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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 = ''
tip

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
tip

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:

info

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
tip
  • 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