1. Winter 2018 CISC101
11/9/2018
CISC101 Reminders
Quiz 1 this week. More info in last Tuesday’s lecture.
Assn 1 now due Monday, Feb. 5, next week.
Exercise 2 is fair game now – work on console input and output techniques.
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

2. Today Conditionals (or “if” statements). Winter 2018
CISC101 - Prof. McLeod

3. So Far… Without conditionals programs are linear:
CISC101
So Far…
Without conditionals programs are linear:
Obtain data from user
Calculations…
Display results
etc.
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

4. Branching Algorithms
If a program could test a condition, then it would have a choice as to its path of execution:
if true
if false
etc.
Winter 2018
CISC101 - Prof. McLeod

5. Conditionals or “Selection Statements”
Python has if, if-else and chained if (if-elif-else) statements.
if statement syntax:
if boolean_expression :
statement1_when_true
statement2_when_true
statement3_when_true …
Example:
if capacitance < 0 :
print("Illegal capacitance")
Winter 2018
CISC101 - Prof. McLeod

6. if-else Statement Syntax of if-else statement: Example:
if boolean_expression :
statement(s)_when_true
else :
statement(s)_when_false
Example:
if stress > maxStress / 1.5 :
result = "failure"
result = "pass"
Winter 2018
CISC101 - Prof. McLeod

7. if Statement, Cont. You will often have to nest if statements: etc.
Winter 2018
CISC101 - Prof. McLeod

8. Nested if Statements
For example, if you have three numbers, a, b, & c and you want to print them to the screen in order: T F
a < b T F T F
b < c
b < c T F T F
a < c
a < c
abc
cba
acb
cab
bac
bca
See SortThree.py
Winter 2018
CISC101 - Prof. McLeod

9. if-elif Statements
In the code in SortThree.py, you might have noticed this construct:
else :
if a < c :
This kind of thing occurs so often that it can be shortened to:
elif a < c :
Winter 2018
CISC101 - Prof. McLeod

10. if-elif Statements, Cont.
This leads to a common code structure often called a “chained if” construct:
if condition1 :
statement(s)
elif condition2 :
elif condition3 :
else :
You can have as many elifs as you want.
The else part is optional.
Winter 2018
CISC101 - Prof. McLeod

11. if-elif Statements, Cont.
There is nothing in this construct that you could not make with normal if-else statements.
For some kinds of conditionals, the if-elif might be easier to put together.
See Part B of assignment 1, for example.
Winter 2018
CISC101 - Prof. McLeod

12. Nested if Statements - Demo
What to wear? Depends on time to destination and outdoor temperature:
If time < 5 minutes, wear shorts & tshirt
if time between 5 and 20 minutes:
if temp >= 5 wear shorts & tshirt
if temp between -10 and 5 wear pants & sweatshirt
if temp less than -10 wear pants & jacket
if time > 20 minutes:
if temp >= 10 wear shorts & tshirt
if temp between 0 and 10 wear shorts & sweatshirt
if temp between – 10 and 0 wear pants & jacket
if temp < -10 wear pants & parka
Winter 2018
CISC101 - Prof. McLeod

13. Nested if Statements – Demo, Cont.
See:
ClothesVersion0.py
ClothesVersion1.py
ClothesVersion2.py
ClothesVersion3.py
All work correctly and all do the same thing.
Which one is “best” and why?
Don’t forget that Python uses “short circuit” evaluation of logical operators – and & or.
Winter 2018
CISC101 - Prof. McLeod

14. Aside – “Short Circuit” Evaluation
In the case of an and, if the LHS (“left hand side”) is already False then the RHS (“right hand side”) is not evaluated.
Similarly for an or, if the LHS is already True, the RHS is not evaluated.
Provides faster code execution!
Winter 2018
CISC101 - Prof. McLeod

15. Building Conditionals
Nesting conditionals leads to the best structure for this example.
If you have to nest conditionals, write the test with the fewest conditions for the outermost if statement.
Make sure you have covered every possible combination of inputs.
When testing, test every possible combination of inputs to make sure your logic is good.
Writing nested conditionals can be tricky!
Winter 2018
CISC101 - Prof. McLeod