Python Lists and Such
CS 4320, SPRING 2015
List Functions
len(s) is the length of list s
s + t is the concatenation of lists s and t
s.append(x) adds x to the end of list s
s.pop(i) removes and returns the element at index I
◦ s.pop() is the same as s.pop(-1)
x in s and x not in s test for containment
del s[i:j:k] removes a slice (or a single element)
s.insert(i,x) inserts x at index i
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Command Line Parameters
When a program is run from the command line, parameters can be added.
Parameters can also be provided when running in PyCharm (most IDE’s support this feature as
well)
The parameters are made available in a list named sys.argv
To use the arguments, include the line
import sys
at the beginning of the script
The values of the arguments are of type string
The parameter at index 0 is always the path to the script
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Examples
Get the list of values from the command line
Apply the various functions and print the results
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Tuples
Tuples are immutable lists
As literals, tuples are given by a comma separated list of values surrounded by parentheses
◦ (1,2,3)
Beware!
◦ (1) is not a tuple, it is simply the integer 1
◦ (1,) is a tuple with one element, the integer 1
Tuples turn up in some places, especially as the return values from some functions.
◦ Just be aware that tuples cannot be modified
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Dictionaries
Also known as ‘hash maps’, ‘associative memory’, and ‘maps’
A dictionary associates keys to values
A literal dictionary uses curly braces with pairs ‘key:value’
Indexing retrieves values for a given key
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Examples
Create some dictionaries and access values
Demonstrate what happens when access is attempted with a key not present
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Dictionary Functions
d.get(key) gets the value associated with key
d.get(key,y) gets the value associated with key, returns y if there is none
key in d returns true if key is in dictionary d
d.keys() returns an ‘iterable’ of the keys in d (will use with loops)
del d[key] deletes the key from the dictionary
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Examples
Use the various functions and display the results
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Comprehensions
A syntactical shortcut for creating lists from other lists
[ expr for var in list ]
This can be extended with more ‘for’ clauses
Filters can be added with ‘if’ clauses
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Examples
Create some list comprehensions
Create a list from a range by applying a simple expression
Modify the comprehension to select based on remainder modulo something
Create a list of pairs (x,y) where x divides y evenly
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