INDEX is not returning a match when using a variable substring

Question

I have compiled code with gfortran and AOCC flang compiler, but it fails for both, is there anything wrong I am doing?

  program find_sub_indx
        implicit none
    !decl
        character(len =30) :: main_string, sub_string
        integer ::  index_1 , index_2
        logical :: back
    !defn   
        main_string = "this is the main string"
        sub_string = "a"
        back = .false.

        index_1 = INDEX(main_string, sub_string, back)  !why does this not work 
        index_2 = INDEX("this is the main string","a", .false.) !this works why?
        print *, "index_1 is " , index_1, index_2
    end program find_sub_indx

Result expected:

index_1 is             14           14

Actual result:

index_1 is             0           14

Is there some standard reference for learning fortran, as I couldn't find the proper definition of intrinsic function used above.

francescalus · Accepted Answer · 2019-07-25 09:44:23Z

1

In the first attempt to use index

INDEX(main_string, sub_string, back)

the variables main_string and sub_string are both of length 30. After the assignment

 sub_string = "a"

the variable sub_string has value starting with a but has 29 trailing spaces after that.

So, the function is evaluated like

INDEX(main_string, 'a                             ', back)

That substring is, of course, not found in main_string and the result is correctly 0.

You can instead use

INDEX(main_string, TRIM(sub_string), back) !or
INDEX(main_string, sub_string(1:1), back)

or declare sub_string to be of length 1.

The literal constant "a" in the second attempt has length 1 and does not have these trailing spaces.

answered Jul 25, 2019 at 9:44

francescalus

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3 Comments

Sameeran Joshi Over a year ago

Thanks,Is there some standard reference for learning fortran, as I couldn't find the proper definition of intrinsic function used above.

francescalus Over a year ago

You can read the language standards (links found here), but your compiler's documentation is likely to provide such descriptions also.

Ian Bush Over a year ago

@SameeranJoshi If you want a book I would recommend oxfordscholarship.com/view/10.1093/oso/9780198811893.001.0001/…

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