2. Basic Elements of Fortran

Transcription

1 2. Basic Elements of Fortran

2 Structure of a Fortran Program 31 characters must be in the 1st line if present declaration section program my_first_program! Declare variables integer :: i, j, k! i, j, k are integer execution section! Interactive input write(*,*) 'Enter the numbers' read(*,*) i, j! Multiply the numbers k = i*j! Interactive output write(*,*) 'i*j = ', k comment after exclamation point termination section stop end program (optional) stop running program, present before end program tell compiler no more statement

3 Goto the folder \GNU_emacs_Fortran Click CMD_emacs_Fortran.bat to open command prompt console and Emacs editor >dir >mkdir examples >cd examples In Emacs editor, create a new file named 2-2.f90 in the folder \examples Copy the program from course webpage and paste on the Emacs editor, and save it. >gfortran 2-2.f90 >dir >a.exe Enter the numbers 2 4 i*j = 8 >a.exe Enter the numbers 2. At line 7 of file 2-2.f90 (unit = 5, file = 'stdin') Fortran runtime error: Bad integer for item 1 in list input

4 Fortran Character Set A ~ Z (uppercase) 26 a ~ z (lowercase) 26 0 ~ 9 (numbers) 10 _ (underscore) * / ** (arithmetic symbols) 5 ( ). =, ' $ :! " % & ; < >? blank (miscellaneous symbols) 17 Total 86 characters can be used in Fortran 95 Upper case and lowercase letters are equivalent in Fortran, i.e. Fortran is case insensitive. But C++ and Java are case sensitive, i.e., A and a are different characters.

5 Fortran Statement Executable statement Non-executable statement : provide information necessary for the proper operation of the program Free-source form statement (90/95/2003) Fixed-source form statement (77 and earlier version) But backward compatible, i.e., you can use fixed-source form in 90/95, but cannot use free-source form in 77.

6 Free-source form statement statement number which gives name of a statement to refer to (1~99999) 12 x = y+z! summation of x and y 12 x = y & +z 132 characters continuation indicator 12 x = y & & +z 40 lines 12 x = y & +z syntax error

7 Fixed-source form statement Do not use!! Just to show you in case your professor is still using Fortran 77. statement label column continuation indicator column instructions column card identification field x = y + z 1 2 x = y 1 + Z A Hollerith card that, when punched, will contain one Fortran statement.

8 Fortran coding sheet Punching machine Card reader What computer (IBM 704) looked like when Fortran was introduced.

9 Constants and Variables legal variables: Time z I_LOVE_you 31 characters; legal characters: a~z, A~Z, 0~9, _ illegal variables: This_is_a_too_long_variable_name (too many characters) 3days (cannot begin with a digit) money$ (cannot use $) Fortran has 5 intrinsic (built-in) types of constants and variables: integer real complex character logical

10 Integer Constant legal integer constants: illegal integer constants: ,357,076 Real Constant legal real constants: illegal real constants: E-3 (=1 x 10-3 ) 124.8E20 (=1.248 x ) 0.15E+1 1,000, E3 10.E1.5

11 A real or floating-point data : E-3 (=1.0 x 10-3 ) 124.8E20 (=1.248 x ) 0.15E+1 A real or floating-point data is stored in a type of scientific notation of base 2 system consisted of two parts, the mantissa and the exponent: mantissa : precision (number of significant digits) exponent : range ( 浮點數 尾數 ) value = mantissa 2 exponent The precision and range of a real value are determined by number of bits allocated. Single-precision real number occupies 32 bits (4 bytes) of memory: 24 bits mantissa: ±2 23 = 8,388,608, i.e., about 6 or 7 significant digits 8 bits exponent: ±2 7 = 128, , i.e., range about ~ Double-precision real number OCCUPIES 64 bits (8 bytes): 53 bits mantissa: ± , i.e., about 14 or 15 significant digits 11 bits exponent: ±2 10 = 1024, , i.e., range about ~

12 computer old Intel 32-bit PC (Pentium) current Intel 64-bit PC declared as double precision Cray (supercomputer) bits per default real number bits in mantissa precision in decimal digits bits in exponent range in exponent ~ ~ ~ ~

13 Character Constants legal character constants: 'This is a test' "This is a test" " " "Dog is man's best friend" '"Who cares? "' illegal constants: 'This is a test" 'This is a test'' Logical Constants (Logical constants are rarely used).true..false.

14 Default and Explicit Variables Typing Default (not specified): (i,j,k,l,m,n) + other characters are integer variables num i38 Others are real variables vel q2 The type of a variables can also be explicitly declared by type declaration statements. integer :: var1, var2, var3 real :: var4, var5 logical :: var6 complex :: var7, var8 optional, but is preferable Character variables must be declared by: optional character(len=10) :: var1, var2 character(15) :: var3

15 Assignment Statements and Arithmetic Calculations Assignment Statements variable = arithmetic expression store the value of expression into the variable location i = i+1 If original i is 3, after the assignment statement i is 4 Arithmetic Operators: + - * / ** wrong operation a*-b a**-2 a*(-b) a**(-2) 2**((8+2)/5) = 2**(10/5) = 2**2 = 4

16 Integer Arithmetic i = 3/4 0 is assigned to i 4/4 1 5/4 1 6/4 1 7/4 1 8/4 2 9/4 2 Real Arithmetic (or floating-point arithmetic) x = 3./ / (precision depends on computers) On some computer, 3.*(1./3.) 1 but 2.*(1./2.) = 1

