Common Python Functions

Mathematical Functions

pow(x, y) computes x raised to the power y.

abs(val) returns the absolute value of val.

round(num) returns the nearest integer to num.

sqrt(n) calculates the square root of n.

input(prompt) reads a line from terminal input.

max(seq) returns the largest item in a sequence.

min(seq) returns the smallest item in a sequence.

len(seq) returns the number of items in a sequence.

Python Sequences

Indexing

Access the first element with seq[0]:

>>> greeting = 'hi,today is Sunday!'
>>> greeting[0]
'h'

Access the last element with seq[-1]:

>>> greeting[-1]
'!'

Access the second-to-last element with seq[-2]:

>>> greeting[-2]
'y'

Slicing

seq[start:end] extracts elements where start ≤ index < end:

>>> items = ['a','b','c','1','2','3']
>>> items[2:4]
['c', '1']

Step Value

The default step is 1; specify a different step as the third parameter:

>>> digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> digits[0:9:1]
[0, 1, 2, 3, 4, 5, 6, 7, 8]
>>> digits[0:9:2]
[0, 2, 4, 6, 8]
>>> digits[0:9:4]
[0, 4, 8]

Sequence Concatenation

Combine two sequences with +:

>>> list_a = [0,2,4]
>>> list_b = [1,3,5]
>>> list_a + list_b
[0, 2, 4, 1, 3, 5]

Sequence Repetition

Repeat a sequence with *:

>>> list_a = [0, 2, 4]
>>> list_a * 3
[0, 2, 4, 0, 2, 4, 0, 2, 4]

Element Deletion

Remove elements using del:

>>> values = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> del values[0]
>>> values
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> del values[0:3]
>>> values
[4, 5, 6, 7, 8, 9]

List Methods

count

Count occurrences of an element:

>>> chars = ['a', 'b', 'c', 'ac', 'bc', 'cd']
>>> chars.count('a')
1
>>> chars.count('ac')
1

append

Add an element to the end:

>>> chars.append('end')
>>> chars
['a', 'b', 'c', 'ac', 'bc', 'cd', 'end']

extend

Append multiple elements from another list:

>>> extra_list = ['extra']
>>> chars.extend(extra_list)
>>> chars
['a', 'b', 'c', 'ac', 'bc', 'cd', 'end', 'extra']

insert

Insert an element at a specific position:

>>> chars.insert(0, 'first')
>>> chars
['first', 'a', 'b', 'c', 'ac', 'bc', 'cd', 'end', 'extra']

pop

Remove and return the last element:

>>> chars.pop()
'extra'
>>> chars
['first', 'a', 'b', 'c', 'ac', 'bc', 'cd', 'end']

remove

Remove the first occurrence of a value:

>>> chars.remove('a')
>>> chars
['first', 'b', 'c', 'ac', 'bc', 'cd', 'end']

reverse

Reverse the list in place:

>>> chars.reverse()
>>> chars
['end', 'cd', 'bc', 'ac', 'c', 'b', 'first']

sort

Sort the list in ascending order:

>>> nums = [1, 5, 9, 2, 6, 8, 0]
>>> nums.sort()
>>> nums
[0, 1, 2, 5, 6, 8, 9]

Strings

String Formatting

Format specifiers control output alignment and precisino:

• % starts the format specifier
• - left-aligns the output
• + shows the sign for numbers
• Width before . sets minimum field width
• Precision after . sets decimal places

Examples:

pi = 3.141592653589793
print("number is %-10f" % pi)
print("number is %10f" % pi)
print("number is %+10f" % pi)
print("number is %010f" % pi)
print("number is %.3f" % pi)

Output:

number is 3.141593   
number is   3.141593
number is  +3.141593
number is 003.141593
number is 3.142

Common format codes:

• %d decimal integer
• %o octal integer
• %x hexadecimal integer
• %f floating-point number
• %s string

Example:

print("%s and %s are all in [a~z]" % ('a', 'b'))
print("price is %d" % 10)
print("pi is %f" % 3.1415926)
print("number 100 equals %d / %o / %x " % (100, 100, 100))

String Methods

find

Locate a substring, returning the lowest index or -1:

>>> text = 'hello,my name is a'
>>> text.find('no')
-1
>>> text.find('name')
9

join

Combine sequence elements with a separator:

>>> letters = ['a', 'b', 'c', 'd']
>>> '+'.join(letters)
'a+b+c+d'
>>> ''.join(letters)
'abcd'

lower

Convert to lowercase:

>>> phrase = 'Hello, My Name Is John'
>>> phrase.lower()
'hello, my name is john'

replace

Replace occurrences of a substring:

>>> phrase.replace('e', 'xxx')
'Hxxxllo, My Namxxx Is John'

split

Divide a string into a list using a delimiter:

>>> phrase.split(' ')
['Hello,', 'My', 'Name', 'Is', 'John']

strip

Remove leading and trailing characters (whitespace by default):

>>> s = ' hi,abc '
>>> s.strip()
'hi,abc'
>>> t = 'hooooooh'
>>> t.strip('h')
'oooooo'