Python[CS61A] Sequences: Videos 정리

Lists 영상

odds = [41, 43, 45, 47]
odds[0]
odds[3] #element selection expression, evaluate the index
len(odds) #call expression

odds[3] - odds[2]
#이걸 index로도 이용 가능
odds[odds[3] - odds[2]]

#list literaLs
digits = [1, 8, 2, 8]
#elements selection syntax
digits[3]
#getitem function (operator module)
getitem(digits, 3)

#Concatenation and Repetition
[2, 7] + digits * 2
add([2, 7], mul(digits, 2))

#nested lists
pairs = [[10, 20], [30, 40]]
pairs[1]
pairs[1][0]

---

Containers  영상

One value that contain others, will an element appear in a list?

 

Container: Built in operators for testing whether an element appears in a compound value.

digits = [1, 8, 2, 8]
# in operator
1 in digits # True
8 in digits # True
5 not in digits # True
'1' in digits # False
[1, 8] in digits # False
[1, 2] in [3, [1, 2], 4] # True
[1, 2] in [3, [[1, 2]], 4] # False

---

For Statements 영상

def count(s, value):
    """Count the number of times that value occurs
    in sequence sequence s.
    """
    total, index = 0, 0
    while index < len(s):
        element = s[index]
        
        if element == value:
            total += 1
        
        index += 1
    return total

count([1, 2, 1, 2, 1], 1)
# output: 3

 

for statement을 이용하면:

def count(s, value):
    """Count the number of times that value occurs
    in sequence sequence s.
    """
    total = 0
    for element in s:
        if element == value:
            total += 1
    return total

count([1, 2, 1, 2, 1], 1)
# output: 3

going to execute a suite of the statement a number of times, which is the number of elements in s.

element will be bound to a different element of s each time

 

For Statement Execution Procedure

pairs = [[1, 2], [2, 2], [3, 2], [4, 4]]
same_count = 0

for x, y in pairs:
    if x == y:
        same_count = same_count + 1

 

---

Ranges 영상

Range is a sequence of consecutive integers.

 

Length: Ending value - starting value

Element selection: starting value + index

 

List constructor: convert to a list when we want to see the elements

range(n)이면 n이 ending value로 취급된다.

range(-2, 2) # -2, -1, 0, 1

list(range(-2, 2)) # [-2, -1, 0, 1]

list(range(4)) # [0, 1, 2, 3]

 

def sum_below(n):
    total = 0
    for i in range(n):
        total += i
    return total
    
sum_below(5) 
# output:10

def cheer():
    for _ in range(3):
        print("Go Bears!")
        
# range(3) means 3 elements in range
# underscore _는 argument name을 신경 쓰지 않는다는 것

---

List Comprehensions 영상

odds = [1, 3, 5, 7, 9]
[x+1 for x in odds]
# output: [2, 4, 6, 8, 10]
[x+1 for x in odds if 25 % x == 0]
# output: [2, 6]

def divisors(n):
    return [1] + [x for x in range(2, n) if n % x == 0]
    
divisors(1)
# output: [1]
divisors(4)
# output: [1, 2]
divisors(8)
# output: [1, 2, 4]

---

Lists, Slices, & Recursion 영상

slice:

s[1:]

slice from index 1 to the end.

a list whose length is one less than the length of s

it contains all of the elements of s except s[0]

Slicing s doesn't affect s

s = [2, 3, 6, 4]
s[1:]
#[3, 6, 4]
s
# [2, 3, 6, 4]

 

Recursion example: sum

Implement sum_list, which takes a list of numbers s and returns their sum. If a list is empty, the sum of its elements is 0.

def sum_list(s):
    """
    sum([2, 4, 1, 3])
    10
    """
    if len(s) == 0:
        return 0
    else:
        return s[0] + sum_list(s[1:])

 

Recursion Example: Large Sums

def large(s, n):
    """Return the sublist of positive numbers s with the 
    largest sum that is less than or equal to n.
    
    >>> large([4, 2, 5, 6, 7], 3)
    [2]
    >>> large([4, 2, 5, 6, 7], 8)
    [2, 6]
    >>> large([4, 2, 5, 6, 7], 19)
    [4, 2, 6, 7]
    >>> large([4, 2, 5, 6, 7], 20)
    [2, 5, 6, 7]
    """
    if s == []:
        return []
    elif s[0] > n:
        return large(s[1:], n)
    else:
        first = s[0]
        with_s0 = [first] + large(s[1:], n - first)
        without_s0 = large(s[1:], n)
        if sum_list(with_s0) > sum_list(without_s0):
            return with_s0
        else:
            return without_s0