Use uninitialised variables for a random effect

If you want non-deterministic behaviour, but it doesn't have to be uniformly random, just do something like print*,i (where i hasn't been previously set) and Fortran (on TIO) will give you an undetermined integer. Or you can do print*,mod(i,5) to get a random integer from 0 to 4. Example

Arithmetic if (Fortran 77)

Rather than

      if (e < 0) then
      A()
      else if (e == 0) then
      B()
      else
      C()
      end if
      D()

you can use

      if(e)1,2,3
    1 A()
      goto 4
    2 B()
      goto 4
    3 C()
    4 D()

Often, one or more of the gotos can be omitted. For example:

      if(e)1,1,2
    1 A()
    2 B()

is equivalent to

      if(e<=0)then
      A()
      endif
      B()

This saves 7 characters: the statement labels themselves are 'free', as the first 6 characters on each line must be present and cannot be used for anything else.

Computed goto

A similar feature is the computed goto, which takes the form

      goto(6,7,8,9)i
      A()
    6 B()
    7 C()
    8 D()
    9 E()

which can be expanded into something like

      if(i==4)then goto 9
      if(i==3)then goto 8
      if(i==2)then goto 7
      if(i==1)then goto 6
      A()
    6 B()
    7 C()
    8 D()
    9 E()

This can be used to implement something like a switch statement.

Some handy print formats:

print"(*(g0))" formats integers to minimum width, instead of their usual fixed 20-odd character width

print'(16Z3.2)' repeat 16 times, convert to hex, 3 characters wide, 0-padded to width 2

Use a function to minimise I/O boilerplate

Setting up variables and reading data from stdin uses up bytes! This is usually allowed to be handled elsewhere, as long as your submission is a working function or subroutine. Example

Use the short form of if when possible

If it's a simple condition, don't do this: if(a.gt.b)then;print*,'yay';endif

Do this instead: if(a>b)print*,'yay'. Example

Use implicit do loops inside print statements

Instead of: do i=1,n;print*,i;enddo

Try this: print*,(i,i=1,n)

You can nest these loops too, using more parentheses (): (reference)

print*,((i*j,i=1,10),j=1,10)

Use macros to #define repeated stuff

Just add the -cpp flag to the gfortran compiler. Easier than subroutines or loops. Example

Leverage Default Behavior:

Don’t declare IMPLICIT NONE unless the problem demands it. Rely on Fortran’s default typing to save characters.

Use Fixed-Form Tricks:

If allowed, use fixed-form Fortran (.f) where columns 1-5 are for labels and 7-72 for code. You can omit spaces and pack statements tightly. For example:

10I=I+1

is shorter than free-form equivalents.

Minimize I/O:

Input/output statements like READ and WRITE can be verbose. Use the shortest form possible:

• READ*,N instead of READ(*,*) N.
• PRINT*,X instead of WRITE(*,*) X.

Exploit Loops:

Fortran’s DO loop can be concise. For example:

DO1I=1,N

in fixed-form is shorter than DO I=1,N with an END DO. If the loop body is one line, skip the END DO and use a label.

Program Structure:

Skip PROGRAM and END PROGRAM if the problem allows an implicit program. A bare subroutine or just executable lines can work: N=5;PRINT*,N instead of: PROGRAM P N=5 PRINT*,N END PROGRAM P

String Handling:

Fortran’s string output can be golfed with concatenation (//) or by avoiding unnecessary quotes. For example, PRINT*,'A',I can sometimes be PRINT*,A,I if A is a character variable.

Know Your Compiler:

Some tricks depend on the compiler (e.g., gfortran vs. ifort). Test minimal syntax like omitting optional keywords or semicolons.