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Chapter 3

Chapter 3. IF & SELECT. Control Constructs: Branches. Definitions: Code: statements or expressions in a program Block: a group of codes Branching: selecting or skipping certain blocks in a program Branching can be done using: IF Statement SELECT CASE. IF.. statement. IF construct

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Chapter 3

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  1. Chapter 3 IF & SELECT

  2. Control Constructs: Branches • Definitions: • Code: statements or expressions in a program • Block: a group of codes • Branching: selecting or skipping certain blocks in a program • Branching can be done using: • IF Statement • SELECT CASE

  3. IF.. statement • IF construct • A block of code is executed if-and-only-if a certain logical expression is true. IF (logical_expr) THEN Statement 1 Statement 2 … END IF • If the logical_expression is true, the program executes the block of codes between IF & END IF

  4. .FALSE. .TRUE. IF.. statement • IF (logical_expr) THEN • Statement 1 • Statement 2 • … • END IF

  5. IF.. statement • Example: Solving quadratic equation • PROGRAM QUADRATIC • IMPLICIT NONE • …… • IF (b**2 – 4*a*c < 0) THEN • WRITE (*,*) ‘There are two complex roots to this equation.’ • END IF • …… • END PROGRAM

  6. .FALSE. .TRUE. IF.. statement • Example: Solving quadratic equation • PROGRAM QUADRATIC • IMPLICIT NONE • …… • IF (b**2 – 4*a*c < 0) THEN • WRITE (*,*) ‘There are two complex roots to this equation.’ • END IF • …… • END PROGRAM

  7. IF, ELSE IF and ELSE • IF: if true then execute, if false then skip • What if we had other situations? • Use ELSE IF & ELSE IF (logical_expr1) THEN Statement 1 Statement 2 … ELSE IF (logical_expr2) THEN Statement 1 Statement 2 … ELSE Statement 1 Statement 2 … END IF

  8. .FALSE. .FALSE. .TRUE. .TRUE. END IF IF, ELSE IF and ELSE • Example:

  9. IF, ELSE IF and ELSE • Example: IF (b**2-4*a*c < 0.0) THEN WRITE(*,*) ‘This equation has complex roots. ’ ELSE IF (b**2-4*a*c > 0.0) THEN WRITE(*,*) ‘This equation has two distinct real roots. ’ ELSE WRITE(*,*) ‘This equation has two identical real roots. ’ END IF

  10. Example 3-2 (The Quadratic Equation) • Problem Design and write a program to solve for the roots of a quadratic equation regardless of type. • Reminder: (Design Procedure) • Problem statement • Defining inputs and outputs • Algorithm design • Task  subtasks • pseudo code or/and flowchart • Turn algorithm into Fortran statements • Test

  11. Example 3-2 (The Quadratic Equation) • Problem Design and write a program to solve for the roots of a quadratic equation regardless of type. • Problem statement (make it more clear) • Design and write a program that calculates the two roots of the quadratic equation a.x2+b.x+c=0. Depending on the three coefficients entered by the user the roots might be real or complex. Furthermore, the two real roots might be distinct or identical. The program should solve for all of these conditions and display the two roots and clearly state the type of the roots.

  12. Example 3-2 (The Quadratic Equation) • Defining inputs and outputs • Inputs Equation coefficients (real) a, b, c • Outputs Roots (real) and statement that describe their type x1, x2 (complex, real distinct or identical)

  13. Example 3-2 (The Quadratic Equation) • Design Algorithm Main tasks Read the input data (a,b,c) Calculate the roots Write the roots (x1, x2)

  14. Example 3-2 (The Quadratic Equation) • Design Algorithm Main tasks Read the input data (a,b,c) Calculate the roots Write the roots (x1, x2) Ask user to enter coefficients Read coefficient a, b, c Calculate discriminant: disc ← b2 - 4 * a * c IF disc > 0 THEN Calculate two distinct real roots Write the distinct real roots ELSE IF disc < 0 THEN Calculate complex roots Write the two complex roots ELSE Calculate one real root Write the repeated root END IF

  15. Ask user to enter coefficients Read coefficient a, b, c Calculate discriminant: disc ← b2 - 4 * a * c IF disc > 0 THEN x1 ← (-b+sqrt(disc))/(2. * a) x2 ← (-b-sqrt(disc))/(2. * a) Write message that equation has two distinct real roots Write x1 and x2 ELSE IF disc < 0 THEN real_part ← -b/(2. * a) Imag_part ← sqrt(abs(disc))/(2. * a) Write message that equation has two complex roots Write the two complex roots ELSE x1 ← -b/(2. * a) Write message that equation has two identical real roots Write x1 END IF Ask user to enter coefficients Read coefficient a, b, c Calculate discriminant: disc ← b2 - 4 * a * c IF disc > 0 THEN Calculate two distinct real roots Write the distinct real roots ELSE IF disc < 0 THEN Calculate complex roots Write the two complex roots ELSE Calculate one real root Write the repeated root END IF

