Strings

Single quoted strings and double quoted strings.
- Let's try the next example.
```
verse0 = "What a wonderful world!"
print(verse0)
verse1 = 'What a beautiful world!'
print(verse1)
```
- How are they different? A single quote symbol can be included in double quoted strings. In the reverse way, a double quote symbol can be included in single quoted strings.
  Let's try the next example.
```
verse0 = "What's your name?"
print(verse0)
verse1 = 'The color of rose: "Red"'
print(verse1)
```
- How to include a single quote symbol in single quoted strings? How to include a double quote symbol in double quoted strings?
- Escape sequences come for the above questions.
  - \' - single quote character
  - \" - double quote character
  - \n - new line character
  - \t - tab character
  - \\ - back slash, '\', character
  - Let's try the next example.
```
verse0 = "What's your name?"
print(verse0)
verse1 = 'What\'s your name?'
print(verse1)
print('\n\n')
verse2 = 'The color of rose: "Red"'
print(verse2)
verse3 = "The color of rose: \"Red\""
print(verse3)
verse4 = "What a beautiful world!\nWhat a beautiful world!"
print(verse4)
```
String over multiple lines
- How to define a string over multiple lines? Let's try the next example. How are the two examples different?
```
verse0 = "What a beautiful world!\
What a wonderful world!"
print(verse0)
verse1 = "What a beautiful world!\n\
What a wonderful world!"
print(verse1)
```
- How to define a string over multiple lines? Let's try another example of three single quotes.
```
verse0 = '''What a beautiful world!

What a beautiful world!'''
print(verse0)
```
- \n\ at the end of line can be ommitteed when we use three single quoted strings.

Comments over multiple lines

We have used single line comments using #.

print("What a beautiful world!\nWhat a wonderful world!")  # What will be printed?

How to use multiple lines for a comment? Let's try the next example.

"""
printBoard(board)
  board: n x n game board
  return: 
"""
def printBoard(board):
    for r in range(len(board)):
        for c in range(len(board)):
            print(board[r][c], end='')  # end='' gives no new line
            if c == 0 or c == 1:
                print('|', end='')
        print()  # new line
        if r != 2:
          print('-+-+-')

'''
An example board, and
print the board
'''
board = [[' ', 'O', ' '], ['X', ' ', 'X'], [' ', ' ', ' ']]  # a tic-tac-toe board
printBoard(board)

Multiple line comments - anything in between """ and """, or anything in between ''' and '''

Strings as sequence data type like lists and tuples

How to access each character in a string - indexing

verse = "What a wonderful world!"
print(verse[2])

How to slice a substring from a string - slicing

verse = "What a wonderful world!"
print(verse[7:16])  # 9 (= 16 - 7) characters from verse[7]
print(verse[:7])  # 7 (= 7 - 0) characters from verse[0]
print(verse[7:])  # all the characters from verse[7]

How to get the length of a string - len()

verse = "What a wonderful world!"
print(len(verse))  # ???

in and not in operations with strings

verse = "What a wonderful world!"
if 'winderful' in verse:
    print('Wonderful verse it is!')
elif 'winderful' not in verse:
    print('Poor verse it is!')

How to insert a string in a string

A case - We have a base sentence that includes name and age. Can we use the base sentence for different people?

Let's try the next example for the above case.

name = 'Dave'
feeling = 'Good'
sentence1 = "Hello " + name + "!" + " Are you feeling " + feeling + ' today?'
print(sentence1)
name = 'Tom'
feeling = 'Excellent'
sentence2 = "Hello " + name + "!" + " Are you feeling " + feeling + ' today?'
print(sentence2)

In the above example, sentences have to be constructed separately.

