Introduction to Programming

This section contains some new information about using python strings.

"Arithmetic" on strings

You've seen the + concatenation operator, which naturally can be extended to the += shorthand operator.

name = "Iron"
# Concatenate "Man" to end of "Iron", reassigning to name:
name += "Man"  #name now stores "IronMan"
# Convert int 3 to string "3", concatenate to end of "IronMan", reassigning to name:
name += str(3)
print(name) # outputs IronMan3

An uncommonly-used but cute feature of python is multiplying a string by an integer. This results in the string being repeated multiple times.

print("Hello!"*5)

outputs

Hello!Hello!Hello!Hello!Hello!

Accessing individual elements of strings

Many of the operations you've learned on lists also work on strings (and some do not).

The len() function returns the number of characters in a string.

greeting = "Welcome to the wonderful world of strings."
print(len(greeting))  # outputs 42, the number of characters in greeting

Just like lists, strings are ordered. So we can use an index to access individual characters in a string. Similarly, they can be sliced.

greeting = "Welcome to the wonderful world of strings."
print(greeting[0])   # outputs the first character, "W"
print(greeting[-1])  # outputs the last character, "."
print(greeting[:10]) # outputs first 10 characters, "Welcome to"
print(greeting[-5:]) # outputs from 5th to last up to last, "ings."

Iterating over the characters in a string (for-loops)

You can use a for loop to traverse the characters in a string, either by iterating over the index values, or iterating over the characters themselves.

Here's an index-based loop that counts the number of "o"s in the string:

count = 0  # initial value, zero o's
# iterate over all index values. i ranges from 0 to 41
for i in range(len(greeting)):
    # greetings[i] is the next character in the string
    if greeting[i] == "o":
        # if the current character is an "o", count it
        count += 1
# output the formatted result
print(f"Number of o's in the greeting is {count}.")

The code below is the content-based version of the same code. This time the variable character iterates over each character in the greeting, starting from W and ending with ., the last character. The variable character takes the value of each character in turn, so we don't need worry about the length of the string, nor keep track of the value of the index.

count = 0  # initial value, zero o's
# iterate over all characters, from "W" to "."
for character in greeting:
    # character is the next character in the string
    if character == "o":
        # if the current character is an "o", count it
        count += 1
# output the formatted result
print(f"Number of o's in the greeting is {count}.")

Strings are immutable

Unlike lists, strings are immutable. This means their contents cannot be changed. While I can access an individual character in a string with an index in square brackets, attempting to modify an individual character fails with a TypeError

greeting = "Welcome to the wonderful world of strings."
greeting[0] = 'V'

outputs

TypeError: 'str' object does not support item assignment

This doesn't mean that the value stored in a variable can't be changed. But because of the immutability of strings, we must re-assign a new string to the variable, rather than changing the contents of the string itself. Here's one possible way to do this:

greeting = "Welcome to the wonderful world of strings."
# Build an entirely new string, starting with "V", then concatenating all but the first character of the original
greeting = "V" + greeting[1:] 
print(greeting)

outputs

Velcome to the wonderful world of strings.