【codewar-6kyu】PaginationHelper

##題目描述

Description:

For this exercise you will be strengthening your page-fu mastery. You will complete the PaginationHelper class, which is a utility class helpful for querying paging information related to an array.

The class is designed to take in an array of values and an integer indicating how many items will be allowed per each page. The types of values contained within the collection/array are not relevant.

The following are some examples of how this class is used:

 1 helper = PaginationHelper(['a','b','c','d','e','f'], 4)
 2 helper.page_count # should == 2
 3 helper.item_count # should == 6
 4 helper.page_item_count(0)  # should == 4
 5 helper.page_item_count(1) # last page - should == 2
 6 helper.page_item_count(2) # should == -1 since the page is invalid
 7 
 8 # page_ndex takes an item index and returns the page that it belongs on
 9 helper.page_index(5) # should == 1 (zero based index)
10 helper.page_index(2) # should == 0
11 helper.page_index(20) # should == -1
12 helper.page_index(-10) # should == -1 because negative indexes are invalid

##思路分析

進入6kyu 以後忽然就吃力了些，感受難起來的並非編碼，而是邏輯的完備性。

細心些，考慮各類很是規狀況，超出處理閾值的狀況，必定要——滴水不漏。

##代碼解析

Python

 1 # TODO: complete this class
 2 
 3 class PaginationHelper:
 4 
 5   # The constructor takes in an array of items and a integer indicating
 6   # how many items fit within a single page
 7   def __init__(self, collection, items_per_page):
 8     self.collection = collection
 9     self.items_per_page = items_per_page
10       
11   
12   # returns the number of items within the entire collection
13   def item_count(self):
14     return len(self.collection)
15   
16   # returns the number of pages
17   def page_count(self):
18     if len(self.collection) % self.items_per_page == 0:
19       return len(self.collection) / self.items_per_page
20     else:
21       return len(self.collection) / self.items_per_page + 1
22     
23       
24   
25   # returns the number of items on the current page. page_index is zero based
26   # this method should return -1 for page_index values that are out of range
27   def page_item_count(self,page_index):
28     if page_index >= self.page_count():
29       return -1
30     elif page_index == self.page_count() - 1:
31       return len(self.collection) % self.items_per_page or self.items_per_page
32     else:
33       return self.items_per_page
34         
35       
36   
37   # determines what page an item is on. Zero based indexes.
38   # this method should return -1 for item_index values that are out of range
39   def page_index(self,item_index):
40     if item_index >= len(self.collection) or item_index < 0:
41       return -1
42     else:
43       return item_index / self.items_per_page
44