Is there any way to use a base sentence and replance name and feeling? Let's try the next example.

base = "Hello %s! Are you feeling %s today?"
print(base)
name = 'Dave'
feeling = 'Good'
sentence1 = base%(name, feeling)
print(sentence1)
sentence2 = base%('Tom', 'Excellent')
print(sentence2)

String methods

.upper() - Convert all the characters to uppercase characters
.lower() - Convert all the characters to lowercase characters

halsays = 'Good morning, Dave!'
print(halsays)
halsays.upper()
print(halsays)  # ???
halsays = halsays.upper()
print(halsays)
halsays = halsays.lower()
print(halsays)

An example how to use .upper() or .lower()

sum = 0
count = 0
while True:
    sum = sum + float(input("GPA: "))
    count += 1
    answer = input("Would you like to keep entering more GPAs? Say Yes or yes! ")
    answer = answer.lower()
    if (answer != 'yes'):  # We don't have to check 'Yes' and 'YES'
        break  # break out of the loop

average = sum / count
print("Average = " + str(average))

.isupper() - is if uppercase characters
.islower() - is if lowercase characters
.isalpha() - is if alphabets
.isalnum() - is if alphabets and numbers
.isdecimal() - is if numbers
.isspace() - is if spaces
.istitle() - is if words that begin with an uppercase letter followed by only lowercase letters

An example how to use .startswith() or .endswith()

title = "MATH 1390 Discrete Structures 2"
if (title.startswith('COMP') or title.startswith('comp')):
    print("Comp course")
elif (title.startswith('MATH') or title.startswith('math')):
    print("Math course")
else:
    print("Not Comp nor Math")
    
if (title.endswith('1')):
    print("First course")
elif (title.endswith('2')):
    print("Second course")
else:
    print("Third or above")

.rjust(), .ljust(), .center - text justification in strings

verse = "What a wonderful day!"  # 21 characters
print('--' + verse.rjust(40) + '--')
print('--' + verse.ljust(40) + '--')
print('--' + verse.center(40) + '--')
print(verse.rjust(40, '-'))
print(verse.ljust(40, '-'))
print(verse.center(40, '-'))

.rstrip(), .lstrip(), .strip - How to remove white spaces in strings

verse = "  What a wonderful day!  "
print('--' + verse.rstrip() + '--')
print('--' + verse.lstrip() + '--')
print('--' + verse.strip() + '--')

Covert a string to a list, and convert a list/tuple to a string

.join() - Converts a list or tuple of string values to a string

student = ['John', '20', 'COMP']
strstudent = ','.join(student)  # comma separated data
print(strstudent)
student = ('John', '20', 'COMP')  # a tuple
strstudent = ' '.join(student)  # space separated data
print(strstudent)
student = ['John', 20, 'COMP']  # ???
strstudent = ','.join(student)
print(strstudent)

How to convert a list of any values to a string?

student = ['John', 20, 'COMP']  # ???
studentstr = []
for item in student:
    studentstr.append(str(item))
strstudent = ','.join(studentstr)
print(strstudent)
# or
studentstr = [str(item) for item in student]  # it is interesting.
strstudent = ','.join(studentstr)
print(strstudent)

Note that any string can be a separator, and only a list or tuple of all string values is converted to a string with .join().

.split() - Converts a string to a list of string values

strstudent = 'John,20,COMP'  # comma separated data
student = strstudent.split(',')
print(student)
strstudent = 'John 20 COMP'  # space separated data
student = tuple(strstudent.split(' '))  # how to convert a string to a tuple
print(student)

Note that any string can be used as a separator with .split().

.partition() - Divides a string to a tuple of two parts with a separator

strstudent = 'John,20,COMP'  # comma separated data
student = strstudent.partition(',')
print(student)
strstudent = 'John 20 COMP'  # space separated data
student = strstudent.partition(' ')
print(student)

Note that any string can be used as a separator with .partition().

Characters as numbers
- When a character is stored in computer memories, the corresponding number, that is called as Unicode code point, of the character is stored. Note that computer memories are arrays of bytes.
- How to see the unicode code point of a character - ord(), chr()
```
print(ord('A'))
print(ord('ㄷ'))  # 'ㄷ' ???
print(chr(65))
```
- Comparison of strings - With the unicode code points, we can say which one of two strings is smaller.
```
name0 = 'Tom'
name1 = 'Hanna'
if (name0 < name1):
    print(name0 + ' is smaller than ' + name1)
else:
    print(name0 + ' is not smaller than ' + name1)
```
Programming exercises
- Write a program that prints the next triangle using text justification.
```
    *
   ***
  *****
 *******
*********
```
- Write a program that prints the next triangle using text justification.
```
*****
 ****
  ***
   **
    *
```
- Write a program that reads a string and converts it to a tuple. E.g., '(10,M)' should be convergted to a tuple ('10','M').
References
- Automate the boring stuff with Python - Manipulating strings

7. Strings