fortran - '*' and '/' not recognized on input by a read statement

Question

I start learning Fortran and I'm doing a little case test program where the user types two real numbers and selects an arithmetic operators (from + - * /). The following error appears when the user selects "*"

F6502 : read <con> - positive integer expected in repeat field

and if the user selects "/" the compiler executes the default case, and displays this message

invalid operator, thanks
the result is 0.000000E+00

The program is as follows.

program operateur
implicit none

   CHARACTER(LEN=1) :: oper 
   real::a,b,res
   print*,'Give the first number a :'
   read*,a
   print*,'Give the second number b :'
   read*,b
   print*,'which operation ?'
   read*,oper
   !print*,'donnez a,b,oper :'
  ! read(*,*)a,b,oper

               select case (oper)

                  case ('+')      
                  res=a+b

                  case ('-')       
                  res=a-b

                  case ('*')       
                  res=a*b


                  case ('/')       
                  res=a/b           

                  case default
                  print*, "Invalid Operator, thanks" 

               end select

   print*,'the result is ',res 

end program operateur

Answer 1

FORTRAN input and output formatting rules are rather involved. Each input and ouptut statement has two arguments that have special meaning. For example

  READ (10,"(2I10)") m,n

The first argument is a file descriptor. Here it is 10. The second argument "(2I10)" is the format specifier. If you give an asterisk (*) as a format specifier you switch on the list-directed formatting mode.

List directed input as the name suggests is controlled by the argument list of the input operator.

1. Why asterisk (*) is special in list-directed input mode?

The input list is split into one or more input records. Each input record is of the form c, k*c or k* where c is a literal constant, and k is an integer literal constant. For example,

  5*1.01

as an instance of k*c scheme is interpreted as 5 copies of number 1.01

   5*

is interpreted as 5 copies of null input record.

The symbol asterisk (*) has a special meaning in list-directed input mode. Some compiler runtimes would report a runtime error when they encounter asterisk without an integer constant in list-directed input, other compilers would read an asterisk. For instance GNU Fortran compiler is known for standards compliance, so its runtime would accept *. Other compiler runtimes might fail.

2. What's up with slash (/)?

A comma (,), a slash (/) and a sequence of one or more blanks ( ) are considered record separators in list-directed input mode.

There is no simple way to input a slash on its own in this mode.

3. Possible solution: specify format explicitly

What you can do to make the runtime accept a single slash or an asterisk is to leave the list-directed input mode by specifying the format explicitly:

read (*,"(A1)") oper

should let you input any single character.