CH11_Data_Collections.md

Data Collections

List

myList = [36, 18, 5, 4]

• a sequence of items stored as a single object
• Items in a list can be accessed by indexing and sublists can be accesed by slicing
• mutable

List Operations

operatonr	meaning
[seq] + [seq]	concatenation
[seq]* [int-expr]	repetition
seq[]	indexing
len([seq])	length
seq[:]	slicing
for [var] in [seq]:	iteration
[expr] in [seq]	membership check(returns a boolean)

List Mehods

• append(x) : adds element x to end of list
• sort() : Sorts(orders) the list
• reverse() : reverses the list
• index(x) : returns index of first occurrence of x
• insert(i, x) : inserts x into list at index i
• count(x) : returns the number of occurrences of x in list
• remove(x) : deletes the first occurrence of x in list
• pop(i) : deletes the ith element of the list and reutrns its value

cf) del : not list method, built-in operation

del myList[1]
del myList[1:3]

Example: Statistic

from math import sqrt

def getNumbers():
    nums = []
    xStr = input("Enter a number (<Enter> to quit) >> ")
    while xStr != "":
        x = float(xStr)
        nums.append(x)
        xStr = input("Enter a number (<Enter> to quit) >> ")
    return nums

def mean(nums):
    total = 0.0
    for num in nums:
        total = total + num
    return total / len(nums)

def stdDev(nums, xbar):
    sumDevSq = 0.0
    for num in nums:
        dev = num - xbar
        sumDevSq = sumDevSq + dev * dev
    return sqrt(sumDevSq / (len(nums) - 1))

def median(nums):
    nums.sort()
    size = len(nums)
    midPos = size // 2
    if size % 2 == 0:
        med = (nums[midPos] + nums[midPos - 1]) / 2.0
    else:
        med = nums[midPos]
    return med

Dictionary

passwd = {"guido":"superprogrammer", "turing":"genius",
          "bill":"monopoly"}
psswd["guido"]
passwd["bill"] = "bluescreen"

• one of the built-in data types for collections

• key-value pair mapping cf) hashes / associative array

• mutable

• order of keys will look essentially random

• keys can be any immutable type, and values can be any type at all

Dictionary Methods

• [key] in [dict] : Return True if dictionary contains the key
• [dict].keys() : returns a sequence of keys
• [dict].values() : returns a sequence of values
• [dict].items() : returns a sequence of tuples (key, value)
• [dict].get([key], [default]) : if dictionary has key reutrns its value, otherwise default
• del dict[key] : deletes the specified entry
• [dict].clear() : deltes all entries
• for [var] in [dict]: : loops over the keys

Example: Word Count

def byFreq(pair):
    reutrn pair[1]
    
def main():
    print("This is program analyzes word frequency in a file")
    print("and prints a report on the n most frequent words.\n")
    
    fname = input("File to analyze: ")
    text = open(fname, 'r').read()
    text = text.lower()
    for ch in '!"#$%&()*+,-./:;<=>?@[\\]^_`{|}~':
        text = text.replace(ch, ' ')
    words = text.split()
    
    counts = {}
    for w in words:
        counts[w] = counts.get(w, 0) + 1
        
    n = eval(input("Output analysis of how many words? "))
    items = list(counts.items())
    items.sort()
    items.sort(key=byFreq, reverse=True)
    for i in range(n):
        word, count = items[i]
        print("{0:<15}{1:>5}".format(word, count))
        
if __name__ == '__main__': main()

추가 참고:Collections module