1.21. builtin functions

This lesson shows the usage of builtin functions available in python. These functions are available in python without importing any module. These are not all the builtin functions but only those which are used frequently.

Important

This lesson is still under development.

slice

a = [1,2,3,4,5,6,7,8,9,10]
print(a[slice(2)])

[1, 2]

print(a[slice(2, 8)])

[3, 4, 5, 6, 7, 8]

print(a[slice(2, 8, 2)])

[3, 5, 7]

a = "This is a string"
print(a[slice(2)])
print(a[slice(2, 8)])
print(a[slice(2, 8, 2)])

Th
is is
i s

a = (1,2,3,4,5,6,7,8,9,10)
print(a[slice(2)])
print(a[slice(2, 8)])
print(a[slice(2, 8, 2)])

(1, 2)
(3, 4, 5, 6, 7, 8)
(3, 5, 7)

a = {1: 'a', 2: 'b', 3: 'c', 4: 'd', 5: 'e'}
# print(a[slice(2)])

Indeed, any python object which can be sliced using [] operator, can also be sliced using slice function. Although, [] operator and slice function look similar however they differ in their behavior. [] operator returns the values whereas slice function returns a slice object. Moreover, slice function is more readable and flexible.

zip

zip is used to iterate over two or more than two sequences.

for i,j in zip([1,2,3], [11, 12, 13, 14]):
    print(i, j)

1 11
2 12
3 13

any

vals = [1,2,3]
print(any([val>3 for val in vals]))

False

vals = [1,2,3, 4]
print(any([val>3 for val in vals]))

True

all

vals = [1,2,3,4,5]
print(all([isinstance(val, int) for val in vals]))

True

vals = [1,2,3,4,5.0]
print(all([isinstance(val, int) for val in vals]))

False

vals = [1,2,3,4,5.0]
print(all([isinstance(val, (int, float)) for val in vals]))

True

vals = [1,2,3,4,'5.0']
print(all([isinstance(val, (int, float)) for val in vals]))

False

Question:

What is the type of the output returned by any and all functions?

sorted

vals = ['a', 'b', 'c']
for val in sorted(vals):
    print(val)

a
b
c

vals = ['a', 'c', 'b']
for val in sorted(vals):
    print(val)

a
b
c

vals = [1,5,3,10.0]
for val in sorted(vals):
    print(val)

1
3
5
10.0

reversed

vals = ['a', 'b', 'c']
for val in reversed(vals):
    print(val)

c
b
a

enumerate

vals = ['a', 'b', 'c']
for idx, val in enumerate(vals):
    print(idx, val)

0 a
1 b
2 c

Question:

What will be the output of the following code?

names = ['jamaludin', 'zaynaldin', 'nurullah shustari', 'kamil dehlavi', 'baqir sadr]
years = ['1385', '1558', '1610', '1809', '1980']
for (idx,name), year in zip(enumerate(names), years):
    print(idx, name, year)

Question:

Consider the following list

dob_years = ['1334', '1506', '1542', '1700s', '1935']

Now modify the for loop in the previous example to also iterate over dob_years list along with names and years.

map

Suppose we have function which takes an input and returns square of it

def square(x):
    return x**2

Now if we want to call this function on several values, we can make a list of all the values on which we want to call it

vals = [1,2,3,4,5]

and then we can call this function in a loss

for val in vals:
    print(square(val))

1
4
9
16
25

An alternative to calling the function in an explicit for loop is to make use of map function.

mapper = map(square, vals)

The map function returns an iterator which we can be converted into a list

print(type(mapper))

<class 'map'>

from collections.abc import Iterator
print(isinstance(mapper, Iterator))

True

We can extract all values from iterator by calling list method on it.

list(mapper)

[1, 4, 9, 16, 25]

we can also provide any builtin function as first argument to map

list(map(float, vals))

[1.0, 2.0, 3.0, 4.0, 5.0]

we can also use map with functions which take multiple input input arguments.

def power(x, n):
    return x**n

vals1 = [1,2,3,4]
vals2 = [1,2,3,4]
list(map(power, vals1, vals2))

[1, 4, 27, 256]

Map has several advantages over an explicit for loop e.g

• It is fast since it is written in C

• It is memory efficient as it returns an iterator

Question:

Convert the years in following list from Hijri to Gregorian 1 calenden using map function

hijri_years = [40, 50, 61, 95, 114, 148, 183, 203, 220, 254, 260]

1

https://github.com/dralshehri/hijridate