Python for loops. Girls Programming Network School of Information Technologies University of Sydney. Mini-lecture 7

Transcription

1 Python for loops Girls Programming Network School of Information Technologies University of Sydney Mini-lecture 7

2 Lists for loops More Strings Summary 2 Outline 1 Lists 2 for loops 3 More Strings 4 Summary

3 Lists for loops More Strings Summary 3 Storing groups of things in variables ˆ Imagine we want to store a list of strings or integers ˆ We can try to do this with variables 1 >>> word1 = 'This' 2 >>> word2 = 'is' 3 >>> word3 = 'a' 4 >>> word4 = 'sentence' ˆ If the list was long we d need thousands of variables ˆ And we d need separate code to do things with each variable: 1 >>> print word1, word2, word3,...

4 Lists for loops More Strings Summary 4 Lists can store multiple things 1 >>> words = ['This', 'is', 'a', 'sentence'] 2 >>> words[0] 3 'This' 4 >>> words[1] 5 'is' 6 >>> ˆ A list is created using square brackets in Python ˆ The words list holds four strings in order ˆ Each item in a list is called an element ˆ Each element can be accessed using an index or subscript ˆ Notice that indices start from zero!

5 Lists for loops More Strings Summary 5 What happens if we fall off the end? 1 >>> words = ['This', 'is', 'a', 'sentence'] 2 >>> words[4] 3 Traceback (most recent call last): 4 File "<stdin>", line 1, in <module> 5 IndexError: list index out of range 6 >>> words[-1] 7 'sentence' 8 >>> ˆ Python complains about accessing off the end of a list ˆ But, negative indices work! ˆ They return elements from the end of the list ˆ What would word[3] and word[-3] return?

6 Lists for loops More Strings Summary 6 You can put anything in a list ˆ You can have a list of integers: 1 >>> primes = [1, 2, 3, 5, 7, 11] ˆ You can have lists with mixed integers and strings 1 >>> l = [1, 'two', 3, 4, 'five'] ˆ But this is almost never a good idea ˆ You should treat every element of the list the same

7 Lists for loops More Strings Summary 7 A Python for loop in action 1 >>> for i in [0, 1, 2]: 2... print i, 'is an element' is an element 5 1 is an element 6 2 is an element 7 >>> ˆ A for loop starts with the keyword for Remember, all Python control structures start with a keyword ˆ Next is the loop variable i which will hold each element ˆ Then the keyword in followed by the list to loop over ˆ Watch out for missing the colon!

8 Lists for loops More Strings Summary 7 A Python for loop in action 1 >>> for i in [0, 1, 2]: 2... print i, 'is an element' is an element 5 1 is an element 6 2 is an element 7 >>> ˆ Each element of the list is assigned to the variable i in turn ˆ Then the body is run ˆ When it runs out of elements in the list, the loop stops ˆ Notice you don t need to update i yourself now

9 Lists for loops More Strings Summary 8 for loops can run over lists of strings too 1 >>> words = ['This', 'is', 'a', 'sentence'] 2 >>> for word in words: 3... print word, 'is an element' This is an element 6 is is an element 7 a is an element 8 sentence is an element 9 >>> ˆ The list can be stored in a variable too (here words)

10 Lists for loops More Strings Summary 9 range and xrange create lists of integers 1 >>> range(10) 2 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 3 >>> for i in range(3): 4... print i >>> ˆ range(n) creates a list up to but not including n ˆ range(n) starts from 0 ˆ This matches list indexing from zero ˆ xrange(n) is a more efficient version for large lists

11 Lists for loops More Strings Summary 10 Appending items to lists ˆ We can add elements to an existing list using append 1 >>> words = ['This', 'is', 'a', 'sentence'] 2 >>> words.append('blah') 3 >>> words 4 ['This', 'is', 'a', 'sentence', 'blah'] ˆ append is a special kind of function called a method ˆ The thing we are operating on appears before the function name ˆ In this case, we re appending to the words list ˆ A period (.) separates the object from the function name

12 Lists for loops More Strings Summary 11 Strings in for loops ˆ You can use strings in for loops in the same way you use lists ˆ Instead of iterating over the elements of the list, you iterate over the characters in the string 1 for letter in "mosey": 2 print letter, ˆ What will be the output of the code above? ˆ You can also access characters in a string by their indices, like elements of a list. 1 >>> last_char = "smiley"[-1] 2 >>> last_char 3 'y'

13 Lists for loops More Strings Summary 12 String Methods ˆ Like the list method append, there are lots of special methods defined for strings. ˆ You can find out about these by using the builtin Python documentation. 1 >>> help(str) ˆ You ll have to scroll a bit to get to the bits you want. Look for stuff like 1 capitalize(...) 2 S.capitalize() -> string 3 4 Return a copy of the string S with only its 5 first character capitalized.

14 Lists for loops More Strings Summary 13 You should now be able to: ˆ Create Python lists ˆ Access individual list elements using indices ˆ Write Python for loops ˆ Append elements to a list ˆ Use help in the Python interpreter