  16. Ask user to enter coefficients Read coefficient a, b, c Calculate discriminant: disc ← b2 - 4 * a * c IF disc > 0 THEN x1 ← (-b+sqrt(disc))/(2. * a) x2 ← (-b-sqrt(disc))/(2. * a) Write message that equation has two distinct real roots Write x1 and x2 ELSE IF disc < 0 THEN real_part ← -b/(2. * a) Imag_part ← sqrt(abs(disc))/(2. * a) Write message that equation has two complex roots Write the two complex roots ELSE x1 ← -b/(2. * a) Write message that equation has two identical real roots Write x1 END IF

  17. Turn it into Fortran statements Ask user to enter coefficients Read coefficient a, b, c Calculate discriminant: disc ← b2 - 4 * a * c IF disc > 0 THEN x1 ← (-b+sqrt(disc))/(2. * a) x2 ← (-b-sqrt(disc))/(2. * a) Write message that equation has two distinct real roots Write x1 and x2 ELSE IF disc < 0 THEN real_part ← -b/(2. * a) Imag_part ← sqrt(abs(disc))/(2. * a) Write message that equation has two complex roots Write the two complex roots ELSE x1 ← -b/(2. * a) Write message that equation has two identical real roots Write x1 END IF

  18. Turn it into Fortran statements PROGRAM roots IMPLICIT NONE REAL :: a, b, c, disc, imag_part, real_part, x1,x2 WRITE (*,*) 'Enter the three coefficints a,b, and c : ' READ (*,*) a, b, c disc = b**2 - 4. *a * c IF (disc > 0.) THEN x1 = (-b + SQRT(disc))/(2. * a) x2 = (-b - SQRT(disc))/(2. * a) WRITE (*,*) ' The equation has two distict real roots.' WRITE (*,*) ' X1= ', x1, ' X2= ', x2 ELSE IF (disc < 0.) THEN real_part = (-b)/(2. *a) imag_part = SQRT(ABS(disc))/(2. * a) WRITE (*,*) ' The equation has two complex roots.' WRITE (*,*) ' X1= ', real_part, '+ i', imag_part, ' X2= ', real_part, '- i', imag_part ELSE x1 = (-b)/(2. * a) WRITE (*,*) ' The equation has two identical real roots.' WRITE (*,*) ' X1= X2 = ', x1 END IF END PROGRAM

  19. GRADES EXAMPLE PROGRAM GRADES IMPLICIT NONE REAL :: GRADE WRITE (*,*) "ENTER YOUR GRADE (out of 100)" WRITE (*,*) "" READ (*,*) GRADE IF (GRADE > 95.0) THEN WRITE (*,*) 'THE GRADE IS A.' ELSE IF (GRADE > 86.0) THEN WRITE (*,*) 'THE GRADE IS B.' ELSE IF (GRADE > 76.0) THEN WRITE (*,*) 'THE GRADE IS C.' ELSE IF (GRADE > 66.0) THEN WRITE (*,*) 'THE GRADE IS D.' ELSE WRITE (*,*) 'THE GRADE IS F.' END IF END PROGRAM

  20. IF, ELSE IF and ELSE • READING ASSIGNMENT: • EXAMPLE 3-3 (PAGE 97) • You should read the above example which illustrates the steps of designing a program using IF statements • Problem statement • Defining inputs and outputs • Algorithm design • Task  subtasks • pseudocode and flowchart • Turn algorithm into Fortran statements • Test

  21. Naming & Nested IFs PROGRAM mixup … IF (test1) THEN … … END IF IF (test2) THEN … … END IF IF (test3) THEN … … END IF END PROGRAM mixup

  22. Naming & Nested IFs PROGRAM mixup … outer: IF (test1) THEN … … middle: IF (test2) THEN … … inner: IF (test3) THEN … … END IF inner … END IF middle … END IF outer … END PROGRAM mixup Nested IF ( one or more IF block inside another one)

  23. Naming & Nested IFs PROGRAM mixup … outer: IF (test1) THEN … … middle: IF (test2) THEN … … inner: IF (test3) THEN … … END IF inner … END IF middle … END IF outer … END PROGRAM mixup Naming IF ( up to 31 alphanumeric)

  24. Naming & Nested IFs What is the advantage of naming IF blocks? outer: IF (test1) THEN … … middle: IF (test2) THEN … … inner: IF (test3) THEN … … END IF inner … END IF middle … END IF outer outer: IF (test1) THEN … … middle: IF (test2) THEN … … inner: IF (test3) THEN … … END IF inner … END IF middle … END IF outer

