Algorithms Programming with Python Module 1 – Python basics

PowerPoint Presentation Algorithms Programming with Python Module 1 – Python basics – Lesson 6 Nguyễn Chí Thức gthucString 2 A string can b.PowerPoint Presentation Algorithms Programming with Python Module 1 – Python basics – Lesson 6 Nguyễn Chí Thức gthucString 2 A string can b.

Algorithms & Programming with

Python

Module 1 – Python basics – Lesson 6

Nguyễn Chí Thức gthuc.nguyen@gmail.com

0986636879

A string can be:

• read from the standard input using the

function input()

• defined in single or double quotes

String length

A string in Python is a sequence of characters

The function len(some_string) returns how

many characters there are in a string:

print(len('abcdefghijklmnopqrstuvwxyz'))

Cast to string

Every object in Python can be converted to

string using the function str(some_object)

So we can convert numbers to strings:

s = str(2 ** 10)

print(s)

print(len(s))

A slice gives from the given string one character

or some fragment: substring or subsequence

Single character:

• S[i] gives ith character of the string

• i (index) is count starting from 0

• i can be counted from the end (negative

index), starting from -1

Slices – single character

Slices – substring

S[a:b] returns the substring of length b - a, starting with the character at index a and lasting until the character at index b, not including the

last one

S = 'Hello’

S[1:4] == 'ell' == S[-4:-1]

S[1:-1] == 'ell'

Slices – substring

• If you omit the second parameter (but

preserve the colon), then the slice goes to the

end of string: s[1:]

• If you omit the first parameter, then Python

takes the slice from the beginning of the

string: s[:-1]

• S[:] matches the string S itself.

Slices – subsequence

• S[a:b:d], the third parameter specifies the

step, same as for function range()

• only the characters with the following index

are taken: a, a + d, a + 2 * d and so

on, until and not including the character with

Immutability of string

• In Python strings are immutable

• Any slice of a string creates a new string and

never modifies the original one

.upper() and lower()

.lstrip(), rstrip() and strip()

.find() and rfind()

str.find(sub[, start[, end]] )

• If substring exists inside the string, it returns

the lowest index where substring is found

• If substring doesn't exist inside the string, it

returns -1

.find() and rfind()

quote = 'Let it be, let it be, let it be'

result = quote.find('let it')

print("Substring 'let it':", result)

.find() and rfind()

Substring 'let it': 11

Substring 'small': -1

Contains substring 'be,'

.find() and rfind()

quote = 'Do small things with great

.find() and rfind()

-1

3

-1

9

.startswith() and endswith()

str.startswith(prefix[, start[, end]])

str.endswith(suffix[, start[, end]])

• Returns True if the string starts with the

specified prefix.

• Returns False if the string doesn't start with

the specified prefix.

.startswith() and endswith()

text = "Python is easy to learn."

result = text.startswith('is easy')

.startswith() and endswith()

False

True

True

.startswith() and endswith()

text = "Python programming is easy."

result = text.startswith('programming is', 7)

print(result)

result = text.startswith('programming is', 7, 18)

print(result)

.startswith() and endswith()

True

False

True

string.count(substring, start= , end= )

• returns the number of occurrences of the

substring in the given string

2

1

str.replace(old, new [, count])

• returns a copy of the string

where old substring is replaced with

the new substring

song = 'cold, cold heart'

print (song.replace('cold', 'hurt’))

replaced_song = song.replace('o', 'e')

print (replaced_song)

song = 'Let it be, let it be, let it be, let

it be'

hurt, hurt heart

celd, celd heart

Let it be, don't let it be, don't let it be, let it be