In computer science, the term character refers, roughly, to a symbol that appears on the keyboard. The following are all characters:

a digit, e.g., '3'
a letter, e.g., 'w'
a period, comma or other punctuation
   space, tab, carriage return e.g., newline is denoted as: '/n'

A string refers to a sequence of one or more characters. In Python, A string literal is defined within single or double quote:

    'david'
    'abc'
    "hello there"
    "327"

"327" is not the number 327. Check out what happens with the following code:
    x = "327"*2
    y = "54"+3

You can use raw_input to read in a string from the user:

   s = raw_input('please enter a string:')

You can use type conversion functions to switch back and forth:
    x = int("345")   # converts string "345" to integer 345
    s = str(345)       # converts integer 345 to string "345"

Internal Representation of a String

Internally, strings are represented using the ASCII or Unicode mapping tables. Each symbol is mapped to a number. For instance,

'a' is represented with 97
'b' is represented with 98

Each string ends with the end-of-string character '/0', which is 0.

ASCII mapping table: http://www.lookuptables.com/ and conversion tool: https://www.branah.com/ascii-converter

What is the ASCII representation of 'cat'?

Unicode is a superset of ASCII, and the numbers 0–128 have the same meaning in ASCII as they have in Unicode. Larger numbers represent symbols in other language.

ASCII: 8 bits per character, Unicode: 16 bits per characters

Python provides the functions ord and chr to convert between number and character
x= ord ('a') # returns 97
c= chr(97) # returns 'a'

Looping through a string

for and in can be used to loop through a string:

for c in s:
# do something with each character c

Demo: Count the lower case letters in the string s

Looping with Indexes

The term "index" refers to a position in a string or a list. If you have the string "xyz", "x" is at position 0, "y" is at position 1, and "z" is at position 2.

You can refer to the ith element in a string or list with brackets, e.g.,

letters = "xyz"
c = letters [1] # what is c?

We'll often use a for loop and a variable named "index" or "i" to loop through elements, e.g.,

for i in range (len(letters)):
# do something with letters[i]

Suppose you wanted to print each letter of a string on a different line, with the number in front:

for i in range (len(letters)):
print i, letters[i]

Demo:

1. print the index of each occurrence of 'z' in a string

2. Count the number of matches in two strings, e.g., if string1="abcdef" and string2="abxdex" the program would print 4.

Building/Modifying a String

You can concatenate (merge) strings with +

"abc"+"def" is "abcdef"

Using +, you can build a string

    s=""
    while (something):
        s = s + item

Demo: Given n, a number input by the user, create a string of n blank spaces.

Modifying a String

You cannot modify a string by using an index. Suppose you wanted to replace the 2nd character of a string. You can't do it simply:

    s ="abcdef'
s[2]='d' # this will give an error

Because of this rule, it is said that string variables in Python are "immutable".

You can get a slice of a string. For instance, the following code gets the first two characters of a string:

startS=s[0:2] # this gives 'ab'

To replace a character, you can concatenate the slice before the character, with the replacement and the slice after the character.

Problems

0. How is "24" represented internally? You'll need the mapping table at: http://www.lookuptables.com/

1. AsciiConvert.py: Write a program that lets the user input a string and prints out the ASCII number equivalent to that string, e.g.,

Please enter a string: abab
97
98
97
98

2. FindIndexes.py: Write a program that lets the user input a string and a symbol, and lists the indexes of all occurrences of the symbol in the string, e.g.,

Please enter a string: xyzxxyz
Please enter the symbol to count: z

z found at index 2
z found at index 6

3. RandomString4: Write a program that creates a random string of length 4 and consisting of the characters 'A' and 'B'. Valid strings include AAAA, BAAA, ABAB.

4. Palindrome.py. Write a program that prints a string backward and says if the string is a palindrome, e.g.,

Please enter a string: aba
the string backward: aba
The string is a palindrome

5. ReplaceX.py. Write code to replace the fourth character of a string with the character "&". So if you have:

    s = "abcdefgh"
    # put your code here
    print s   # this should print "abc&efgh"

You'll need to get slices of the string, e.g., s[0:4].

6. ReplaceX2.py. Write code that replaces the ith character of the string s with a replacement r. The user should enter s, i, and r, e.g.,

please enter a string s: abcdef
please enter the index to replace: 3
please enter replacement character: 8
The new string is abc8ef