constant number before a statement in fortran code

I came across something and I am unable to find any explanation for it, consider the following two code snippets.

THIS RUNS

Program Hello

implicit none

print *, "hello world"

1 print *, "one"

print *, "bye world"

End Program Hello

output:
hello world
one
bye world

THIS DOES NOT COMPILE

Program Hello

implicit none

print *, "hello world"

1 
print *, "one"

print *, "bye world"

End Program Hello

error:\
main.f95:16:1:\
   16 | 1\
      | 1\
Error: Statement label without statement at (1)

Anyone knows what is happening here? Why does the first one compile and run?

PS: it is not useful code, however I saw "1 continue" in some Fortran code, and was just wondering what that even achieves?

EDIT: SO it is likely a statement label, How are Fortran statement labels used? are they purely cosmetic or actually provide some usage?

The 1 here is not a numeric constant; it is a statement label. (Recall that expressions like the actual constant 1 cannot appear in arbitrary places, or as the only part of a statement as can be found in some other languages.)

A statement label (Fortran 2018 6.2.5):

> provides a means of referring to an individual statement

That's it: it's a label to say that the print statement in the first example can be referred to be a reference to the label 1.

A statement label consists of between one and six digits, at least one of which is not a zero. Within a scope, they must be unique (ignoring leading zeros).

A label must be followed by a statement with non-blanks. This makes

1

invalid.

Not all statements may be labelled, and not all statements are usefully labelled. Labels are meaningful to the program only in the following cases:

1. As a statement to be "jumped" to (go to, etc.)
2. Necessarily as part of a format statement (so it can be referenced)
3. In a non-block DO construct (obsolete in Fortran 2018)

In other cases a statement label is meaningful only to programmers (say as documentation).

1 continue

is commonly used in cases like the non-block DO:

do 1 i=1, 10
1 continue

(but we don't do these any longer).

Or similarly as a do-nothing jump target:

read(unit, err=1)
...
1 continue

(if we aren't using iostat=... which we should).

Fortran 2018 in the section about statement labels:

> 6.2.5 Statement labels
>
> 2 If a statement is labeled, the statement shall contain a nonblank
> character. ...

You cannot have a label on an empty line, it is not valid Fortran. You can put the CONTINUE statement to your lable, it does nothing.

It is quite common to find CONTINUE statements with labels terminating a DO loop in old source codes.

Other than that, stament labels were also commonly used for FORMAT statements and as targets of GO TO. There are other, less common, use cases too.

So, regarding your edit:

DO 10 I = 1,99
  PRINT *, I
10 CONTINUE

is a do loop and

GO TO 10

is a jump to that end line with the statement.