String

String is a collection of items of same data type stored at contiguous memory locations.

(deja vu enough?) Well, string is array.

Python strings are "immutable" which means they cannot be changed after they are created.

Declare

To declare a string, you can assign variable to empty string

s = ''

mẹo

You can use s = '' to delete all characters of the string

Basics

Since, string is array, it has array's basic function.

Get array size

Use len(s)

s = 'Python'
len(s)


> s: 6

Check empty string

Check the len.

s = 'Python'
if len(s) == 0:
  print('Nothing is here!')
else:
  print('String is not empty!')

Insert

Use slicing array. Common pattern is str[:pos] + something + str[pos:]

s = 'Python'
s2 = ' is great!'
s = s[:6] + s2 + s[6:]


> s: 'Python is great'

Delete

Same as insert, use slicing array to delete. Common pattern is str[:start] + str[end:]

s = 'Python is great'
s = s[:6] + s[14:]

> s: 'Python'

Find

Use find(string). You can user find(string, start, end) to find in a specific range.

s = 'Python'
f = s.find('t')
nf = s.find('z')
r = s.find('t', 0, 1)


> f: 2
> nf: -1
> r: -1

Get sub-string

Use str[start:end]

'Python'[1:3]


> yt

Concat

Use + operator.

'Python ' + 'is great' + '!'


> Python is great!

Validation of string

ASCII tables

Lookup for ASCII table here or simply here.

Check if character is a digit

You can use ord(char) to get ASCII decimal number of a character.

Based on the ASCII table, you can see digits will have decimal number from 48, which is 0 to 57, which is 9.

if ord(str[0]) >= 48 and ord(str[0]) <= 57:
    # it is a digit, do something

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Remember number for ASCII '0' is 48.

There are digits from 0 to 9. So you can add more up to 9, which come up to 57.

You can use chr(number) to convert ASCII code to a character.

zero = chr(48)


> 0

Built-in functions

Python has some built-in libraries to do fun thing on string. How to use them is alike. Below is some examples:

thông tin

Python built-in functions can not only check a character but also a whole string.

isalpha

Use built-in function str.isalpha(). It will return true if string is alpha, false if not alpha or empty.

str = 'Python is great!'
res = str[1].isalpha()


> res: true

isdigit

islower

isupper

Normalization

string to number

• int(string): int to string.
• float(string): float to string.

number to string

str(value): number to string

case-sensitive

Use built-in functions:

• char.lower(): returns a string where all characters are lower case.
• char.upper(): returns a string where all characters are upper case.

or you can change its ASCII number

s = "python"
c = chr(ord(s[2]) - 32)


> c: T

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• char_ascii + 32: transform to lowercase
• char_ascii - 32: transform to uppercase

Input & output

In Python,

s = input() # then insert "Python is fun!"


> s: Python is fun!

To get each word, you can use string.split()

line = input().split() # insert "Python is fun!"
word = line[0]


> word: Python