Decision Making

Decision Making

In this chapter you will look at how computer programs make decisions based on numbers and variables. This is done with a statement called the IF-THEN-ELSE statement. This is a very useful statement. It will be an important building block in building up large programs.

Chapter Topics:

• Two-way Decisions.
• QBasic IF-THEN-ELSE statement.
• Outline of a two-way decision.
• Relational Expressions.
• Relational Symbols.
• Example Programs.

QUESTION:

You are driving in your car and it starts to rain. The rain falls on your windshield and makes it hard to see. What should you do with the windshield wipers?

The windshield wipers are controlled with an ON-OFF switch. The decision to turn the switch on looks like this:
In this picture of a decision, you are supposed to start at the top, then follow the line to the question:

is it raining?

The answer to the question is either TRUE or FALSE.

If the answer is TRUE, 
     follow the line labeled TRUE,
     do the directions in the box "turn wipers on",
     follow the line to "continue"

If the answer is FALSE
     follow the line labeled FALSE,
     do the directions in the box "turn wipers off",
     follow the line to "continue"

QUESTION:

How many ways can you go from "start" to "continue"?

The "windshield wiper" decision is a two-way decision (sometimes called a "true-false" decision.) It seems small, but in programming very complicated decisions can be made up of many small decisions.
Here is a QBasic program that includes a two-way decision:

PRINT "Enter a Number"
INPUT NUMBER
'
IF NUMBER >= 0 THEN
  PRINT "The number is zero or positive"    ' true branch
ELSE
  PRINT "The number is negative"    ' false branch
END IF
'
PRINT "Bye"     ' this statement is always done
END

The words IF, THEN, ELSE, and END IF are brackets that divide the part of the program into two branches. The ELSE is like a dividing line between the "true branch" and the "false branch".

• The IF statement always asks a question (usually about the number in a variable).
  • The answer will be TRUE or FALSE.
• If the answer is TRUE only the true-branch is executed.
• If the answer is FALSE only the false-branch is executed.
• No matter which branch was chosen, execution continues with the statement after the END IF.

A two-way decision is like picking which of two roads to take to the same destination. The fork in the road is the IF statement, and the two roads come together just after the END IF statement.

QUESTION:

The user runs the program and enters "12". What will the program print?

Here is the program again, done as a chart. Because the answer to the question was "true", the path on the left was done.
The "two-way split" of the program is easy to see in a two dimensional chart. It is harder to see this in a program.

QUESTION:

The user runs the program and enters "-5". What will the program print?

Here is the program again with some added statements:

PRINT "Enter a Number"
INPUT NUMBER
'
IF NUMBER >= 0 THEN
  PRINT "The number is zero or positive"    ' true branch
  PRINT "Positive numbers are > 0"          ' true branch
ELSE
  PRINT "The number is negative"    ' false branch
  PRINT "Negative numbers are < 0"  ' false branch
END IF
'
PRINT "Bye"     ' this statement is always done
END

The statements in the true branch are executed when the question in the IF statement is TRUE. There can be as many statements as you want in the true branch. The true branch consists of the statements between the IF statement and the ELSE statement.
Of course, the statements in the false branch are executed when the question in the IF statement is FALSE. There can be as many statements as you want in the false branch. The false branch consists of the statements between the ELSE statement and the END IF statement.

QUESTION:

In answer to the question, the user enters a 17. What will the new program print?

Here is how an outline of how to make a two-way decision:

... statements done before the decision
'
IF condition THEN
    ....  ' true branch
    ....  
    ....  ' true branch
ELSE
    ....  ' false branch
    ....
    ....  ' false branch
END IF
'
... statements done after the branch comes back together

Here are some details:

• The IF condition THEN must be on one line.
• The condition compares variables and values.
  • The statements in the true branch must be one per line.
  • There can be as many statements in the true branch as you need.
• The ELSE statement must be by itself on a line.
  • The statements in the false branch must be one per line.
  • There can be as many statements in the false branch as you need.
• The END IF statement must be by itself on a line.

The condition looks like the part of a DO WHILE condition statement that compares what is held in a variable with other values. You can use the same comparisons: <, <=, =, and so on.

QUESTION:

Is the following program correct?

PRINT "Enter a Number"
INPUT NUMBER
'
IF NUMBER >= 0 THEN
  PRINT "The square root is:", SQR( NUMBER )
ELSE  PRINT "There is no square root"
  PRINT "Run the program again."
END IF
'
PRINT "Bye" 
END

The IF, THEN, ELSE, and END IF are like brackets that emphasize the parts of the two-way branch. The QBasic system requires that you put the ELSE and END IF on their own lines to keep the true and false branches clear.
PRINT "Enter a Number" INPUT NUMBER ' IF NUMBER >= 0 THEN PRINT "The square root is:", SQR( NUMBER ) ELSE PRINT "There is no square root"       <---- Wrong!!! PRINT "Run the program again." END IF ' PRINT "Bye" END So the above program is incorrect because the ELSE was not alone on its line.

QUESTION:

How would you fix the above program?