An Introduction To Software

Development Using Python

Spring Semester, 2015

Class #5:

IF Statement,

Relational Operators

Decisions!

• One of the essential features of computer programs is their ability to make decisions

• Example: what a car does at a stoplight depending on the color of the light.

• A program can take different actions depending on inputs and other circumstances.

Image Credit: www.xconomy.com

Say Hello To The “IF” Statement

• In Python, an IF statement is used to implement a decision.

• When a condition is fulfilled, one set of statements is executed. Otherwise, another set of statements is executed

Image Credit: www.clipartpanda.com

Example Of Using IF

Is room too

cold?

Set room temperature to

85 degrees

Set room temperature to

72 degrees

Yes No

Syntax Of Python IF Statement

Room Temperature Python Code

thermostatTemperature = 0

if measuredTemp < 70 :

thermostatTemperature = 85

else:

thermostatTemperature = 78

Compound IF Statementsthermostat Temperature = 0

if measuredTemp < 70 :

thermostatTemperature = 85

firePlace = 1

hotChocolate = 1

print(“The room will be heated”)

else :

thermostatTemperature = 78

print(“The room will be cooled”)

Statement

Block

Statement

Block

Sample IF Problem

The variables fuelAmount and fuelCapacityhold the actual amount of fuel and thesize of the fuel tank of a vehicle.

If less than 10 percent is remaining in the tank, a status light should show a red color; otherwise it shows a green color.

Simulate this process by printing out either "red" or "green"

Image Credit: www.pubzi.com

Tabs

• Blockstructured code has the property that nested statements are indented by one or more levels:

if totalSales > 100.0 :

discount = totalSales * 0.05

totalSales = totalSales − discount

print("You received a discount of $%.2f" % discount)

else :

diff = 100.0 − totalSales

if diff < 10.0 :

print("purchase our item of the day & you can receive a 5% discount.")

else :

print("You need to spend $%.2f more to receive a 5% discount." % diff)

Image Credit: www.clipartpanda.com

Tabs

• Python requires blockstructured code as part of its syntax. The alignment of statements within a Python program specifies which statements are part of a given statement block.

• How do you move the cursor from the leftmost column to the appropriate indentation level? A perfectly reasonable strategy is to hit the space bar a sufficient number of times.

• With most editors, you can use the Tab key instead. A tab moves the cursor to the next indentation level. Some editors even have an option to fill in the tabs automatically

Image Credit: imgarcade.com

Coding Best Practice:Avoid Duplicates In Branches

• Look to see whether you duplicate code in each branch. If so, move it out of the if statement. Here is an example of such duplication:

if measuredTemp < 70 :thermostatTemperature = 85

print(“New Room Temperature:", thermostatTemperature )else :

thermostatTemperature = 78

print(“New Room Temperature:", thermostatTemperature )

Image Credit: www.canstockphoto.com

Coding Best Practice:Avoid Duplication In Branches

• The output statement is exactly the same in both branches. This is not an error—the program will run correctly. However, you can simplify the program by moving the duplicated statement, like this:

if measuredTemp < 70 :thermostatTemperature = 85

else :thermostatTemperature = 78

print(“New Room Temperature:", thermostatTemperature )

Image Credit: www.canstockphoto.com

Conditional Expressions• Python has a conditional operator of the form:

value 1 if condition else value2

• The value of that expression is either value1 if the condition is true or value2 if it is false. For example, we can compute the actual temperature number asthermostatTemperature = 85 if measuredTemp < 72 else 78

which is equivalent as:if measuredTemp < 72 :

thermostatTemperature = 85

else :thermostatTemperature = 78

• Note that a conditional expression is a single statement that must be contained on a single line or continued to the next line.

• Also note that a colon is not needed because a conditional expression is not a compound statement

Image Credit: www.fotosearch.com

Relational Operators

Equality

Testing

Requires

2 “=“

Note: relational operators have a lower precedence than arithmetic operators

Examples Of Relational Operators

The Python ATM Machine

Image Credit: www.canstockphoto.com

Create software that will provide an ATM user

with the proper change for any dollar amount

up to $200.

Example: Run the code for $200 and for $19

ATM Machine Algorithm

• is the amount > $100, then provide a $100 bill & subtract $100

• is the amount > $100, then provide a $100 bill & subtract $100

• is the amount > $20, then provide a $20 bill & subtract $20 [repeat 4 times]

• is the amount > $10, then provide a $10 bill and subtract $10

• is the amount > $5, then provide a $5 bill and subtract $5

• provide the remaining amount in single $1 bills

Image Credit: www.gograph.com

What’s In Your Python Toolbox?

print() math strings I/O IF/Else

What We Covered Today

1. If / Else

2. Relational Operators

Image Credit: http://www.tswdj.com/blog/2011/05/17/the-grooms-checklist/

What We’ll Be Covering Next Time

1. Nested Branches

2. Multiple Alternatives

Image Credit: http://merchantblog.thefind.com/2011/01/merchant-newsletter/resolve-to-take-advantage-of-these-5-e-commerce-trends/attachment/crystal-ball-fullsize/