1

Sequences

• A sequence is a list of elements

• Lists and tuples– Lists mutable

– Tuples immutable

• Sequence elements can be indexed with subscripts– First index is 0

– Access element with sequenceName[ subscript ]– Subscripts may be negative

– Slicing works same way as with strings

2

Creating Sequences

• Creations– Strings

• Use quotes: s = ""

– Lists• Use brackets• list1 = []• list2 = [ 1, 7, 66 ]

– Tuples• Use parentheses• tuple1 = ()• Tuple2 = 5,• tuple3 = ( 19, 27 )

startlist = [‘John’,‘George’, …,‘Wayne’] # long list

for i, name in enumerate( startlist ): # here we get both the subscript and the item: print “%s has index %d” %( name, i )

for name in startlist: # here we get the item directly, but not the subscript print name

for item in .. ?

Two ways of accessing all elements in a sequence

aList = [] # create empty list

# add values to list

for number in range( 1, 11 ):

aList += [ 1.0/number ]

Adds the values to the list (by adding a new list with one element to the existing list in each round)

Adding items to a list

• Using the + operator, you need to add a list to a list:

aList += [ number ]

• Using the append method, you add an element to the list:

aList.append( number )

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Using Lists and Tuples

• Only difference between lists and tuples is:– Lists are mutable, tuples are immutable

>>> aList = [3, 6, 9]>>> aList[2] = 141>>> aList[3, 6, 141]

>>> aTuple = (3, 6, 9)>>> aTuple[2] = 141Traceback (most recent call last): File "<stdin>", line 1, in ?TypeError: object doesn't support item assignment

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Passing a list as an argument to a function

• sum and max functions

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Output

Top 3 of the grades [10, 5, 11, 5, 13, 5] are:[13, 11, 10]Average grade: 5

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Explanation

[ , , , , , ]grades

10 5 11 5 135

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Explanation

[ , , , , , ]grades

10 5 11 5 135

a

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Better way of finding topN from a list of numbers

1. Keep a sorted list of current top-N

2. Go through given list one number at a time

3. If number is greater than current N’th greatest,

insert in correct position

Algorithm:

Don’t find the max N times: Inefficient!

13topN

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Better way of finding topN from a list of numbers

Want to compare to top‘s N’th element, so top must be initialized with N values

Make sure any value can enter top

top 11 10 5 4

5n i

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Output

- current top4: [10, 4, 4, 4] - current top4: [10, 5, 4, 4] - current top4: [11, 10, 5, 4] - current top4: [11, 10, 5, 5] - current top4: [13, 11, 10, 5]Top 4 of the grades [10, 5, 11, 5, 13, 5] are:[13, 11, 10, 5]Average grade: 8.17

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Functions working on lists

• map

• reduce

• filter

• zip

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Unelegant way of finding length of longest string

Don’t call len twice!

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yElegant way using map

• map: applies given function (first argument) to all elements in given sequence (second argument), returns list with results

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Sequence unpacking – testing a function

• for/else construct

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New implementation of function, same test

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Dictionaries

• Dictionaries– Mapping that consists of unordered key-value pairs– Each value is referenced though its key – Curly braces {} are used to create a dictionary– Creating a dictionary: { key1:value1, … }

or: dict( [ (key1, value1), …] )– Keys must be immutable values such as strings,

numbers and tuples– Values can be anything– Add item to dictionary d with d[key] = value

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Dictionary example: DNA dinucleotides

List comprehension:Create list of all tuples a, b where a and b are from nuc

Dictionary method fromkeys:Create dictionary with keys from the given list and all values 0Iterate through s, update dictionary

value for each dinucleotide key

Dictionary method iteritems:Iterate through (a sorted copy of) all of d’s key/value pairs. The keys are tuples, the values are integers.

Add report string to histogram list, convert to string and return

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Output

Input DNA string: atcagcgatattcgatcagctagAG : 3AT : 4CA : 2CG : 2CT : 1GA : 2GC : 2TA : 2TC : 3TT : 1

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On to the exercises..