This module includes:

1. Start with strings

We learned that string is a sequence of characters. You can treat it like a list of single-character elements.

▷ Indexing

Use square brackets [] to retrieve a single character. Indices start at 0; negative indices count from the end.

sample = "sample"

print(sample[0])    # → 's'   (first character)
print(sample[1])    # → 'a'   (second character)
print(sample[-1])   # → 'e'   (last character)

▷ Slicing

Retrieve a substring by specifying a start and end index: [start:end] grabs characters at positions start up to (but not including) end.

s = "sample"

print(s[0:1])      # → 's'
print(s[0:2])      # → 'sa'
print(s[1:-1])     # → 'ampl'
print(s[1:])       # → 'ample'
print(s[:2])       # → 'sa'

▷ Concatenation

Join strings with the + operator:

a = "super"
b = "awesome"

print(a + b)         # → 'superawesome'
print(a + " " + b)   # → 'super awesome'

▷ Membership test

Use the in operator to check if a substring exists:

word = "awesome"

if "a" in word:
    print("Contains 'a'!")   # → Contains 'a'!
else:
    print("No 'a' found.")

2. Lists

A list is an ordered, mutable collection of any Python objects. Lists use square brackets and commas.

platforms = ["Windows", "Mac", "Linux"]

▷ Indexing and slicing

Just like we used indexing and slicing for strings, lists also support the same rules:

print(platforms[1])    # → 'Mac'
print(platforms[1:])   # → ['Mac', 'Linux']

▷ Concatenation

Combine lists with +:

numbers = [1, 2, 3]
combined = platforms + numbers
print(combined)
# → ['Windows', 'Mac', 'Linux', 1, 2, 3]

▷ Mutability

  • .append(x) adds an element to the end.
  • lst[i] = x replaces the element at index i.
platforms.append("Android")
print(platforms)
# → ['Windows', 'Mac', 'Linux', 'Android']

platforms[0] = "FreeBSD"
print(platforms)
# → ['FreeBSD', 'Mac', 'Linux', 'Android']

3. Conditional Statements

Branch your code using if, elif, and else. Conditions use comparison or membership operators.

x = 2

if x < 2:
    print("x is less than 2")
elif x == 2:
    print("x is exactly 2")   # → x is exactly 2
else:
    print("x is greater than 2")
  • Examples with strings and lists:
# String comparison
s = "python"
if s == "python":
    print("Match!")           # → Match!

# Membership test in list
langs = ["Python", "Java", "C++"]
if "Python" in langs:
    print("Found Python!")   # → Found Python!

4. Loops

Loops repeat a block of code for each item in an iterable.

▷ for loops

for loops are used when you know how many times you want to repeat something. It iterates over a sequence (like a list, string, or range).

words = ["read", "code", "repeat"]

for w in words:
    print(w)
# → read
# → code
# → repeat

You can nest conditionals inside loops:

for w in words:
    if "e" in w:
        print(w)
# → read
# → repeat

▷ while loops

while loops are used when you want to keep repeating until a condition is no longer true. The number of repetitions is not fixed in advance.

count = 0
while count < 3:
    print(count)
    count += 1
# → 0
# → 1
# → 2

# you might want to test the different between for vs. while loops by running the following code!
words = ["read", "code", "repeat"]
while w in words:
    print(w)

5. Exercises

  1. String indexing and slicing

    • Assign "programming" to s.
    • Print the first character, last character, and substring "gram".

  2. List operations

    • Create fruits = ["apple", "banana", "cherry"].
    • Append "date", replace "banana" with "blueberry", and print fruits[1:].

  3. Conditional logic

    • Given n = 7, write an if block that prints whether n is even or odd.

  4. Loop practice

    • Write a for loop over the list [2, 4, 6, 8, 10] that prints each number divided by 2.

  5. Combined task

    • Write a function filter_long_words(words, min_length) that returns a list of words whose length ≥ min_length.
    • Test it on ["data", "analysis", "python", "AI"] with min_length=5.