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柴柴のSequences Data abstraction

2021-10-28 7:25:03

1. Sequences

Copy a list 

listC = list(listA) / listA[:]

def sum_nums(nums):
"""Returns the sum of the numbers in NUMS.
>>> sum_nums([6, 24, 1984])
2014
>>> sum_nums([-32, 0, 32])
0
""" 
if (nums == []):
    return 0
else:
    return nums[0] + sum_nums( nums[1:] )

When recursively processing lists, the base case is often the empty list and the recursive case is often all-but-the-first items.

If a recursive function needs to keep track of more state than the arguments of the original function, you may need a helper function.

 2. Data abstraction 

data abstraction lets us manipulate compound values as units, without needing to worry about the way the values are stored. 拆分compound

A pair abstraction

If we needed to frequently manipulate "pairs" of values in our program, we could use a pair data abstraction.

pair(a, b)constructs a new pair from the two arguments.
first(pair)returns the first value in the given pair.
second(pair)returns the second value in the given pair.

Rational numbers

Constructorrational(n,d)constructs a new rational number.
Selectorsnumer(rat)returns the numerator of the given rational number.
denom(rat)returns the denominator of the given rational number.

Reducing to lowest terms

from math import gcd def rational(n, d):

"""Construct a rational that represents n/d in lowest terms."""

        g = gcd(n, d)

                return [n//g, d//g]