Fortran is a programming language that was invented back in 1954 by IBM. Fortran is faster and more efficient than any other programming language, making it well-suited for tasks such as scientific simulations and large-scale numerical modeling.

These notes are based on the book "FORTRAN 77 for Humans" written by Rex L. Page and Richard L. Didday and published in 1980.

• g95 -o NAME SOURCE.f compile source files into executable named NAME
• program code must start on 7th column (for historical reasons)
• comments start with C

• INTEGER standard signed integer, can have +/- in front
• REAL signed floating point numbers, can use scientific notation eg. 6.0123E+23
• CHARACTER*n string of character enclosed by single quotes must specify n, which is length of string when declaring

• PRINT *, list prints each of the values in the list of a line

• READ *, list places data into list from the data line (user input)

• END tells the compiler to stop doing statements in program

• Operations are +, -, *, /, ** (last is exponentation)

• If your line is too long, then put & in 6th column of next line:

  PRINT *, 'THIS LONG LINE IS',
  &'CONTINUED HERE'
  

• IF (e1 rel e2) GOTO s e1 and e2 are expressions, both of the same type rel is relational operator s is statement label (line number) decide if relationship is true, if yes proceed to line s

• Relational operators: .LT. less than .LE. less than or equal to .EQ. equal to .NE. not equal to .GE. greater than or equal to .GT. greater than

• indent if loop blocks, so starting after IF and going until last GOTO:

  100  IF (test for exit) GOTO 200
           fortran operation
           .
           .
           GOTO 100
  200 continue program  
  

• can do while loops (if statement at beginning) or do-while (if statement end)

• pre-test loops are safer and clearer

• Do loops are like for loops:

        INTEGER TOP = 5
        DO 10 N=1, TOP,1
  10    PRINT *, N
  

  Code above loops ten times. After DO the first value is line number after which the loop goes to the next iteration. N is the iterator that increases or deacreases, top is ending value and last integer is increment.

• CONTINUE statement that does nothing, used as terminal statement in the range of DO-loops

• Nested loops are possible. Note that both line statements point to line 10:

      DO 10 I=1, 10, 1
          DO 10 J=1, 10, 1
  10          PRINT *, I*J
  

• If-else structure in fortran:

  IF (test condition) THEN
      fortran operation 1
  ELSE
      fortran operation 2
  ENDIF
  

• Can also use multiple else-if with ELSE IF structure. Add THEN after each ELIF

• .EQV. equivalence operator, used to compare boolean values

• .NEQV. nonequivalence operator

• May omit ELSE in IF-THEN statement.

• LOGICAL data type to hold booleans

• Use INT(x) to truncate real number x or NINT(x) to round

• MOD(x, n) is modulus operator

• INTEGER ARR(n) declare an array of length n

• ARR(1) access first element (indexing starts from 1 not 0)

• MUL(3,3) to create 2-dimensional array. first row, then column

• To loop arrays use the following structure (reads 20 ints from user):

      DO 100 I=1,20
          READ *, ARR(I)
  100     CONTINUE            
  

• Substrings can be accessed with STR(j:k). STR is string, j and k are indices

• multiline code can be written with + sign in between:

  IF (PHRASE .EQ. 'HECK'
  + .OR. PHRASE .EQ. 'DARN')
  

• when assigning substrings, can't reference same portions eg. N(1:9) = N(5:15)

• INDEX('ABCDEF', 'C') operator returns 3 as a result. Returns 0 if not found. - A = B // C concatenates B with C and stores it in A

• Fortran uses subroutines and functions
• CALL AVG(a1, a2) calls subroutine AVG with arguments a1 and a2
• SUBROUTINE AVG(a1) declaration of subroutine
• Subroutines must end with RETURN and `END statements
• Function returns one value, subroutines can return any number of values through it's arguments. Choosing which to use is matter of taste.
• type FUNCTION f(p1) declares function f that takes in one parameter returns variable with name f at RETURN statement
• first write main program, then function/subroutine definitions

• Format PRINT statement by replacing * by line where FORMAT is stated:

      PRINT 10, SNGLSP, 'STRING'            
  10  FORMAT(A,T25, A)        
  

  This will print the text STRING in 25th column.

• SNGLSP is carriage control character that can be the following:

  • move down the page one line before printing
  • 0 move down two lines
  • 1 move to the top of next page
  • + overprint previous line

• Looks like SNGLSP is not needed when printing to terminal, so this is enough:

      PRINT 10, 'STRING'            
  10  FORMAT(T25, A)        
  

• Different data descriptors: A for chars, I for ints, F for reals (I4 or F6.2)

• OPEN(speclist) open files with this command
• CLOSE() disconnect file
• READ/WRITE() read or write files