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MET 50

MET 50. Fortran programming basics (2). Programming Basics. A few extra things from last week… For a program written in Fortran 90, use the extension “.f90” This tells the compiler: “ Hey – this is a Fortran 90 code”. *.f might be interpreted at Fortran 77. Programming Basics.

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MET 50

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  1. MET 50 Fortran programming basics (2)

  2. Programming Basics A few extra things from last week… • For a program written in Fortran 90, use the extension “.f90” • This tells the compiler: “Hey – this is a Fortran 90 code”. • *.f might be interpreted at Fortran 77. MET 50, FALL 2011, CHAPTER 2 PART 2

  3. Programming Basics • When writing a number like: 6.7371E6, always write this as: • (real number) E (integer) • So 6.371E6 is OK • But 6.371E1.5 is not OK. MET 50, FALL 2011, CHAPTER 2 PART 2

  4. Programming Basics Real versus Integer: it is dangerous to mix real and integer variables in Fortran Example: REAL :: A=10.0, B=4.0, C INTEGER :: K=4, L C = A/B ! Produces C = 10./4. = 2.5 MET 50, FALL 2011, CHAPTER 2 PART 2

  5. Programming Basics REAL :: A=10.0, B=4.0, C, D INTEGER :: K=10, L=4, KL C = A/B ! Produces C = 10./4. = 2.5 KL = K/L ! Produces KL = 10/4 = 2 ! Rounded down to nearest integer D = A/L ! A/L = 10.0/4 = 2.5 ! A/L is treated as REAL ! D is REAL and has value 2.5 MET 50, FALL 2011, CHAPTER 2 PART 2

  6. Programming Basics Example: REAL :: A=2.0, B=4.0, C INTEGER :: K=4, L C = A**(3/2) ! 3/2 = 1 (INTEGER!) ! C = A**1 = A = 2.0 ! but (2)**(3/2) = SQRT(2**3) = 8 So…you get an error… Should be: C=A**(3./2.) MET 50, FALL 2011, CHAPTER 2 PART 2

  7. Programming Basics • Order of operations in Fortran: • Stuff inside parentheses is done first. • Inside parentheses, the order is: • Exponentiation (A**2) – right  left • A**2**3 is computed as • A**(2**3)=A**8 • Multiplication & division: right  left • Addition & subtraction: right  left MET 50, FALL 2011, CHAPTER 2 PART 2

  8. Programming Basics Example: X = SQRT(B**2 – 4.0*A*C) X = SQRT(B**2 – 4.0*A*C) X = SQRT(B**2 – 4.0*A*C) X = SQRT(B**2 – 4.0*A*C) X = SQRT(B**2 – 4.0*A*C) X = SQRT(B**2 – 4.0*A*C) MET 50, FALL 2011, CHAPTER 2 PART 2

  9. Programming Basics Page 25, Q 10. ((2 + 3)**2) / (8 - (2 + 1)) =((5)**2) / (8 - (3)) = (5**2) / (5) = (25) / (5) = 5 MET 50, FALL 2011, CHAPTER 2 PART 2

  10. Programming Basics But…caution about parentheses… ((2 + 3)**2) / (8 – (2 + 1)) = (5**2) / (7) = (25) / (7) • 5 Parentheses matter!!! MET 50, FALL 2011, CHAPTER 2 PART 2

  11. Programming Basics And… ((2 + 3)**2) / (8 – (2 + 1) Will not run! Why?? Parentheses really matter!!! MET 50, FALL 2011, CHAPTER 2 PART 2

  12. Programming Basics PRINT*, READ* statements: We have met the PRINT* statement. PRINT*, VAR prints the value of “VAR” PRINT*, VAR, TAR prints the values of “VAR” and “TAR” Quantities are printed to the screen (only)…”hardcopy”? MET 50, FALL 2011, CHAPTER 2 PART 2

  13. Programming Basics Results may be printed “ugly”, such as: 4.500000 7.800000 We can make things a bit nicer, as in: PRINT*, ‘value of VAR is’, VAR, ‘value of TAR is’, TAR Would  value of VAR is 4.500000 value of TAR is 7.800000 MET 50, FALL 2011, CHAPTER 2 PART 2

  14. Programming Basics We can make things better still using the WRITE command (Chapter 5)(or sooner!) READ* statement is the simplest way to input data to a program. In lab-02, you ran “add2.f” to add 2 numbers. The code prompted you for two numbers. How? Using the READ* statement. MET 50, FALL 2011, CHAPTER 2 PART 2

  15. Programming Basics Example: REAL :: A, B, C READ*, A, B C=A*B PRINT*, A, B, C As this code runs, it will stop – waiting for YOU to enter values of A and B at the “READ” command. Codes in this class will run FAST (since they are very small), so if the code stops, it is either expecting input – or it’s broken  MET 50, FALL 2011, CHAPTER 2 PART 2

  16. Programming Basics To help you see what is going on in your code, it is good practice to add some PRINT statements: PRINT*, ‘enter values for A and B’ READ*, A, B Or: PRINT*, ‘enter first number’ READ*, A PRINT*, ‘enter second number’ READ*, A MET 50, FALL 2011, CHAPTER 2 PART 2

  17. Programming Basics This is a style thing! There is a more powerful READ statement – Cht. 5 MET 50, FALL 2011, CHAPTER 2 PART 2

  18. Programming Basics Comment statements: Comment statements are vital! Use to explain what this section of code does. ! State @ top of code what the program does! ! Read in parameter values ! Main computation ! Write results MET 50, FALL 2011, CHAPTER 2 PART 2

  19. Programming Basics Next lecture? “selective execution” Fortran equivalent to: “IF it’s Sunday, sleep in. ELSE, set alarm.” MET 50, FALL 2011, CHAPTER 2 PART 2

  20. Programming Basics Next lab? Practice finding problems with REAL and INTEGER numbers mixed. READ* and PRINT* statements MET 50, FALL 2011, CHAPTER 2 PART 2

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