📝 Week 2 - Lab Intro

In this lab introduction we will briefly revisit some of the key topics from lecture and discuss the map function, which is used in the lab assignment.

Strings

The string data type stores a string of characters. Create a string by enclosing text in single or double quotes.

s1 = "This is an example string"
s2 = 'So is this'

Find the string length with len().

alphabet = "abcdefghijklmnopqrstuvwxyz"
print(len(alphabet))

The position of each letter in a string is called its index and starts at 0

You can also use negative indices. A negative index counts from the end.

# Get the 1st letter of the alphabet 
print(alphabet[0])

# Get the 4th letter of the alphabet 
print(alphabet[3]) 

# Get the 26th letter of the alphabet 
print(alphabet[25])

# Get the 26th letter of the alphabet
print(alphabet[len(alphabet)-1])

# Get the 26th letter of the alphabet
print(alphabet[-1])

# Get the 25th letter of the alphabet
print(alphabet[-2])

# Get the 1st letter of the alphabet
print(alphabet[-len(alphabet)])

You can slice strings by supplying indices in square brackets.

To slice a list L from a start index start up to (but not including) index stop with a step size of step, do the following:

L[start:stop:step]

# Slice from index 0 up to (not including 5)
print(alphabet[:5]) 

# You can also write this as:
print(alphabet[0:5]) 

# Slice from index 5 to the end
print(alphabet[5:])

# You can also write this as:
print(alphabet[5:len(alphabet)])

# Slice from index 0 to the end, stepping by 2
print(alphabet[::2])

# You could also write this as:
print(alphabet[0:len(alphabet):2])

# Reverse a string
print(alphabet[::-1])

You can use f-string formatting to easily include variable values in a string

year = 2024
year_message = f"The current year is {year}."

print(year_message)

Lists and dictionaries

Lists are ordered collections of data. Dictionaries are a data structure that have (key, value) pairs.

# this is a list
L = ["abc", 42, True] 

# this is a dictionary
D = {
    "color": "green",
    "number": 12,
    "result": True
}

Square brackets are used to access values in a list or dictionary.

# get the list item at index 1
print(L[1])

# get the dictionary item at key "color"
print(D["color"])

Map function

We can use the map function to apply the same function to all elements of a list.

Example: squaring a list of numbers

# a simple fucntion that squares a number
def square(x):
    return x**2 

L = [0,1,2,3,4,5,6,7,8,9,10]

print(map(square, L))

The map function returns a map object that can be converted to a list as follows:

squared_L = list(map(square, L))
print(squared_L)