  25. END IF missing Naming & Nested IFs What is the advantage of naming IF blocks? outer: IF (test1) THEN … … middle: IF (test2) THEN … … inner: IF (test3) THEN … … END IF inner … END IF middle … END IF outer outer: IF (test1) THEN … … middle: IF (test2) THEN … … inner: IF (test3) THEN … … END IF inner … END IF middle … END IF outer END IF missing Compiler

  26. Naming & Nested IFs PROGRAM GRADES IMPLICIT NONE REAL :: GRADE WRITE (*,*) "ENTER YOUR GRADE (out of 100)" WRITE (*,*) "" READ (*,*) GRADE IF (GRADE > 95.0) THEN WRITE (*,*) 'THE GRADE IS A.’ ELSE IF (GRADE > 86.0) THEN WRITE (*,*) 'THE GRADE IS B.’ ELSE IF (GRADE > 76.0) THEN WRITE (*,*) 'THE GRADE IS C.’ ELSE IF (GRADE > 66.0) THEN WRITE (*,*) 'THE GRADE IS D.’ ELSE WRITE (*,*) 'THE GRADE IS F.’ END IF END IF END IF END IF END PROGRAM PROGRAM GRADES IMPLICIT NONE REAL :: GRADE WRITE (*,*) "ENTER YOUR GRADE (out of 100)" WRITE (*,*) "" READ (*,*) GRADE IF (GRADE > 95.0) THEN WRITE (*,*) 'THE GRADE IS A.' ELSE IF (GRADE > 86.0) THEN WRITE (*,*) 'THE GRADE IS B.' ELSE IF (GRADE > 76.0) THEN WRITE (*,*) 'THE GRADE IS C.' ELSE IF (GRADE > 66.0) THEN WRITE (*,*) 'THE GRADE IS D.' ELSE WRITE (*,*) 'THE GRADE IS F.' END IF END PROGRAM

  27. Special IF construct • One line statement that is equivalent to if block with one statement • IF (logical _expr) Statement • IF (mark > 95) grade= ‘A’

  28. SELECT CASE • Another branching method • Used to select a block of code to execute based on the value of a single integer, character, or logical expression • General Form: SELECT CASE (Case_expr) CASE (selector_1) Statement 1 Statement 2 … CASE (selector_2) Statement 1 Statement 2 … CASE DEFAULT Statement 1 Statement 2 … END SELECT

  29. GRADES EXAMPLE PROGRAM GRADES IMPLICIT NONE INTEGER :: GRADE = 98 WRITE (*,*) "ENTER YOUR GRADE" WRITE (*,*) "" READ (*,*) GRADE SELECT CASE (NINT(GRADE)) CASE (101:) WRITE (*,*) 'GRADE CANNOT EXCEED 100%' CASE (95:100) WRITE (*,*) 'THE GRADE IS A.' CASE (86:94) WRITE (*,*) 'THE GRADE IS B.' CASE (76:85) WRITE (*,*) 'THE GRADE IS C.' CASE (66:75) WRITE (*,*) 'THE GRADE IS D.' CASE DEFAULT WRITE (*,*) 'THE GRADE IS F.' END SELECT END PROGRAM

  30. SELECT CASE • Forms of case selectors: (RANGES) • CASE ( value1 : value2 ) value1 to value2 • CASE ( value1 : ) value1 and above • CASE ( : value2 ) value2 and below • CASE ( value ) a single value • CASE (value1, value2, value3, value4) a list of values

  31. SELECT CASE • Forms of case selectors: (RANGES) • CASE ( 1 : 10 ) • CASE ( 10 : ) • CASE ( : 10 ) • CASE ( 7 ) • CASE ( 3, 4, 7) • Reading assignment: • Example 3-5 (Page 107): Selecting day of the week

  32. Example: Month’s Name

  33. Example: Month’s Name program month implicit none CHARACTER(LEN=9) :: month_name INTEGER :: month_number WRITE (*,*) "Enter the month of the year (1-12)" WRITE (*,*) "" READ (*,*) month_number SELECT CASE (month_number) CASE (1) WRITE (*,*) 'JANUARY' CASE (2) WRITE (*,*) 'FEBRAURY' CASE (3) WRITE (*,*) 'MARCH' CASE (4) WRITE (*,*) 'APRIL'

  34. Example: Month’s Name CASE (5) WRITE (*,*) 'MAY' CASE (6) WRITE (*,*) 'JUNE' CASE (7) WRITE (*,*) 'JULY' CASE (8) WRITE (*,*) 'AUGUST' CASE (9) WRITE (*,*) 'SEPTEMBER' CASE (10) WRITE (*,*) 'OCTOBER' CASE (11) WRITE (*,*) 'NOVEMBER‘ CASE (12) WRITE (*,*) 'DECEMBER' CASE DEFAULT WRITE (*,*) ‘Error: out of range' END SELECT end program

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