1. CISC101 Reminders Quiz 1 grading underway Next Quiz, next week.
Winter 2018 CISC101
12/7/2018
CISC101 Reminders
Quiz 1 grading underway
Next Quiz, next week.
Next Assignment (Assn 2) due Friday, next week – the 16th.
The program we wrote last lecture is linked to the lecture notes page.
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

2. Today Four more loop keywords.
Demo: Calculating natural logs. An advanced example.
Another in-class program?
The for loop.
Winter 2018
CISC101 - Prof. McLeod

3. Four More Loop Keywords
break, continue, else, pass
else and pass are used in other places too.
Should not be used very often!
Winter 2018
CISC101 - Prof. McLeod

4. Use of break and continue With Loops
CISC101
Use of break and continue With Loops
Don’t use these keywords unless you feel it makes your code easier to read and debug!
break exits a loop immediately
continue jumps to the next iteration immediately.
See BreakContinueDemo.py
Does break kick you completely out of a nested loop?
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

5. Use of else and pass With Loops
else with a loop gives you a chance to see how the loop exited - if a normal exit, the else is executed.
See LoopElseDemo.py
pass is used when a statement is required, but you don’t have anything you want to do!
(Or you have not yet written the code.)
See PassDemo.py
Winter 2018
CISC101 - Prof. McLeod

6. Natural Logs Demo
Uses a numerical technique to estimate the value of natural logs.
Explain what natural logs are first, and then look at the infinite series (a Taylor’s series) used for the calculation.
How to stop the calculation loop?
You are not responsible for the math or the code in this demo
Winter 2018
CISC101 - Prof. McLeod

7. Another Demo - Natural Logs (Math!)
Consider log10 first:
This is the power of 10 that gives you a certain number.
For example log10(100) is 2 since 102 is 100.
Natural logs don’t use base 10, they use base e, where e is
The natural log function is usually called “ln()” (but in Python it is math.log(). Base 10 log is called as math.log10().)
Winter 2018
CISC101 - Prof. McLeod

8. Demo – Computing Natural Logs
Here is a series to compute natural logs:
or:
Using x = 1/3 will provide ln(2), for example.
Winter 2018
CISC101 - Prof. McLeod

9. Computing Natural Logs, Cont.
To calculate the natural log of any number, y, where y is greater than one, you can first calculate x using:
As you can see, x will always be less than 1.
Winter 2018
CISC101 - Prof. McLeod

10. Computing Natural Logs, Cont.
The natural log is not defined for values of y less than or equal to zero.
But if y is between zero and one – a fractional value – calculate the log of 1/y and negate it:
Winter 2018
CISC101 - Prof. McLeod

11. Natural Logs Demo
Write a program to estimate the natural log of a supplied number, y, where the value must be greater than zero. Display the result and the number of iterations required. Compare your estimated value against math.log().
Winter 2018
CISC101 - Prof. McLeod

12. CISC101
Natural Logs Demo, Cont.
The input part is just like what we have done before, but how does the calculation part work?
Since you can’t sum to infinity, how do you decide when to stop?
(Note that this is a pretty advanced example!!)
See ComputingLogs.py
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

13. Demo Summary
The demo showed how a loop can be used to carry out a serious calculation.
And we saw another way to stop a loop which works because floats are limited in size.
You should be able to write any of the code in the demo – no advanced code was used, but you do not need to understand the math!
Winter 2018
CISC101 - Prof. McLeod

14. In-Class Problem 2
Display all positive numbers in-between two supplied numbers that divide both numbers evenly. “Even division” means zero remainder after division.
Winter 2018
CISC101 - Prof. McLeod

15. Problem 2 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 2018
CISC101 - Prof. McLeod

16. 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.
Winter 2018
CISC101 - Prof. McLeod

17. 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 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

18. 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 2018
CISC101 - Prof. McLeod

19. Some New Syntax
Two new things snuck their way into the code on the previous slide:
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 2018
CISC101 - Prof. McLeod

20. 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
See RangeForLoopsDemo.py
Winter 2018
CISC101 - Prof. McLeod

21. 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 way gives you the element at the location specified. The other way gives you all elements between the first location and one less than the last location.
We will look at this operator again later.
Winter 2018
CISC101 - Prof. McLeod