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CIT 590 Intro to Programming Lecture 5 – completing lists
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Textbook sections skipped Writing a function that calls itself (Recursion) – postponed till later Raising exceptions – postponed Please read your textbook and work through the examples by actually typing them up.
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Agenda Lists The concept of side effect Be careful with list manipulations Strings Tuples Hoping to complete Chapter 4
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lists ls = [1, 4, 5] Very similar to strings in terms of some of the basic operations Slicing is your best friend What is [2, 3, 4] + [7, 8, 9]? ls * 3 will repeat the list 3 times List functions Append – add elements Extend - add this other list at the end Index - find and return index Iterating over the elements of a list for listelement in lst: Python has no issue with mixed types >>> [1, 0.45, True, 'abc', [1,2,5]] [1, 0.45, True, 'abc', [1, 2, 5]]
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List functions work by side effect ls.append() automatically changes the value of ls The function append actually returns the value ‘None’, so if you were to do ls = ls.append(5) or ls = ls.extend([5, 6, ‘t’]) Disaster! >>> lst = [45, '6', True] >>> lst.append(lst.append(576)) What will print lst give me now?
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Fixed sized lists (arrays?) You can initialize a fixed length list by doing something like [None] * 10 [0] *5 Fixed length lists behave very similar to the concept of arrays found in other programming languages
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Range Range basically produces a list range(1,10) 1 through 9 range(1,17,4) – in steps of 4 [1, 5, 9, 13] Xrange on the other hand, is a better thing to use when you are not interested in actually creating the list.
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Assignment by reference as opposed to value(copy) A variable is always assigned by reference in Python If you’ve seen pointers in C, this is equivalent to that concept You can make a true copy by doing a complete slice Function arguments are also passed by reference Be very careful when doing this See sneakyAppend for an example
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Identity and equality Use the identity testing operator ‘is’ if you want to ensure two variables hold references to the same object Use ‘==‘ operator if you want to ensure that the two variables have the same value, even if they are not the same object a is b implies a == b, but not necessarily the other way around
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Tuples Defined using () Immutable - once defined I cannot change individual elements Similar to the list function there is a tuple function which makes a tuple out of a string or for that matter any list Very useful when you want to return multiple things from a function – getMaxAndMin.py Why not return things as a list? The mutability of a list makes it a somewhat dangerous datastructure
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Strings are somewhat like lists but … Strings are immutable Lists are mutable If a is a list I can do things like a[0] = ‘hahaha’ When you do b = a for a list you have to careful String to list conversion list(‘computer science rocks!’) Stringifying a list ''.join(lst) Join is a powerful function that basically uses the string as a separator between elements of the list argument '||'.join(['45','56','56'])
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Strings and cool string functions Split Join Both split and join will work essentially with any symbol for concatenation or separation Grab sentences using the ‘.’ separator
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Sets They do not allow repeated elements set() Since sets are unordered collections, they do not support things like indexing However they do support most standard set operations – union, intersection, difference. Best way to eliminate dupes?
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A few examples from the book Encryption (simpleEncryption.py)
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Hands on example.. Most common starting letter for first names of students in this class Any guesses?
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