1. Winter 2019 CISC101
2/17/2019
CISC101 Reminders
Quiz 1 marking complete. Grades are available in onQ.
Assignment 1 sample solution posted.
Assignment 2 due a week from today.
Quiz 2 next week, as well. Topics listed in Tuesday’s lecture. “for” loops will *not* be on the quiz. See next slide.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

2. Quiz 2 Topics, Updated All Python.
Winter 2019 CISC101
2/17/2019
Quiz 2 Topics, Updated
All Python.
Up to and including today’s lecture:
Style
Expressions
Console I/O
Conditionals
while Loops
Does not include the for loop, “turtle”, range() BIF or slice operator.
Same format & rules as for Quiz 1.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

3. Today Continue Iteration or “Loops”: Code a program in class.
Winter 2019 CISC101
2/17/2019
Today
Continue Iteration or “Loops”:
Code a program in class.
The “Other” loop syntax – the “for” loop.
Use the turtle to illustrate more examples of iteration. (If we have time).
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

4. In-Class Problem
Display all positive numbers in-between two supplied numbers that divide both numbers evenly. “Even division” means zero remainder after division.
Winter 2019
CISC101 - Prof. McLeod

5. Problem Algorithm
Obtain both numbers from the user. The values must be >= 1. Assume user enters an integer, but loop back if value not legal.
Determine which of the two numbers is the smaller one.
Loop a divisor from 1 to the smaller number:
If the divisor divides both numbers evenly (use %) then display the divisor.
Winter 2019
CISC101 - Prof. McLeod

6. The Other Loop… The other kind of looping syntax uses the for keyword.
It can be more powerful, particularly with collections – producing very compact code.
But you could almost always use a while loop instead – it would just take more code to carry out the same operation.
We’ll make more use of the for loop later in the course, after we have discussed collections.
Winter 2019
CISC101 - Prof. McLeod

7. for variable_name in iterable :
CISC101
The for Loop
Syntax:
for variable_name in iterable :
You make up variable_name
iterable can be a collection, such as a string, list, tuple, set or dict. It can also be an object produced by certain BIFs such as reversed() or range(). It can even be a file!
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

8. The for Loop, Cont.
It is still interchangeable with a while loop. These two “snippets” act in the same way:
testString = "I like watching videos!"
i = 0
while i < len(testString) :
print(testString[i])
i = i + 1
for aChar in testString :
print(aChar)
Winter 2019
CISC101 - Prof. McLeod

9. Some New Syntax
Two new things snuck their way into the code on the previous slide:
The len( ) BIF is used with collections and gives you the number of elements in the collection. For a string, it gives you the length of the string.
The square brackets: [ ] is called the “slice operator” and returns a single element (or character) or even a range of elements or characters from a collection.
Winter 2019
CISC101 - Prof. McLeod

10. The range() BIF
This BIF generates an iterable and is often used with a for loop.
Remember our while loop code to display 1 to 20?:
i = 1
while i < 21 :
print(i)
i = i + 1
Equivalent code using the range() BIF instead:
for i in range(1, 21) :
Winter 2019
CISC101 - Prof. McLeod

11. range(start, finish, step)
The range() BIF, Cont.
Full 3 parameter syntax:
range(start, finish, step)
start is the start value for the sequence.
finish the sequence will stop one short of this value.
step the step value to be used for the sequence.
Winter 2019
CISC101 - Prof. McLeod

12. The range() BIF, Cont. 2 parameter syntax: range(start, finish)
step is assumed to be 1.
1 parameter syntax:
range(finish)
start is assumed to be 0.
See RangeForLoopsDemo.py
Winter 2019
CISC101 - Prof. McLeod

13. Aside - Slice Operator
It is placed after a collection (a list, tuple, string, etc.) or a variable that is a collection type.
Used as in:
[ # ] or [ # : # ]
The first syntax gives you the single element at the location specified. The other syntax gives you all elements between the first location and one less than the last location.
We will look at this operator again later.
Winter 2019
CISC101 - Prof. McLeod

14. Turtle Graphics The “Turtle” is an easy way of drawing to the screen.
CISC101
Turtle Graphics
The “Turtle” is an easy way of drawing to the screen.
One use is to illustrate how loops work.
See the first section in “Program Frameworks” in the Python Standard Library.
To start you’ll need to import the turtle module:
from turtle import *
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

15. A Few turtle Commands To make the turtle look like a “turtle”:
shape(“turtle”)
To set the line thickness and line colour:
pensize(5)
pencolor(“green”)
The turtle starts out at the “home” position, (0, 0), facing right or “east”.
To draw, just issue commands to move the turtle.
Winter 2019
CISC101 - Prof. McLeod

16. A Few More turtle Commands
Turning:
right(num_degrees) # clockwise
left(num_degrees) # counter-clockwise
Moving in the direction he is pointing;
forward(length)
Changing speed:
speed(5) # 0 is no animation, 1 slow 10 fastest
Winter 2019
CISC101 - Prof. McLeod

17. turtle Demos
Start with five demo programs that illustrate turtle commands.
These five do not have any iteration.
Start with TurtleDemo1.py
Winter 2019
CISC101 - Prof. McLeod

18. Filling Commands Use begin_fill(), then draw, followed by end_fill()
to fill a closed, drawn area.
See TurtleDemo2.py
See TurtleDemo3.py to see how to draw a filled circle.
Winter 2019
CISC101 - Prof. McLeod

19. Regular Polygons
Modify how circle() is invoked to draw regular polygons instead of circles.
See TurtleDemo4.py
Draw text, too!
See TurtleDemo5.py
Fun!
Winter 2019
CISC101 - Prof. McLeod