Problem Decomposition

Welcome to the second part of the Python curriculum. Until now, we learned the tools: variables, loops, functions, lists, dictionaries. Now we learn how to think with them

The most important skill in programming is not knowing the syntax. It's knowing how to break a problem into smaller steps. This is called decomposition

Let's say someone asks you: "Write a program that finds the most common word in a sentence"

If you try to write the whole thing at once, you'll get lost. Instead, think about it step by step

Step 1: How do I get the individual words from a sentence? → use .split()

sentence = "the cat sat on the mat the cat"
words = sentence.split(" ")
print(words)

Output ['the', 'cat', 'sat', 'on', 'the', 'mat', 'the', 'cat']

Step 2: How do I count how many times each word appears? → use a dictionary

counter = {}
for word in words:
    if word in counter:
        counter[word] = counter[word] + 1
    else:
        counter[word] = 1
print(counter)

Output {'the': 3, 'cat': 2, 'sat': 1, 'on': 1, 'mat': 1}

Step 3: How do I find which word has the highest count? → loop through the dictionary

best_word = ""
best_count = 0
for word, count in counter.items():
    if count > best_count:
        best_count = count
        best_word = word
print(f"Most common: {best_word} ({best_count} times)")

Output Most common: the (3 times)

Each step is simple on its own. The trick is not trying to solve everything at once. Instead:

Understand what the problem is asking
Break it into smaller sub-problems
Solve each sub-problem one at a time
Combine the solutions

This is how professional programmers think. Nobody writes a program in one go. They build it piece by piece, testing each piece along the way

Let's do another one. "Given a list of numbers, find all pairs that add up to a target"

Step 1: What does "all pairs" mean? → every combination of two numbers

numbers = [1, 3, 5, 7, 2]
target = 8

Step 2: How do I check every pair? → nested loop

for i in range(len(numbers)):
    for j in range(i + 1, len(numbers)):
        if numbers[i] + numbers[j] == target:
            print(f"{numbers[i]} + {numbers[j]} = {target}")

Output

1 + 7 = 8
3 + 5 = 8

Why range(i + 1, len(numbers))? Because we don't want to pair a number with itself, and we don't want to count the same pair twice (3+5 and 5+3 are the same pair)

Mission: Log Analyzer

The station log, stored in a variable named log, is a single string of event codes separated by spaces. Break the work into three functions, then use them together:

split_codes(log): returns the list of codes (use .split(" "))
count_codes(codes): returns a dictionary mapping each code to how many times it appears
most_common(counts): returns the code with the highest count

In the main program, call the functions in order, then print:

Codes: then the total number of codes
each code as code: count (loop over the dictionary)
Most common: then the most common code

Output

Codes: 6
alpha: 3
beta: 2
gamma: 1
Most common: alpha