CST-170 Chapter 4 - Making Decisions - Part 1

Chapter 4 discusses making decisions.

before we get into the two types of

decision structures or selection

structures, we need to understand what

a boolean expression is. A boolean

expression is a comparison. The results

of the boolean expression is going to be

either a TRUE or FALSE. Either the

expression is TRUE or the expression is

FALSE. These boolean expressions are used

in every selection structure.

The question that is being asked in

the diamond symbol of your flow chart,

that question is really a boolean

expression. Let's look at the first type

of selection structure. It's called a

dual alternative or binary, 'bi',

being two, because it provides an action

for each of the two outcomes.

If the expression, remember the expression

will be written inside the diamond of

your flow chart, if the expression

results to TRUE or YES, we follow this

plan or path of action. The alternative

is if the expression evaluates to FALSE

or NO then you follow a different plan

of action. In pseudo code a dual

alternative selection is often written as

an IF THEN ELSE statement.

The second type of selection structure

is the single alternative or unary

selection, single being one action or

path to follow. If the expression

evaluates to TRUE, you follow the path

and perform some sort of action. If the

expression evaluates to false then there

is no special action to take; we simply

exit the structure. You could write your

expression in such a manner that the plan

of action is executed from the FALSE

results but that's what we call negative

logic and we are going to discuss avoiding

that. So the action does not always have

to happen if the expression is TRUE but

you do want to be careful about how

you write your expressions. You want to

make sure your logic is clear and

understandable so sometimes that negative

logic can confuse people. The single

alternative selection when written in

pseudo code is often written as an IF THEN

statement. If the expression is TRUE

then do this. That's it. There's no else.

The IF THEN ELSE decision, remember this

is the dual alternative selection, the IF

line, if the condition is TRUE then the

THEN clause. The THEN clause holds the

action or actions that execute when the

tested condition in the decision is TRUE.

If your boolean expression evaluates to

TRUE you do the actions within the THEN

clause. If the boolean expression

evaluates to FALSE the THEN clause is

skipped and the actions in the ELSE

clause are executed.

Your boolean expression does not have to

be an 'equal' symbol comparison. You

don't have to write your boolean

expression as 'value1 = value2' and

remember we want to make sure that we

are talking about comparisons now, we

quite often use the '=' sign to do

comparisons. Don't confuse that with

chapter 2 when we discussed assignment

statements and the assignment operator

was also the '=' symbol.

These are comparisons; however, we are

not going to limit ourselves to just

'equal to' type comparisons. Most

programming languages support six

different types of comparison operators.

All of these comparison operators will

result in a TRUE or FALSE. You compare

the two values by using either variables

or constants or a combination of a

variable and a constant. The term

'trivial expression' is when you compare

two constants. In this example

the digit 20, 20 equals 20 that's always

going to be TRUE. It's never going to

change. So quite often in our

expressions we use variables because the

data in the variable changes. It's not

uncommon to compare a variable value to a

constant. It's not uncommon to compare

a variable to a variable and when I say

that I mean the data in one variable

compared to the data in another variable.

But it is very trivial to compare a

constant to a constant because a constant

never changes. Avoid comparing a

constant to a constant.

Here are the six relational operators

that our commonly accepted.

the equivalency operator or the equal to,

greater-than, less-than, greater-than or

equal-to, less-than or equal-to, and then

not-equal-to. Now once again I'm going

to say that depending on your language

when you actually write the code these

symbols may be slightly different

depending on the language.

Any logical situation can be expressed

with only the three types of comparisons,

equal-to, greater-than, less-than. So

technically we could argue that you could

throw out the greater-than equal-to and

the lesser-than equal-to, but that's kind

of a debatable discussion. Now here it

says it makes the code more readable.

I would argue that really depends on the

individual. If you want to say

Ms. Veral's video on boolean expressions,

she talks about asking the computer her

birth date. Is your birthday on or before

July 1st? Well if you want to be

inclusive, including July 1st, you could

just ask if it were before July 2nd.

That would include July 1st but not

include July 2nd. It's arguable whether

or not the less-than or equal-to, greater-

than or equal-to makes your code more

readable. Just know that we always have

the option that there's a work around

for these greater-than or equal-to,

less-than or equal-to.

And a not-equal operator, again, debatable

but I'm going to agree, probably the most

confusing of comparisons.

But the not-equal-to is asking if two

variables are not equal.

Here's some example flow chart and

example pseudo code using a negative

comparison. Is the customer code not

equal to 1. If the expression evaluates

to TRUE or YES, the customer code is not

1 and the discount is 0.25. If the

customer code not-equal to 1 evaluates to

FALSE, which think carefully that means

the customer code is equal to 1, then

the discount is 0.50. Here's that pseudo

code to match it. Customer code not

equal to 1 then discount is 0.25

otherwise discount 0.50. You can simply

rewrite this to be using positive logic

rather than negative logic. Instead of

saying not-equal-to. I want you to know

this before we change slides. Not-equal

to one, discount 0.25. Equal-to 1

discount 0.5. Notice that we're going to

keep the negative or the FALSE path on the

left and the TRUE path on the right.

But notice the question will change and

the actions will change.

Customer code is-equal-to 1, YES

discount 0.50. Customer code not-equal-to

1, discount 0.25. Look at the pseudo

code. Now I'm going to back up a slide.

Compare this pseudo code using straight

forward or positive logic, compared to

the same logic, excuse me, the same

actions but in negative logic. I would

argue that using the not-equal-to causes

a little bit more confusion. You are

certainly welcome to disagree.

Avoiding a common error with relational

operators, be very careful, read your code

do a desk check. I'm sorry, not your code.

Read your logic, walk through your pseudo

code, walk through your flow chart doing

a desk-check. Make sure you're using the

correct operators. A very common error

is missing that boundry or limit for a

selection. Again if we refer to the

birth date July 1st, you ask is your

birthday before July 1st. Well do you

want to include July 1st in your question?

That's where you've gotta ask. That's that

boundary or limit their talking about.

If I ask you to pick a number between 1

and 5. Between 1 and 5, does that mean

2, 3, or 4? Or are we being inclusive,

1, 2, 3, 4 and 5 pick one of those.

That's what we are talking about with the

boundary or the limit. So be careful

that you use the correct relational

operator in your boolean expressions.