If-Then-Else Statements

If-Then-Else Statements

Simple selection occurs when a choice is made between two alternative paths, depending on the result of a condition being true or false. If-Then-Else Statements are one way of implementing selection.

The general form of the If-Then-Else statement is:

If condition is true Then

code for true alternative

Else

code for false alternative

End If

After the If-Then-Else statement processing continues with the next statement.

For example a bank charges $5 service fee if an account balance is less than $500 and $2 otherwise. The code would be:

If AccountBalance < 500 Then

Charge = 5

Else

Charge = 2

End If

If the condition was true, say the account balance was $300 (ie less than 500), the path taken by the code would be:

If AccountBalance < 500 Then

Charge = 5

Else

Charge = 2

End If

The else statements would be skipped.

If the condition was false, say the account balance was $800 (ie not less than 500), the path taken by the code would be:

If AccountBalance < 500 Then

Charge = 5

Else

Charge = 2

End If

The if statements would be skipped.

The condition (AccountBalance < 500 in the above example) can use one of the following:

Equal to / =
Less then
Greater than
Less than or equal to / <=
Greater than or equal to: / >=
Not equal to

Some examples include:

Comparison / Some Valid Values
Age < 5 / 0 1 2 3 4
Age <= 5 / 0 1 2 3 4 5
Age > 5 / All ages except 5

If then..else if

If this condition is true Then

code for true condition

Elseif this condition is true Then

code for true condition

Elseif this condition is true Then

code for true condition

Elseif this condition is true Then

code for true condition

Elseif this condition is true Then

code for true condition

Elseif this condition is true Then

code for true condition

Else

code for false condition (all of the above are false)

End If

Loops

All loops have two parts:

1.  the body of the loop (the statements being repeated)

2.  a termination condition that stops the loop

If the termination condition does not work the loop will continue for ever.

For Loops

The format of the for loop is:

For variable = start value To end value Step change value

statements that compose body of loop

Next variable

where

variable / counter variable in the loop
start value / beginning value of the counter variable
end value / ending value of the counter variable
change value / amount the counter variable changes each time through the loop
Note that if the change value is omitted it defaults to 1.
Next variable / the end of the For loop
The name of the variable can be omitted.

An example loop is

Do-while and Do-Until Loops

With a For loop the number of times it is to be executed is known. When this is not known a Do While and Do Until loop should be used.

Do Until and Do While loops are done until some condition becomes true (for Do Until ) or false (for Do While). If the condition does not become true or false as required loop will not terminate

Pre-test loops - test at start

Do Until (or While) condition

body of loop

Loop

Post-test loops - test at end

Do

body of loop

Loop Until (or While) condition

The following diagram shows the logic used in these loops:

Notes prepared by Jodi Tutty