Transcript Lecture 4
CIT 590 Intro to Programming Lecture 4 How are assignments evaluated • Pay attention to what the HW says it is trying to teach you about • ‘modular programming’ • ‘lists’ • If it is a mathematical/scientific program we will care more • • • • • about correctness. A math error in HW1 is more pardonable than one in HW2 If it is a game, the fun element is important. If it is a user interface (we’ll have one later) – make it look nice! Pay a ton of attention to function names and specs Style is hard to define but you know bad style when you see it Plagiarism is often detectable and will be dealt with harshly • Do not post your question on stack overflow! Agenda • Lists • The concept of side effect • Be careful with list manipulations • Tuples 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 • Count – count the number of occurrences of an element lists • 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]] 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? 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 Range, zip • Range basically produces a list • range(1,10) • 1 through 9 • range(1,17,4) – in steps of 4 • [1, 5, 9, 13] • Just like range produces lists you can use the function zip for tuples >>> zip([1,2],[1,2]) [(1, 1), (2, 2)] 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 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 Sorted lists • You can use the in-built ‘sorted’ function • You can use ls.sort • You can get fancy and use ls.sort with an argument • First time we are passing a function as an arguments • For writing a function that works as an argument to sort def sortingFunc(x, y): #return 0 when x and y are ‘equal’ # return -1 when x is ‘less’ than y # return 1 otherwise • fancySort 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 Strings are somewhat like lists but … • Strings are immutable and 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. • Fine to do it for strings • For lists you often actually want b = a[:] • 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']) 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 Sets • They do not allow repeated elements • set() • Since sets are unordered collections, they do not support things like indexing • However they do support things like union and intersection A few examples from the book • Palindrome (palindrome.py) • Encryption (simpleEncryption.py) Hands on example .. Most common starting letter for first names of students in this class Any guesses?