17 Hierarchy of Operators (order of arithmetic operators) same as the rules of algebra distance = 0.5*accel*time**2 = 0.5*accel*(time**2) (0.5*accel*time)**2 a*b+c*d+e/f**g = (a*b)+(c*d)+(e/(f**g)) a*(b+c)*d+(e/f)**g = (a*(b+c)*d)+(e/f)**g a*(b+c)*(d+e)/f**g = ((a*(b+c))*(d+e))/(f**g) a**b**c = a**(b**c) (a**b)**c = 4 9 = = 4096

18 Mixed-mode arithmetic (Do NOT use whenever possible) The computer converts the integers into real numbers, and real arithmetic is used. 3/2 1 3./ / / integer (= 0) 1+1/ / / real (=0.25) Mixed-mode exponentiation var = y**n = y*y*y*...*y (This, however, is preferable!) But n times var = y**x = e**(x*log(y)) (-2.0)**2 4 (-2.0)**2.0 some compiler will result in runtime error

19 Intrinsic Functions (some) Function name & argument Argument type Result type sqrt(x) SQRT(X) (x 0) R R abs(x) R/I R/I sin(x) (x in radians) R R cos(x) (x in radians) R R tan(x) (x in radians) R R exp(x) R R alog(x) R R alog10(x) R R int(x) integer part of x R I nint(x) nearest integer to x R I real(i) I R fraction(x) = x-int(x) R R mod(a,b) = a-int(a/b)*b remainder R/I R/I max(a,b) R/I R/I min(a,b) R/I R/I asin(x) R R acos(x) R R atan(x) R R

20 PROGRAM my_first_program! Purpose:! To illustrate some of the basic features of a Fortran program.! Declare the variables used in this program. INTEGER :: i, j, k! All variables are integers! Get the variables to multiply together. WRITE(*,*) 'Enter the numbers to multiply: ' READ(*,*) i, j! Multiply the numbers together. k = i * j! Write out the result. WRITE(*,*) 'Result = ', k! Finish up. STOP END PROGRAM

21 List-directed Input and Output read (io_unit, statement_no_format) write(io_unit, statement_no_format) * means to read from standard input device * means free-format input (or list-directed input) The types of the variables in the input variable list determine the required format of the input data. read(*,*) i, j read(*,*) k, l begin to read another line of data write(*,*) " Output: ", i, j, k, l > 1, 2, 3, 4 > 5, 6, 7 > Output: input in console output in console

22 Initialization of Variables 3 ways to initialize a variable: integer :: i i = 1 integer :: i read(*,*) i integer :: i = 1 real :: pi pi = acos(-1.) real :: pi read(*,*) pi real :: pi = acos(-1.) To get the initialized variable as a constant throughout the program, use named constant as: real, parameter :: pi = acos(-1.) real, parameter :: pi = acos(-1.) pi = 3.14 real, parameter :: pi = 3.14 pi = acos(-1.) These are not allowed since pi has been assigned as a constant parameter.

23 IMPLICIT NONE Statement Disable the default typing provisions of Fortran. If you use implicit none statement, you must explicitly declare EVERY variable in the program otherwise it is considered as an error. Good reasons to use implicit none statement: catch typos at compilation time, and make the code more manageable program test_1 real :: time =10.0 write (*,*) 'Time = ', tmie end program test_1 Program test_1 will be compiled successfully, and produce output: Time = E+00. program test_2 implicit none real :: time =10.0 write (*,*) 'Time = ', tmie end program test_2 In contrast, program test_2 will be compiled with error!

24 Program Example: 5 T t Convert temperature T F in degrees of Fahrenheit ( F) to an absolute 9 temperature t k in Kelvin (K). T F = 5 9 t k program temp_conversion! Purpose: To convert an input temperature from degrees! Fahrenheit to an output temperature in kelvins. implicit none! Force explicit declaration of variables! Declare variables, and define each variables when it is declared real :: temp_f! Temperature in degrees Fahrenheit real :: temp_k! Temperature in kelvins! Prompt the user for the input temperature. write(*,*) 'Enter the temperature in degrees Fahrenheit: ' read (*,*) temp_f! Convert to kelvins. temp_k = (5./9.)*(temp_f-32.) ! Write out the result. write(*,*) temp_f, ' degrees Fahrenheit = ', temp_k, ' kelvins '! Finish up. end program

25 In Emacs editor, create a new file named 2-6.f90 in the folder \examples Copy the program from course webpage and paste on the Emacs editor, and save it. >gfortran 2-6.f90 >dir >a.exe Enter the temperature in degrees Fahrenheit: degrees Fahrenheit = kelvins > >gfortran 2-6.f90 -o 2-6.out >dir >2-6.out Enter the temperature in degrees Fahrenheit: degrees Fahrenheit = kelvins >

26 Exercise 1 Write a Fortran program which prompts to input your name (in English) and ID as in the example program, and output as follow: My name is your name. My student ID is your id. Exercise 2 Write a Fortran program to compute the area of a triangle determined by jointing the three points (x1,y1), (x2,y2) and (x3,y3) given by a user who executes the program. Use implicit none statement in your program. Exercise 3 Write a Fortran program to computer the logarithm of a number x to an arbitrary base b, i.e., log b x, using the intrinsic function ALOG10(x). Test the program by calculating the logarithm to the base e of number x using ALOG(x). Use implicit none statement in your program.

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