[PDF] Appendix C - A Fortran Primer: (and cheat sheet)

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Appendix CA Fortran Primer: (and cheatsheet)This section will provide a basic intro to most of the commonly occuring features

of Fortran that you will need for the course. This list is by nomeans exhaustive, but it should be enough to get you where you need to go. For more information, We have extensive fortran manuals scattered about the observatory. By the end of this section you should understand the basic data types, the structure of arrays, standard arithmetic operations, loops and conditional statements,subroutines and functions, and basic IO. This section will also briefly discuss how to build programs using makeand how to debug them usingdbxtool/debugger.

C.1 Basic Fortran Concepts and Commands

Data typesfor the most part there will be only three basic types of data you will have to deal with, integers, floating point numbers and characters. In fortran these data types are declared as integerexact whole numbers (-3, 0, 5 234), usually stored in 4 bytes realinexact representation ofdecimal numbers (3.1415, 27.2, 1.e23). Usu- ally stored as 4 bytes, good for about 6-9 significant digits,ranges from about10-38-1038 double precisionsame as real but much larger range and precision. Usually stored as8bytes, good for about 15-17 significant digits, ranges from about10-308-10308. You can get more precision than this but you should have a good reason. character, character*neither a single character or a string of char- acters of lengthn. Constant and Variable namesA constant or variable can have any name with up to 32 characters (To play it safe though, Standard Fortranallows only

6 characters). The name must start with a letter, but can havemost of the

19 20 alphanumeric characters in it. Fortran also has the annoying feature ofim- plicit typingso that if undeclared, any variable name that starts with the lettersithroughnare assumed to be integers, everything else is assumed real. Rigorous programming practice suggests that you start every program withimplicit noneand declare every variable. It's annoying but it keeps you honest. I will try to do this in my programs. A sample program declara- tion might look like implicit none integer npnts, n real c, dcdt real car(1000),tar(1000) character*40 fileout arraysPerhaps the most important feature for modeling is arrays. Arrays are sim- ply ordered sets of numbers that can be addressed by index. Every array has a name (e.g.carortarabove) and a length (here bothcarandtarare ar- rays of real numbers of length 1000). Arrays can be of integers, reals, double precision or even characters (fileoutis actually an array of 40 characters). Each member of an array can be addressed as by specifying its index in the array (car(10),tar(n)etc.). Arrays can also have up to 7 dimensions. A two dimensional arraya(10,10)can be thought of as 10 1-dimensional arrays of 10 numbers each (a total of 100 elements). Most importantly in fortran, the leading index increases fastest i.e. fora(i,j), thei=1andi=2 are next to each other in memory. Simple operationsinteger and floating point numbers and arrays of them can all be operated on by standard mathematical options. The most commonly used arithmetic are =assignmentx=y **exponentiala=x**y /, *divide, multiplya=x/y, or a=x*y +, -add subtracta=x+y, or a=x-y The order of evaluation is standard algebraic and parentheses can be used to group operands. In addition to the simple operations, Fortran also has some built in intrinsic functions. The most commonly occuring are trigonometric functionssin(x), cos(x), tan(x),asin(x),acos(x), atan(x),atan2(x)(inverse trig functions)sinh(x),cosh(x),tanh(x) etc. exponential functionsexp(x),log(x)(natural log),log10(x)(logbase ten),sqrt(x)square-root. conversion functionsthese functions will convert one data type to another, e.g.int(x)returns the integer value ofx,real(?)converts anything

Fortran21

to a real,dble(?)converts anything to a double (also operations for complex numbers) misc. functionssee table 6.1 Program flow controlAny program will just execute sequentially one line at a time unless you tell it to do something else. Usually there are only one of two things you want it to do, loop and branch. The control commands for these are doloopsDo loops simply loop over a piece of internal code for a fixed number of loops and increment a loop counter as they go. do loops come in the standard line number flavour do 10 i=1,n,2quotesdbs_dbs3.pdfusesText_6

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