Read substrings from a string containing multiplication [duplicate]

Question

I am a scientist programming in Fortran, and I came up with a strange behaviour. In one of my programs I have a string containing several "words", and I want to read all words as substrings. The first word starts with an integer and a wildcard, like "2*something".

When I perform an internal read on that string, I expect to read all wods, but instead, the READ function repeatedly reads the first substring. I do not understand why, nor how to avoid this behaviour.

Below is a minimalist sample program that reproduces this behaviour. I would expect it to read the three substrings and to print "3*a b c" on the screen. Instead, I get "a a a".

What am I doing wrong? Can you please help me and explain what is going on?

I am compiling my programs under GNU/Linux x64 with Gfortran 7.3 (7.3.0-27ubuntu1~18.04).

PROGRAM testread

IMPLICIT NONE
CHARACTER(LEN=1024):: string
CHARACTER(LEN=16):: v1, v2, v3

string="3*a b c"

READ(string,*) v1, v2, v3

PRINT*, v1, v2, v3

END PROGRAM testread

Answer 1

You are using list-directed input (the * format specifier). In list-directed input, a number (n) followed by an asterisk means "repeat this item n times", so it is processed as if the input was a a a b c. You would need to have as input '3*a' b c to get what you want.

I will use this as another opportunity to point out that list-directed I/O is sometimes the wrong choice as its inherent flexibility may not be what you want. That it has rules for things like repeat counts, null values, and undelimited strings is often a surprise to programmers. I also often see programmers complaining that list-directed input did not give an error when expected, because the compiler had an extension or the programmer didn't understand just how liberal the feature can be.

I suggest you pick up a Fortran language reference and carefully read the section on list-directed I/O. You may find you need to use an explicit format or change your program's expectations.