1. Nate Brunelle Today: Slicing, Debugging, Style
CS1110
Nate Brunelle
Today: Slicing, Debugging,
Style

2. Questions?

3. Last Time
While loops
For loops

4. Collections Order doesn’t matter, No Repetition
Order Matters, Repetition ok
Examples:
String
List
Tuple
Range
Counting starts at 0
collection[index] gives a specific value in the collection
Order doesn’t matter, No Repetition
Examples:
Set
Dict

5. Range range(x) range(x,y)
Gives all integers from 0 (inclusive) to x (exclusive)
range(5) -> 0,1,2,3,4
range(x,y)
Gives all integers from x (inclusive) to y (exclusive)
Range (2, 6) -> 2,3,4,5
[lower, upper)

6. hello Strings Collection of characters Order Matters Repetition ok
X = “hello”
X[0] -> h
hello 1 2 3 4
Indices
Indexing

7. More on Indexing s = ‘bananarama’ Indices are always int
S[1.0] -> error
Start counting indices at 0:
S[0] -> ‘b’
S[1] -> a
S[2] -> n ….
len(s) gives the number of things in the collection
len(s) -> 10
The last thing in s can be identified by: s[len(s)-1]
Use negative indices to index from the end
S[-1] is the last thing
S[-2] is the second from last thing
S[-x] is the same as s[len(s)-x]

8. Slicing s = ‘bananaarama’ Allows you to refer to a range of indices
Give a start index and an end index
Evaluates to a new collection from start (inclusive) to end (exclusive)
s[start:end]
s[2:5] -> ‘nan’
S[2:len(s)] -> ‘nanarama’
s[start:]
Goes from start index (inclusive) to the end of the collection
s[2:] -> ‘nanarama’
S[x:] the same as s[x:len(s)-1]
s[:end]
Goes from the beginning of the collection to the end index (exclusive)
S[:5] -> ‘banan’
S[:x] the same as s[0:x]
s[start:end:step]
Extended slice
Goes from the start index (incluvie) to the end index(exclusive), but keeping only every stepth thing

9. Debugging
“bug”- an error in your code

10. Test Case Selection Exceptional Cases: Equivalence Classes: Edge Cases
What happens when we get bad arguments?
Not important yet
Equivalence Classes:
Groups of things such that if our code behaves correctly on one of them, it will behave correctly on the whole group
Edge Cases
Boundary between Equivalence Classes
Tip: Make sure you know the answer!
Pow(1.479, )

11. Equivalence Classes for Absolute Value
1.0
8.5 8
13.2
Positive 13
0.0 -8
-8.5
Negative
-13
-13.2
int
float
-1.0

12. Equivalence Classes for Median?
median(a,b,c)
Equivalence Classes:
Positive values, negative values
a>b>c vs b>a>c ..
A == b == c

13. Which thing in the middle?
b smaller
a smaller
a bigger a
b bigger b
a bigger
a smaller c

14. Style You should write code to be read by humans Docstrings Names Pep8
Have information at beginning of functions to describe what it does
Names
A name should describe the meaning
Can be hard
Make sure there is one meaning per variable
Pep8
Other Python style guidelines
Talks about things that aren’t names or docstrings, but make code ugly