b.stefanuk.ca

Appearance

Python - Key Keywords and Builtins ​

Builtin Functions ​

max(A) and min(A) ​

Returns the largest (max) or smallest (min) element of A.

  • Independent elements may be with these builtins instead of an array, e.g., max(3,4,5)
python
out = min([4,7,10])
assert(out == 10)
out = min([4,7,10])
assert(out == 10)

any(A) and all(A) ​

any(A) returns true if any of the elements are non-zero, otherwise false all(A) returns true if all of the elements are non-zero, otherwise false

  • Independent elements may be with these builtins instead of an array, e.g., any(-12,0,0)
python
out_any = any([0,0,2])
assert(out_any == True)
out_all = all([1,2,3])
assert(out_all == True)
out_any = any([0,0,2])
assert(out_any == True)
out_all = all([1,2,3])
assert(out_all == True)

ord(a) ​

Returns the Unicode integer associated with character a

python
out = ord('a')
assert(out == 97)
out = ord('a')
assert(out == 97)

set(A) and frozenset(A) ​

Returns a set (of unique values) from the array A

python
out = set([1,1,2])
assert(out == {1,2})
out = set([1,1,2])
assert(out == {1,2})

filter(f, A) ​

Returns only the elements from A that aren't filtered the boolean function f

python
func = lambda x: x > 4
out = list(filter(func, [1,3,5,7,9]))
assert(out == [5,7,9])
func = lambda x: x > 4
out = list(filter(func, [1,3,5,7,9]))
assert(out == [5,7,9])

map(f, A) ​

  • Applies the function f to each element of A, returning an object of the same length
  • Additional arrays maybe used, in which case f must have the same number of arguments
python
A = [1,2,3,4,5]
out = list(map(lambda a: a**2, A))
assert(out == [1,4,9,16,25])
A = [1,2,3,4,5]
out = list(map(lambda a: a**2, A))
assert(out == [1,4,9,16,25])

hash(s) ​

  • Returns the hash code of a given immutable object.

iter(obj, end) ​

  • Returns an iterator to the object obj, an optional custom sentinal (stop value) can be passed via end
  • Use next(iterator) to call the iterator's internal __next__() method, returning the next item in the iterator
python
A = [1,2,3,4,5]
out = iter(A)
assert(next(A) == 1)
assert(next(A) == 2)
A = [1,2,3,4,5]
out = iter(A)
assert(next(A) == 1)
assert(next(A) == 2)
Tip

On variables of type dict use d[key] = d.get(key, default) to create default values for keys that may not exist instead of using an if-else statement. Alternatively use d.setdefault(key, default)

python
d = {}
d['a'] = d.get('a', 1)
d = {}
d['a'] = d.get('a', 1)

Another good approach is to simply use the collections.defaultdict(type) type where unregistered keys will take on the default of type when accessed, e.g., int=0, str='', list=[].

From functools ​

reduce(f(acc,k), A, a0) ​

  • Accumulates a value acc by applying the function f to each element k of array A
  • a0 is an initial value such that acc = a0 before the first iteration
  • f must be a function of two variables, e.g., lambda acc, k: acc*k
python
A = [1,2,3,4,5]
a0 = 0
out = reduce(lambda acc, k: acc+k, A, a0)
assert(out == 15)
A = [1,2,3,4,5]
a0 = 0
out = reduce(lambda acc, k: acc+k, A, a0)
assert(out == 15)

From itertools ​

chain(*A) ​

Flattens the array A by first unpacking it with *

python
A = [[1,2,3], [4,5,6]]
out = chain(*A)
assert(out == [1,2,3,4,5,6])
A = [[1,2,3], [4,5,6]]
out = chain(*A)
assert(out == [1,2,3,4,5,6])

Operators ​

>> and << ​

Bit Shift Operator

python
x = 9 # in binary 00001001
xr = x >> 1 # in binary 00000100 = 4
xl = x << 1 # in binary 00010010 = 18
x = 9 # in binary 00001001
xr = x >> 1 # in binary 00000100 = 4
xl = x << 1 # in binary 00010010 = 18