Computer Programming 12
Lesson 5 - Decision Structure
By: Dan Lunney
If /Then / Else statement
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Decision logic structure is used
to make a decision that has 2
outcomes: true or false
We test which it is by using the
if/then/else statement
IF <condition> THEN
<True set of instructions>
ELSE
<False set of instructions>
If/Then/Else Flowchart
Start
TRUE
FALSE
IF <condition (s)>
Instructions if
conditions are
false
Instructions
if conditions
are true
Exit
Nested If/Then/Else
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We can have a second
if/then/else statement as the set
of instructions for the True
and/or False condition of
another if/the/else statement
This is called a nested
if/then/else
Nested If/Then/Else Flowchart
Start
FALSE
if
TRUE
FALSE
TRUE
if
Instructions
Instructions
Instructions
Three Types of Logic
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Straight-through (no else part)
Positive Logic
Negative Logic
Straight-Through Logic
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All if statements are executed
sequentially
There is no ELSE branch to be
executed
This type of logic is the least
efficient because we need to
write a code statement for every
possible decision
Straight Logic Flowchart
T
IF
F
T
IF
F
T
IF
F
Positive Logic
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Most common type as this is
how we think
Continues to processing
decisions until a decision is true
then it stops executing
Positive Logic Flowchart
Start
TRUE
if
FALSE
TRUE
FALSE
if
Instructions
Instructions
Exit
Instructions
Negative Logic
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
Works the opposite as Positive
Logic
Continues to process decisions
until a decision is false then it
stops executing
Negative Logic Flowchart
Start
FALSE
if
TRUE
FALSE
TRUE
if
Instructions
Instructions
Instructions
Sample Problem – Lesson 5
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What would the flowcharts and
algorithms of the following problem
look like in straight logic, positive
logic, and negative logic?
People pay $7 for movies if they are
under 16, $10 if they are between 16
and 65, and they pay $5 if they are
greater than 65.
See sample problem sheet