http://bit.ly/ds2oo1 practicum 4 Read final project spec for class next week
DS 2001:Programming Practicum Fall 2020 — Felix Muzny Practicum 4: Lists! Trace the value of x as the given code executes. What is printed by this code x = 4 x DX when it runs? ls = ['cat', 'bat', 'horse'] $ for i in range(len(ls)): - 61 x = x + (ls[i] % 2) -7 lends Cia ' cat ' print(x) 42 ERROR ! 1
Functions + Scope • Function parameters "live" only inside the function they belong to (they are "local" to that function) • Variables and parameters defined in one function are not accessible in another function • I strongly recommend that you only define variable inside of functions from now on • This will keep your program structure more organized, and make it easier to track what variables have what values at di ff erent points. def square1(num): squared = num ** 2 print(squared) # ERROR print(num) 2
Functions: tracing program execution • When we run programs, we can trace the execution by looking at what lines run in what order. This gets complex when we have multiple functions! Order of execution 1 def add(num1, num2): 2 a = num1 + num2 - 11 3 return a 4 4 def sub(num1, num2): 58 7 5 a = num1 - num2 6 6 return a 1 10 7 def main(): oats 2 8 plussed = add(10, 5) Is 3 9 minussed = sub(10, 5) - q 10 print("done!") 11 main() → 3
Functions: tracing program execution • When we run programs, we can trace the execution by looking at what lines run in what order. This gets complex when we have multiple functions! 1 def add(num1, num2): 11 2 a = num1 + num2 7- 3 return a 8 4 4 def sub(num1, num2): 56 5 a = num1 - num2 6 return a ← 5 45 7 def main(): 7 8 plussed = add(sub(10, 5), sub(11, 3)) 6 9 extra_plus = add(plussed, plussed) 1 10 print("done!") 11 main() 23 4
lists: a summary • Lists are a container type that holds other values (ints, strings, other lists, etc) • We can think of two dimensional lists as tables of data with each sublist as one row and indexes of the sublists corresponding to columns data = [[ ], 1 "Anuj" 15 "Mon and Sat" [ ], 2 "Kaushik" 14 "Tue and Wed" [ ], 3 "Niyati" 14 "Tue and Fri" [ ]] 4 "Satya" 15 "Tue and Fri" OT ) # 1st row print ( data [ Anuj print ( data Co ] [ IT ) # print ( data E3 ] [ IT ) # Satya 5
lists: common patterns • iterating over a list can be done in two ways: by element or by index • iterate by element if you need a quick interface to the values in the list • iterate by index if you need location information or to modify the list elements ls = ['cat', 'bat', 'dog'] # iterate by element ' } for element in ls: print(element) print C element t " s # iterate by index } for i in range(len(ls)): ls[i] = ls[i] + 's' " bats - I print l l s " ) # C " " cats 6 . . ,
lists: common patterns • Accumulating an answer by visiting each element in a list def number_uppercase(ls): count = 0 for element in ls: if element . is upper l ) : + I = count count return count 7
lists: common patterns • Iterating over two-dimensional lists data = [[1, 3, 5], [2, 4, 6]] P t P loop # Z - # I for row in data: print("row:", row) for item in row: # 2 - print(item) 8
lists: common patterns • Iterating over two-dimensional lists, using range so that we have access to the indices data = [[1, 3, 5], [2, 4, 6]] Ed , [ 1,03 ] PPP for row_index in range(len(data)): T print("row:", row_index) ] ) ) - index ←¥ CdataCrow 8 for col_index in _______________: E 't ] print(____________) dataflow - indexed - indeed 9
lists: common patterns • lists are mutable --this means that they are going to act di ff erently than the types that we are used to when we pass them as parameters and update them. odds k ¥ 7 x TE def main(): def main(): odds = [1, 3, 5] x = 1 other - odds - xD other - other_odds = odds other_x = x odds[0] = odds[0] + 2 x = x + 2 print(odds) print(x) print(other_odds) print(other_x) main() main() 10
lists: common patterns • lists are mutable --this means that they are going to act di ff erently than the types that we are used to when we pass them as parameters and update them. def list_add_mutate(ls, num): - ls[0] = ls[0] + num def main(): odds = [1, 3, 5] list_add_mutate(odds, 2) # [ 373,5 ] print(odds) main() 11
lists: common patterns • lists are mutable --this means that they are going to act di ff erently than the types that we are used to when we pass them as parameters and update them. def list_add_pure(ls, num): - new_ls = [:] # get a copy of ls # update copy new_ls[0] = new_ls[0] + num return new_ls def main(): odds = [1, 3, 5] a- updated_odds = list_add_pure(odds, 2) # Ct , 3,5 ] print(odds) # E3 , 3,5 ] print(updated_odds) main() 12
lists: common patterns • slicing (isolating a part of a list and returning it to us as a new list) list_part = list_variable[begin_index:end_index] ls = ['cat', 'hat', 'horse'] print(ls[1:2]) print(ls[1:3]) print(ls[1:]) print(ls[:2]) print(ls[:]) 13
Practicum today - coding • For practicum today, you'll be writing a series of functions that work with lists . • Each time you write a function, you'll write one or more corresponding function calls in your main function. • Your main function will go at the bottom of your file so that the other function definitions can be read in by python first. • Remember to work together and ask us questions—those of you who are remote, we'll put you in groups and ask you to screenshare with each other, as usual. 14
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