通过python中的嵌套列表进行二进制搜索

Question

I have a homework question asking:

Write a function called readCountries that reads a file and returns a list of countries. The countries should be read from this file (countries.txt), which contains an incomplete list of countries with their area and population. Each line in this file represents one country in the following format:

 name, area(in km2), population

When opening the file your function should handle any exceptions that may occur. Your function should completely read in the file, and separate the data into a 2-dimensional list. You may need to split and strip the data as appropriate. Numbers should be converted to their correct types. Your function should return this list so that you can use it in the remaining questions.

I have a text file called "countries.txt" with a list of a bunch of countries, their area, and their population.

Sample of "countries.txt":

Afghanistan,    647500.0,   25500100
Albania,    28748.0,    2821977
Algeria,    2381740.0,  38700000

This is the code I have and it works:

def readCountries(filename):
    '''read a file and print it to the screen'''
    countryList = []
    for line in open(filename):
        with open(filename) as aFile:
            countries = aFile.read()
            countryList.append(line.strip().split())
    aFile.close()

    return countryList 

Sample of output when I ran the question:

>>> countryList = readCountries("countries.txt")
>>> countryList
[['Afghanistan,', '647500.0,', '25500100'], ['Albania,', '28748.0,', '2821977'], ['Algeria,', '2381740.0,', '38700000']

The next question asks:

Write a function called printCountry that takes a string representing a country name as a parameter. First call your answer from question 1 to get the list of countries, then do a binary search through the list and print the country's information if found. And should print out:

 printCountry("Canada") Canada, Area: 9976140.0, Population: 35295770 printCountry("Winterfell") I'm sorry, could not find Winterfell in the country list.

But I can't figure it out.

When I tried to do the coding for this question, I typed:

countryList = readCountries("countries.txt")  
def printCountry(name):
    lo, hi = 0, len(countryList) - 1
    while lo <= hi:
        mid = lo + (hi - lo) // 2
        country = countryList[mid]
        test_name = country[0]
        if name > test_name:
            lo = mid + 1
        elif name < test_name:
            hi = mid - 1
        else:
            return country[0] + ", Area: " + str(country[1]) + ",    Population: " + str(country[2])
    return "I'm sorry can not find " + str(name)

and the result was:

>>> printCountry("Canada")
'Sorry can not find Canada'

even though Canada is in the text. Where did I go wrong?

Answer 1

Your binary search code is (mostly) ok, but there are a couple of problems in your code that reads in the list of countries.

Your file opening & reading code is strange. It's like you've combined two different approaches to reading data, so you are opening the file multiple times.

Fortunately, the effects of these lines:

with open(filename) as aFile:
    countries = aFile.read()

don't affect the output of the readCountries function because you don't do anything else with countries .

Also, in the description of your assignment it says to "strip the data as appropriate. Numbers should be converted to their correct types", which your code doesn't do. And as my hint above implied, that means the country names in your list still had the commas attached to them, so the binary search couldn't find them (unless you included the comma in the search name).

Anyway, here's a cleaned up version that's designed to run on Python 2.6 or later.

from __future__ import print_function

def readCountries(filename):
    countryList = []
    with open(filename) as aFile:
        for line in aFile:
            line = line.strip().split()
            #Remove anny trailing commas from each field
            line = [s.rstrip(',') for s in line]
            #Convert area to float and population to int
            line = [line[0], float(line[1]), int(line[2])]
            #print line
            countryList.append(line)
    return countryList

countryList = readCountries("countries.txt")

def printCountry(name):
    lo, hi = 0, len(countryList) - 1
    while lo <= hi:
        mid = lo + (hi - lo) // 2
        country = countryList[mid]
        test_name = country[0]
        if name > test_name:
            lo = mid + 1
        elif name < test_name:
            hi = mid - 1
        else:
            print('  {0}, Area: {1}, Population: {2}'.format(*country))
            break
    else:
        print("  I'm sorry, could not find {0} in the country list.".format(name))

#tests
printCountry("Canada")
printCountry("Winterfell")

print('- ' * 20)

#make sure we can find the first & last countries.
printCountry("Afghanistan")
printCountry("Nowhere")

Here's the data file I ran it on:

countries.txt

Afghanistan,    647500.0,   25500100
Albania,    28748.0,    2821977
Algeria,    2381740.0,  38700000
Canada,     9976140.0,  35295770
Nowhere,    1000.0      2345678

And this is the output it produced:

  Canada, Area: 9976140.0, Population: 35295770
  I'm sorry, could not find Winterfell in the country list.
- - - - - - - - - - - - - - - - - - - - 
  Afghanistan, Area: 647500.0, Population: 25500100
  Nowhere, Area: 1000.0, Population: 2345678

Answer 2

确保在对列表进行二进制搜索之前对列表进行排序：

countryList.sort()