Computers and FORTRAN Language Fortran 95/2003. Dr. Isaac Gang Tuesday March 1, 2011 Lecture 3 notes. Topics:

Transcription

1 Computers and FORTRAN Language Fortran 95/2003 Dr. Isaac Gang Tuesday March 1, 2011 Lecture 3 notes Topics: - Program Design - Logical Operators - Logical Variables - Control Statements Any FORTRAN program can be made up of only three control statements: These are: - Sequential where statements executed one after another - Selection were statements are executed based on some condition - Repetition where statements are executed repeatedly until some condition become false/true - When given a program never try to tackle the whole thing. Top-down design break it into the smallest parts possible, solve the small parts and then put it all together. By breaking the program into smaller pieces you can test each small piece. When each small piece works, put them all together and the whole thing will work. Steps to make writing a program simple State the problem that you want to solve Describe what inputs and outputs are needed Design the algorithm o Algorithm - step by step method of how to find the solution. o Pseudo-code write out in simple English statements and/or a flow chart. Turn the algorithm into Fortran statements Test the resulting Fortran program (Below, you design 3 programs using flowchart).

2

3 Logical expressions: Logical expressions allow the computer to know how to compare things To perform a task a specific number of times, the computer has to know: when to start and when to stop Requires knowing the start time and when this start time is equal to the stop time Moreover, logical expressions are required to use loops and branches Programs become user friendly and extensible with the use of: Branches user friendly Consider all possible inputs (upper or lower case, real or integer, etc.) Certain segments of code can be executed and others omitted User cannot break the program Loops - extensible

4 Segments of code can be repeated Can ask the user for the number of items to read Branches and loops require knowledge of Logical constants and variables Logical Operators Relational and Combinational Order of logical expressions Logical Constants and variables: Logical Value.TRUE..FALSE. Constant Variables that can contain one non-changing logical data value Variable A variable that stores a logical data value Declaring Same as any other data type Constant LOGICAL, PARAMETER :: PASSED_CON =.TRUE. Variable Initialize at declaration LOGICAL :: passed =.TRUE. Initialize in the executable part of the program LOGICAL passed passed =.TRUE. Initialize with a read LOGICAL passed WRITE(*,*) Do you think you passed the course? READ(*,*) passed Two types of logical operators: Relational operators Work on two numerical or character variables and yields a logical result Mostly used for direct comparisons Example: does x = y Combinational Works on two or more logical objects and produces a logical result Operators that work to combine or manipulate relational operators Example: does x=y and y=z The below tables shows relational and combinational operators:

5 Example: Expressions are evaluated in the order: arithmetic operators relational operators.not. Operators.AND. operators working left to right.or. operators working left to right.eqv. And.NEQV. operators working left to right Parentheses can be used to change the order of operations If present they are evaluated first Evaluate the following expressions < > 6 * >= 6 * (3 2) grade1 = 80 grade2 = 75 grade 3 = 99 (grade3 > grade1).and. (grade2 > grade3) (NOT. (grade3 < grade1.

6 (grade1 > 90).OR. (grade2 > 90).OR. (grade3 > 90) A programming structure that allows certain pieces of code to be executed based on a Condition. Four Types: If Statement If this expression is true, do this If Else Statements If this expression is true, do this, otherwise, do that If Else If Statements If this expression is true, do this, else if this is true to that Case Statement If this expression fits into this category to do this, else, do this An if statement is only executed when the logical expression is true IF (logical_exp) THEN Do some things END IF If the logical_exp is FALSE then the block of code is skipped and the program continues past the END IF segment Write a program that allows a user to enter two numbers and see if their sum is equal to 100. If their sum is 100, then we want to output yes, the sum of x and y is 100. What are the inputs? What are the outputs? What do we need to consider in each case? In what order does all of this need to be done? Sometimes if something in one expression is not true (IF) we want to do something else (ELSE) or test another condition (ELSE IF). Exampl If the user is prompted to enter their favorite element from the periodical table and you are going to print the name and information for that element. name IF (logical_expression) THEN ELSE IF (logical_expression_2) THEN ELSE END IF name

7 Note: name is optional but each if statement must have a unique name. From the previous example, change it so that it displays the sum is not 100 if the sum is not 100. What are the inputs? What are the outputs? What do we need to consider in each case? In what order does all of this need to be done? Then write a program that determines what the letter grade of a numeric value would be. What are the inputs? What are the outputs? What do we need to consider in each case? In what order does all of this need to be done? There is also an IF operator that will perform only one task IF (logical_expression) Statement Although the IF statement and IF block is useful there could be problems when having several layers of them Nested loops one loop inside of another Better to a SELECT CASE if there are multiple IF/ELSE/ELSEIF conditions The SELECT CASE can be used to match integer, character, or logical expressions name SELECT CASE (case_expr) CASE (value_1) CASE (value_2) CASE DEFAULT END SELECT name The values can either be a list or a range (:-1) (1:) (1, 2, 3, 4) Monday Case expression can be a variable or expression Rewrite the determine grade program above using the select case statement: Still have the same inputs. Have it produce the same outputs. What is the approach to solving this?