        if(input=='P'.or.input=='p')
          write(*,*)'P command activated'
        end if
      end do
      stop
      end program MENU

But when I go to compile it with gfortran (ahh, gfortran -o menu menu.f)

I get

menu.f:9.72:

if(input=='P'.or.input=='p')
1
Error: Cannot assign to a named constant at (1)
menu.f:11.11:

end if
1
Error: Expecting END DO statement at (1)

Which I dont understand, since I thought == was a comparison operator, not the assignment one. Is it absolutely necessary to declare another character string , assign 'P' or 'p' to it, and check it in that case?

 

[xwb]

You are missing the then

Also, since input has been declared as character(len=5), trim it before comparing. Unlike C, there is no string terminator. Fortran strings are space padded so it will be comparing 'P ' against 'P'.

 

[BruceJohnJennerLawso]

So is the then keyword required for all if statements, or just ones that compare strings?

How exacty do I "trim" the input variable?

 

[xwb]

Not always, depends on how you write the if statement.

! Then not required
if (condition) dosomething
! Then  required
if (condition) then
    dosomething
end if

Trimming

if(trim(input)=='P'.or.trim(input)=='p') then
    ...

 

[salgerman]

not equal" is working for me, I had to change the "do while" around:

PROGRAM MENU
      implicit none
      character (LEN=5):: input
      write(*,*)'==Age Database=='
      do ! while(input/='Q'.or.input/='q')
      
        write(*,*)'==Main Menu=='
        write(*,*)'> '      
        read(*,*)input
        if ( input=='P' .or. input=='p') then
          write(*,*)'P command activated'
        end if
        
        if ( input=='Q' .or. input=='q') exit
      end do
      stop
end program MENU

No need for trim, by the way, in this case; but do learn about it.

 

[salgerman]

Sorry...I meant to say that "not equal" it's not working for me

 

[xwb]

It is a logic error. Should be

    do while (input /= 'Q' .and. input /= 'q')

 

[salgerman]

doh!