Chapter 3Program Design

And

Branching Structures

Design approaches On-the-fly OK with simple programs Simple programs can be debugged easily Top-down design Necessary for large/complex programs Task subtasks Test each subtask individually, then combine all Debugging is easier

Top-down-design steps

1. Clearly state the problem

2. Define required inputs and produced outputs

3. Design algorithm to be used

4. Turn algorithm into Fortran statements

5. Test program

1. Clearly state the problem Write a program which calculates the total

energy of a falling object. Not clear enough Write a program which prompts the user to enter

the parameters of a falling object (height, mass, and velocity), calculates the total energy of the object (potential + kinetic) and prints on the screen for the user the total energy of the object.

Clearer

2. Inputs & Outputs In the previous example:

Inputs: mass height velocity Outputs: Total energy of the falling object

3. Algorithm

Prompt (ask) user for inputs Compute the total energy using the equations: Gravity = 9.8 Potential energy = mass * gravity * height Kinetic energy = 0.5 * mass * velocity2

Total energy = Potential energy + Kinetic energy

4. Algorithm Fortran statementsPROGRAM Energyimplicit none

REAL,parameter :: gravity=9.8REAL :: height, mass, velocityREAL :: Potential_Energy, Kinetic_Energy, Total_Energy

write (*,*) "Please enter the height, mass, and velocity of the object"read (*,*) height, mass, velocity

Potential_Energy = mass * gravity * heightKinetic_Energy = 0.5 * mass * velocity**2Total_Energy = Potential_Energy + Kinetic_Energy

write (*,*) "The toal energy of the object = ", Total_Energy, "Joul."END PROGRAM

5. Testing Test in the LAB For big programs: ALPHA release First complete version Tested by the programmer and close friends All possible ways of using the program are tested BETA release Serious bugs in ALFA release is removed Tested by users who need the program Program is put under many different conditions Released for general use Released for everyone to use

Standard Forms of Algorithms:Pseudocode and Flowcharts

An algorithm is composed of constructs. Constructs can be described using:

Pseudocode Flowcharts

Advantages: Standard form: Easy to understand by others Making changes in the program is easier Debugging is easier

Pseudocode: Describe algorithm using a mix of Fortran

language and English language Example:

Prompt user to enter height, mass, and velocityRead height, mass, and velocityGravity 9.8 Potential energy mass * gravity * heightKinetic energy 0.5 * mass * velocity2

Total energy Potential energy + Kinetic energyWrite Total energy on the screen

Standard Forms of Algorithms:Pseudocode and Flowcharts

Flowcharts: Describe algorithm graphically Standard shapes are used for each type of construct

Standard Forms of Algorithms:Pseudocode and Flowcharts

LOGICAL constants take one of 2 values: true / false

Example: Logical, parameter:: correct = .TRUE. Logical, parameter:: wrong = .FALSE.

LOGICAL constants are rarely used

Logical Variables and Constants

LOGICAL variables are declared like other variables Example: LOGICAL :: var1, var2, var3 LOGICAL :: var4

LOGICAL variables are more used than LOGICAL constants

Logical Variables and Constants

Invalid syntax correct = .TRUE incorrect = FALSE.

ERRORS …

Logical statement form: Logical_variable_name = logical experession

Example: PROGRAM PASS IMPLICIT NONE CHARACTER (len=4) :: PASSWORD LOGICAL :: CHECK WRITE (*,*) “ What is the password? “ READ (*,*) PASSWORD

CHECK = ( PASSWORD == ‘EASY’ ) WRITE (*,*) CHECK END PROGRAM

Logical Statements

Relational operators: compare two operands and produce logical results (T/F) A1 op A2 A1 & A2: can be either numerical or character op: == /= > < >= <= Examples: 3 < 4 .TRUE. 3 <= 4 .TRUE. 4 <= 3 .FALSE. ‘A’ < ‘B’ .TRUE. 4 < ‘A’ ????? ILLEGAL (ERROR)

Logical Statements.. Relational Operators

Example: PROGRAM PASS IMPLICIT NONE INTEGER :: x, y LOGICAL :: compare

WRITE (*,*) “Enter numbers (x, y) to check if “ WRITE (*,*) “x > y “ WRITE (*,*) “ “ READ (*,*) x, y

CHECK = (x > y) WRITE (*,*) “ The statement x > y is “, CHECK END PROGRAM

Logical Statements.. Relational Operators

Combinational operators: compare two operands and produce logical results (T/F) A1 op A2 A1 & A2: logical operands (.TRUE. / .FALSE.) op: .AND. .OR. .EQV. .NEQV. .NOT Truth table for binary combinational logic operators: L1 .FALSE. .FALSE. .TRUE. .TRUE. L2 .FALSE. .TRUE. .FALSE. .TRUE. L1 .AND. L2 .FALSE. .FALSE. .FALSE. .TRUE. L1 .OR. L2 .FALSE. .TRUE. .TRUE. .TRUE. L1 .EQV. L2 .TRUE. .FALSE. .FALSE. .TRUE. L1 .NEQV. L2 .FALSE. .TRUE. .TRUE. .FALSE.

L1 .TRUE. .FALSE. .NOT. L1 .FALSE. .TRUE.

Logical Statements.. Combinational Operators

Exercise:

L1 = .TRUE.

L2 = .TRUE.

L3 = .FALSE. Logical expression .NOT. L1 .FALSE. L1 .OR. L3 .TRUE. L2 .NEQV. L3 .TRUE.

Logical Statements.. Combinational Operators

When evaluating an expression, follow these rules: 1. Arithmetic operations (e.g. (3 + 4 * ( 2 / 5)) 2. Relational logic operations (e.g. ( 3 > 4 ) ) 3. Combinational logic operations, evaluate in this order:

.NOT. (left to right) .AND. (left to right) .OR. (left to right) .EQV. and .NEQV. (left to right)

Parenthesis can change order of evaluation

Logical Statements.. Evaluation order

Exercise:

L1 = .TRUE.

L2 = .TRUE.

L3 = .FALSE. Logical expression .NOT. ( 3 > 4 ) .TRUE. L3 .OR. ( (2 * 5) < 12 ) .TRUE. L2 .NEQV. ( L3 .AND. ( 3 /= 4)) .TRUE.

Logical Statements.. Evaluation order