Kurage Notes

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Sequences (L11) ​

Two basic conditions of a sequence:

  1. Length
  2. Element selection

Lists ​

List is a kind of compound value, a container.

python
>>> list1 = [1, 1, 4, 5, 1, 4]
>>> for i in range(len(list1)): print(list1[i])
1
1
4
5
1
4

Operations ​

Info ​

python
>>> len(list1)
6
>>> list1[2], operator.getitem(list1, 2)
2, 2

The index of a list is "zero-based", it should be viewed as the offset to zero:

python
>>> list1[0]
1
>>> list1[-1]
4
>>> list1[-8]
1

Concatenation and Repetition ​

python
>>> [3, 6, 4] + list1
[3, 6, 4, 1, 1, 4, 5, 1, 4]
>>> list1 * 2
[1, 1, 4, 5, 1, 4, 1, 1, 4, 5, 1, 4]
"""
add and mul also works
"""

Nested Lists ​

python
>>> nested = [[8, 1, 0], [8, 9, 3]]
>>> nested[1]
[8, 9, 3]
>>> nested[1][2]
0

in and not in: Membership ​

python
>>> a = [1, 3, 5]
>>> 1 in a
True
>>> 3 not in a
False

in looks for individual elements

Slicing ​

A slice of a sequence is any contiguous span of the original sequence, designated by a pair of integers.

python
>>> digits = [1, 8, 2, 8]
>>> digits[0:2]
[1, 8]
>>> digits[1:]
[8, 2, 8]
>>> digits[:]
[1, 8, 2, 8]
>>> digits[1:-1]
[8, 1, 8]

Aggregation ​

sum(iterable[, start]) ​

python
'''
sum(iterable[, start])
Add all elements in ITERABLE to START (default 0)
'''
>>> sum([[1, 2, 3], [4]], [])
[1, 2, 3, 4]

max(iterable[, key=func]), min(iterable[, key=func]) ​

python
"""
max(iterable[, key=func])
Return the element X in the ITERABLE that makes the return value of KEY(X) largest (default lambda x: x)
"""

all(iterable), any(iterable) ​

python
"""
Return True if bool(x) is True for all values x in the iterable.
If the iterable is empty, return True.
"""

For Statement ​

python
>>> RIM = ["YJSP", "RU", "YMN", "BNKRG", "KNN", "SNNN"]
>>> for i in RIM:
... 	print(i)
... 
YJSP
RU
YMN
BNKRG
KNN
SNNN

Execution Rule ​

A for statement consists of a single clause with the form:

python
for <name> in <expression>:
    <suite>

A for statement is executed by the following procedure:

  1. Evaluate the header <expression>, which must yield an iterable value.
  2. For each element value in that iterable value, in order:
    1. Bind <name> to that value in the current frame.
    2. Execute the <suite>.

Sequence Unpacking ​

python
pairs = [[1, 1], [4, 5], [1, 4], [1, 9], [1, 9]]
same = count = 0
>>> for x, y in pairs:
... 	if x == y:
... 		same_count += 1
>>> same_count
1
  • The sequences in the parent sequence must be in fixed-lengt
  • A name for each element in a fixed-length sequence

Range ​

A range is a sequence of consecutive integers.

python
>>> range(-2, 2) # Integers in [-2, 2)
range(-2, 2)
>>> list(range(-2, 2))
[-2, -1, 0, 1]
>>> list(range(4))
[0, 1, 2, 3] # Starting at 0
  • Length: ending value - starting value
  • Element selection: starting value + index

If you don't want use the name in for ​

python
>>> for _ in range(3):
... 	print("ok")
ok
ok
ok

Use the underline _ as a blank to indicate that you don't want to use the name.

Sequence Processing ​

List Comprehensions ​

python
>>> [x for x in range(5)]
[0, 1, 2, 3, 4]
>>> [x + 1 for x in range(5)]
[1, 2, 3, 4, 5]
>>> [x for x in range(5) if 8 % x == 0]
[1, 2, 4]

HOF ​

python
>>> def apply_to_all(map_fn, s):
... 	return [map_fn(x) for x in s]
>>> def keep_if(filter_fn, s):
... 	return [x for x in s if filter_fn(x)]

Conventional Names:

python
apply_to_all = lambda map_fn, sequence: list(map(map_fn, sequence))
keep_if = lambda filter_fn, sequence: list(filter_fn, sequence)

reduce ​

python
>>> def reduce(reduce_fn, sequence, initial):
... 	reduced = initial
... 	for x in sequence:
... 		reduced = reduce_fn(reduced, x)
... 	return reduced
... 
>>> reduce(mul, [2, 4, 8], 1)
64 # 1 * 2 * 4 * 8

recursively apply a function reduce_fn Conventional Name:

python
>>> from functools import reduce
>>> from operator import mul
>>> def product(s):
... 	return reduce(mul, s)
... 
>>> product([1, 2, 3, 4, 5])
120

String ​

String also supports length and element selection

python
>>> city = 'Shanghai'
>>> len(city)
8
>>> city[4]
g

Strings can be executed by exec(string)

Membership ​

in matches substrings

python
>>> 'here' in "Where?"
True

Multiline Literals ​

Span multilines by triplequote '''

python
>>> """The Zen of Python
... claims, Readability counts.
... Read more: import this."""
'The Zen of Python\nclaims, "Readability counts."\nRead more: import this.'

String Coercion ​

python
>>> yjsp = [1, 1, 4, 5, 1, 4]
>>> str(yjsp) + ' yjsp'
'[1, 1, 4, 5, 1, 4] yjsp'

Executing an Expression in the String Format ​

eval(expression)

Dictionaries ​

Collections of key-value pairs

python
>>> value1, value2 = 1, 2
>>> dic = {'key1': value1, 'key2': value2}
>>> dic['key1']
1
>>> list(dic)
['key1', 'key2']
>>> list(dict.keys())
['key1', 'key2'] # The same
>>> 'key1' in dic
True
>>> dic.values()
dict_value([1, 2]) # It is a iterable value
>>> list(dic.values())
[1, 2]

dict.get(key[, default]) method ​

python
>>> dic.get('key1')
1
>>> dic.get('key3') # Returns nothing
>>> dic.get('key3', 114514) # If the key 'KEY3' doesn't exist, return the next parameter 114514

Dictionary Comprehensions ​

python
{<key exp>: <value exp> for <name> in <iter exp>[ if <filter exp>]}