Python. Olmo S. Zavala R. Python Data Structures. Lists Tuples Dictionaries. Center of Atmospheric Sciences, UNAM. August 24, 2016

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1 Center of Atmospheric Sciences, UNAM August 24, 2016

2 are the most versatile datatype available in. It can be seen as a container of other types of variables. are identified with square brackets and its content is separated by commas, for example: x = [1, 3,..., n] functions len(list) Length of the list list.append(x) Adds item to the list list.extend(list) Appends items for another list list.insert(i,x) Inserts x at index i list.remove(x) Removes x if exists. list.pop([i]) Removes or pops list.clear() Clears the list list.index(x) Gets the index where x is at list.count(x) Counts the number of times x is in the list list.sort(reverse=false) Sorts the list list.copy() Makes a shallow copy

3 List slicing and splicing List slice and splice list[start:end] From start to end list[start:end:step] From star to end with step list[:] All the list list[-1] Last index of the list Example mylist = [1, name,3,true] mylist[0] 1 mylist[1] [1] mylist[0:2] [1, name ] mylist[0:3:2] [1,3] mylist[-1] True

4 multiple dimensions Each element of a list can also be a list. In this way we can simulate multidimensional matrices. functions x = [ [1,2,3], [4,5,6] ] x[0] [1,2,3] x[0][0] 1 x[0][1] 2 x[-1][1] 5 Two dimensional array

5 exercises Define: x = [ [1,2,3], [4,5,6],[7,8,9] ] 1 Make a function that receives a lists and prints the first row of the list. 2 Make a function that receives a list and a number n and prints the n row of the list. 3 Make a function that receives a list and a number n and prints the n column of the list. 4 From the previous function, print an error string if the list does not has that dimension. 5 Make a function that receives two lists and computes the dot product. 6 From the previous function, print an error string if the two lists do not have the same dimension.

6 are immutable sequences. They are created using parenthesis rather than square brackets and are smaller objects than lists. If you have data that will not change, you should use tuples. Uses same slicing and splicing techniques than lists. Examples x = (1,3,...,n) x = ( Text,1,True) x = (1,2,3,4,5,6,7,8,9) x[0:3] (1,2,3)

7 are list that are indexed by keys (string or numbers). are defined with curly braces. x = { mykey: MyValue } x = { name : Usain, last name : Bolt } Dictionary functions len(dict) Length of the dictionary del dict[key] Deletes content from dictionary dict.get(key) Gets value of key dict.has_key(key) Boolean about the existence of the key dict.values() Returns a list of the values dict.keys() Returns a list of the keys dict.pop([i]) Removes or pops dict.update(otherdict) Adds otherdict into dictionary dict.clear() Clears the list dict.copy() Makes a shallow copy

8 Looping dictionaries There are several ways to iterate over a dictionary. Assume we have defined the following dictionary: mydict = { "name": "Usain", "last name":"bolt"} Then, you can loop with: Looping dictionaries for var in mydict: var will contain all the keys of the dictionary. for var1,var2 in mydict.items(): var1 has keys and var2 has values Examples d = {1:"First Val", 2:"Second Val"} for mykey in d: mykey mykey 1 in the first loop mykey 2 in the second loop for myk, myv in d.items(): myk 1 and myv "First Val" in the first loop myk 2 and myv "Second Val" in the first loop

9 Dictionary exercises 1 Make a function that prints Key: key Value: value for each key,value of a dictionary. 2 Make a function that adds all the values of a dictionary. 3 Make a function that adds 2 to each value of the dictionary (assuming each value is a number).