Chapter 2 — Key Takeaways (Setting Up)

A one-page reference for the toolchain. Keep the two build commands and implicit none where you can see them; they are the muscle memory the rest of the book assumes.

Install gfortran (want version 10+)

Verify: gfortran --version → read the version number off the first line. Full detail in Appendix C.

The skeleton of every program

program name
  implicit none            ! always — no exceptions
  ! ... declarations, then statements ...
  print '(a)', "text"      ! flush-left; print * adds a leading blank
end program name            ! the name must match `program name`

• Files are .f90 (free-form). Keywords lowercase. Indent two spaces (for humans; the compiler ignores it).

The compile–link–run cycle

  source  ──compile──▶  object  ──link──▶  executable  ──run──▶  output
 hello.f90            hello.o             hello (.exe)        ./hello

Read the error's stage: has no IMPLICIT type / syntax error = compile. undefined reference to … = link. Different stage, different fix.

The five flags that matter early

Two profiles (memorize):

$ gfortran -std=f2018 -Wall -g -fcheck=all file.f90 -o file    # develop
$ gfortran -std=f2018 -Wall -O2 file.f90 -o file               # release

Rule: develop with checks on; run with optimization on. Never quote a timing measured with -fcheck=all. (Deeper flags — -O3 -march=native -flto — are Chapter 30.)

Free-form vs fixed-form

You cannot paste fixed-form into a .f90 file unchanged — convert it (Part IV).

implicit none — the non-negotiable habit

• Goes immediately after the program/module/subroutine/function line, before declarations.
• Turns off implicit typing (undeclared names typed by first letter: i–n → integer, else real).
• Why it is mandatory: without it, a mistyped variable name is silently invented as a new, uninitialized variable → a wrong answer with no error. With it, that typo is a compile-time error.

The one rule of this chapter: implicit none, in every program unit, always.

Numbers, the house style (previewed; full story Ch. 3)

use, intrinsic :: iso_fortran_env, only: dp => real64
real(dp) :: x
x = 1.5_dp          ! declare reals as real(dp); suffix literals with _dp

• print '(a, f0.2)', 'x = ', x → text plus a real with 2 decimals, minimum width, no leading blank.

Common pitfalls

• Confusing "it compiled" with "it is correct" — the compiler checks grammar, not intent.
• Forgetting implicit none (silent typo bugs) or -Wall/-fcheck=all (declining free help).
• Pasting fixed-form code into a .f90 file.
• Timing a -fcheck=all build and calling Fortran slow.
• Forgetting the ./ to run a program in the current directory on Linux/macOS.

Project piece added this chapter

The driver: program heat — a banner that compiles cleanly, in heat-solver/heat.f90. Built with the development profile; verified by hand (no arithmetic, so the banner prints verbatim). Every future checkpoint grows this file or links new files to it, all the way to the Chapter 38 capstone.