18. Fortran programming style

There are many different programming styles, but we will try to give some general guidelines that are fairly non-controversial.


To ensure portability, use only standard Fortran 77. The only exception we have allowed in this tutorial is to use lower case letters today.

Program structure

The overall program structure should be modular. Each subprogram should solve a well-defined task. Many people prefer to write each subprogram in a separate file.


Let us repeat this: Write legible code, but also add comments in the source explaining what is happening! It is especially important to have a good header for each subprogram that explains each input/output argument and what the subprogram does.


Always use proper indentation for loops and if blocks as demonstrated in this tutorial.


Always declare all variables. Implicit type declaration is bad! Try to stick to maximum 6 characters for variable names, or at the very least make sure the 6 first characters are unique.


Never let functions have "side effects", i.e. do not change the value of the input parameters. Use subroutines in such cases.

In the declarations, separate parameters, common blocks, and local variables.

Minimize the use of common blocks.


Minimize the use of goto. Unfortunately it is necessary to use goto in some loops since while is not standard Fortran.


In many cases it is best to declare all large arrays in the main program and then pass them as arguments to the various subroutines. This way all the space allocation is done in one place. Remember to pass the leading dimensions. Avoid unnecessary "reshaping" of matrices.

Efficiency concerns

When you have a double loop accessing a two-dimensional array, it is usually best to have the first (row) index in the innermost loop. This is because of the storage scheme in Fortran.

When you have if-then-elseif statements with multiple conditions, try to place the most likely conditions first.

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