Introduction to FORTRAN 77/90 Brent Minchew November 30, 2011 FORTRAN Brief • Originally an acronym (FORmula TRANslation) that evolved into a name: Fortran • First written in 1957 at IBM for use with punch cards • Latest standard version is Fortran 2003 (though the latest version on many compilers is Fortran 90 or, possibly, Fortran 95) • In general, newer versions are backward compatible, at least to Fortran 77 11/30/2011 Intro to Fortran 2 Fortran Brief (continued) • Differences between versions – Fortran 77 is the foundation of most legacy code – Fortran 90 added array operations, structures, and dynamic memory allocation, among other features – Fortran 95 was a minor upgrade to Fortran 90 – Fortran 2003 adds other useful features such as dynamic character allocation and some intrinsic functions 11/30/2011 Intro to Fortran 3 Fortran Brief (continued) • Advantages: – Tried and true (more or less) – Low-level language…which means it’s fast – Large libraries (BLAS, LAPACK, HSL, etc.) – Lots of legacy code (particularly Fortran 77) • Disadvantages: – All variables must be initialized – Few intrinsic functions and libraries can be a headache to implement – Debugging can be relatively painful 11/30/2011 Intro to Fortran 4 Things to keep in mind about Fortran • Write the code in any text editor (e.g. vim, emacs, Notepad++, etc.) • Compile on your favorite compiler (gfortran, Intel, etc.) • Not case sensitive • Most compilers limit lines to 72 columns; expandable to 132 columns with the proper compiler options – This includes spaces but not comments – Lines can be appended using & to end one line and begin the next • Comments begin with ! (some compilers still support beginning comments with C as well) – There is no terminating command as in C/C++ – Everything that follows ! on a given line is commented • Array indices begin with 1 (as in Matlab) • By default, operations on arrays are element by element (unlike Matlab) • Spaces and tabs have no meaning in free form (recommended) 11/30/2011 Intro to Fortran 5 The best way to learn is to do… 11/30/2011 Intro to Fortran 6 Example Code Outline • Takes in a command file from a command line argument • Reads the command file which: – Specifies an array of complex data (name and dimensions) – Assigns an output • Parses the command file • Calculates the amplitude and phase of the complex data • Writes amplitude and phase to file • Calculates mean and standard deviation of phase prints to the screen 11/30/2011 Intro to Fortran 7 Getting started • Every program needs a name implicit none – requires you, the • programmer, to initialize all variables…it is best to start every program with implicit none • Everything else is initialization which must be done at the beginning of the program • Variable types are: – Character – Integer (4 or 8 byte) – Floating Point (4 or 8 byte) – Complex (8 or 16 byte) • Structures are also available with Fortran 90 and newer (note the use of % instead of . for structure variables) 11/30/2011 Intro to Fortran 8 Making the code user-friendly • Unless you plan to wrap your code in another code, it’s a good idea of give yourself reminders of how to run the code • This example code takes in only one command line input…anything else causes the code to bomb and to output simple … usage instructions 11/30/2011 Intro to Fortran 9 Parsing the command file Parsing code: Command file: • Code searches for a divider (in this case ::) • If divider is found, the line is split into an inquiry (left of the divider) and a user response (right of the divider) • The inquiry (label) is read into case select and variables are assigned accordingly •Read statement converts string input to requisite variable (number) format 11/30/2011 Intro to Fortran 10 Allocating memory and opening files • Allocation can be done anywhere in the code, so long as it’s done before the respective array is used • Allocation only needs to be done once • Deallocation is also an option…should be used sparingly unless memory is a limiting factor • Files only need to be opened once (I prefer to do this all together towards the beginning of the program) • By default, all files are closed when the program exits but you can use the close command (e.g. close(11)) • Open command always begins with unit number (any integer except 6), then file name; the rest is optional – Access = data input mode – Form = file format (‘unformatted’ is for binary files) – Status = file status when opened – Recl = record length (specified here as bytes*number_of_entries) • // tie up string fragments 11/30/2011 Intro to Fortran 11 Do loops and if statements • Do loops (same concept as for loops in Matlab) are initilized in order: – do loop_variable=begin,end,step – The default step = 1 (no need to specify) – Important note: when the do loop exits, loop_variable = end+step • Read and write statement can contain implicit do loops (in this example these are mm=1,cols) • If statements must have the form: – If (condition) command (or then for block statements) – When elseif or else statements are needed, if statements must be in block form (if then else) – Elseif must have the form elseif (condition) then • Note that end statements go with opening statements (enddo or end do for do loops, etc.) 11/30/2011 Intro to Fortran 12 Input/Output • Read command: – Read(unit,rec) for unformatted files (if … files are formatted, rec is replaced by a format command) • Unit = integer defined in open statement • Rec = record number; record length was specified in open statement – Read(unit,*,options) is a wildcard format not suitable for unformatted files, but is good for input commands (see parsing section) • Write command: – Same form as read for unformatted and formatted data – Write(*,format,options) = write(6,format,options) = write to screen in given format with options – ‘\b\b\b…’ is a crude carriage return command (\b = backspace one space) – Advance=‘no’ does not advance one line after writing 11/30/2011 Intro to Fortran 13 Functions • Functions must be … initialized at the beginning of the program and defined … after the end of the program (or in a separate … file that is compiled with the driver) • They can appear in an algebraic statement • Variables in parenthesis are inputs • Function name is output • Functions must have end statement … • Return statement tells the program to return to the calling program 11/30/2011 Intro to Fortran 14 Subroutines • Must be called … • Do not need to be initialized … within the program • Are defined after the end of the program (or in a separate file that is compiled with the driver) • Important! Outputs and inputs are passed in the same group…Fortran does not distinguish by default – Can use intent command in subroutine initialization to specify input and output variables – In some cases, it may be a good idea to pass copies of variables 11/30/2011 Intro to Fortran 15 … Format commands … • Format commands can appear anywhere (I like to put them at the end) • The line number must be unique (i.e. can’t have two 100 format calls) • Different formatting cells are separated by a comma • Cells are defined by a single letter denoting type (a for text, i for integer, f for fixed point, e for exponential form, etc.) and a number specifying the cell width (in spaces). Fixed point and exponential form require a decimal point followed by the number of decimal places • A more thorough description of format commands is here: http://www.cs.mtu.edu/~shene/COURSES/cs201/NOTES/chap05/format. html 11/30/2011 Intro to Fortran 16 17 Intro to to Fortran Intro Endings … … … … … … … In Fortran: if it begins,InFortran: it must end. … 11/30/2011 • Compilers • Once the code is written it needs to be compiled • Many compilers are available, each with various options • GNU is open source and is probably installed on all GPS computers (Getting started wiki: http://gcc.gnu.org/wiki/GFortranGettingStarted) • GNU compile command for the example program (in Linux) is: gfortran example.f –ffixed-line-length-0 –ffree-form –O3 –o fortex Where: –ffixed-line-length-0 specifies that the line length in unlimited (in practice this means that the max line length is 132) –ffree-form removes significance of character placement in the line –O3 is an optimization routine (optional) –o fortex is the executable 11/30/2011 Intro to Fortran 18 Intrinsic Functions • Generally easier to use than external functions because they don’t require building libraries, additional compiler options, etc. • Limited in number and applicability • Complete list: http://gcc.gnu.org/onlinedocs/gfortran/Intrins ic-Procedures.html 11/30/2011 Intro to Fortran 19 That’s it • Feel free to ask me any questions about Fortran • I would appreciate feedback on the class (organization, content, etc.) • Feel free to use/modify/distribute the example code 11/30/2011 Intro to Fortran 20 .