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Asked: 2026-06-18T22:42:44+00:00

program Test implicit none character (LEN=100) :: input character (LEN=100) :: output print *,Please

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program Test

implicit none
character (LEN=100) :: input
character (LEN=100) :: output
print *,"Please input your message: "
read *, input

For every character, I encrypt it in Ceaser’s Cipher

Calculations

print *,"This is the output: "
write (*,"(2a)") "Message = ", out

end program Test

This doesn’t work entirely.

For every character in the input, I convert it using the modulo(iachar()) functions. It works up until the print, I followed the debugging, the encryption is fine.
But the issue with the output lies in LEN=100. The do loop will go through 100 times converting nonexistent characters into garbage, breaking the program at output with UNDEFINED TYPE.

So if I input “test”, it will encrypt CBNC*GARBAGE-TO-100* and not output. If I define length as 4, and do it, it works. but I want to be able to do it without defining a length. Any way around this?

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1 Answer

  1. Editorial Team

    The read statement should pad input out to the full length of the variable (100 characters) with blanks, rather than adding “garbage”. The LEN_TRIM intrinsic function will give the significant length of the variable’s value – i.e. the length excluding trailing blanks. You may need to remember this significant length of the input string for when you print the output string.

    (Note the rules on list directed input (indicated by the * in the read statement) can be a little surprising – a format of “(A)” may be more robust, depending on the behaviour your want.)

    In terms of avoiding fixed length strings in the context of reading input – Fortran 2003 introduces deferred length character, which greatly helps here. Otherwise see Reading a character string of unknown length for Fortran 95 possibilities. One complication is that you are reading from the console, so the backspace statement may not work. The work around to that follows a similar approach to that linked, but necessitates piecewise building the input string into an allocatable array of character at the same time as the input record length is being determined. Sequence association is then used to convert that array into a scalar of the right length. Comment or ask again if you want more details.

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