Broken or Fixed — Computer Science Functions

What is a function?

In computer science, a function is a reusable block of code that:

takes inputs (often called parameters or arguments),
performs a computation, and
produces an output (often called a return value).

Functions help you write code that is easier to test, reuse, and understand.

Inputs → Output

Many functions can be thought of like math: you feed in values and get a result.

// Example (JavaScript)
function add(a, b) {
  return a + b;
}

add(2, 3); // 5

Side effects

Some functions also change the outside world (printing, saving files, network calls). Those changes are called side effects.

# Example (Python)
def greet(name):
    print("Hello,", name)  # side effect: prints to screen

Broken or Fixed: a simple checklist

When you say a function is broken, it typically fails one of these:

Correctness

Returns the right output for typical inputs
Handles edge cases (empty, negative, very large)
Avoids off-by-one errors

Reliability

Doesn’t crash unexpectedly
Validates input types/ranges
Has clear error handling

Performance

Runs quickly enough for the use case
Uses memory reasonably
Scales with input size

Clarity

Readable name and parameters
Small and focused responsibility
Documented assumptions

A function is fixed when it meets the expected behavior consistently and predictably.

Examples: broken vs fixed

Example 1: division

// Broken: doesn't handle divide-by-zero
function divide(a, b) {
  return a / b;
}

// Fixed: validates input and gives a clear error
function divide(a, b) {
  if (b === 0) throw new Error("Cannot divide by zero.");
  return a / b;
}

Example 2: summing an array

# Broken: fails on empty list, and assumes numeric inputs
def sum_list(nums):
    total = nums[0]
    for n in nums[1:]:
        total += n
    return total

# Fixed: handles empty list and validates numbers
def sum_list(nums):
    total = 0
    for n in nums:
        if not isinstance(n, (int, float)):
            raise TypeError("All elements must be numbers.")
        total += n
    return total

Key ideas (quick reference)

Parameters vs arguments

Parameters are names in the function definition. Arguments are values you pass in.

Pure functions

A pure function returns the same output for the same input and has no side effects.

Big-O

Big-O describes how runtime/memory grows as inputs get larger (e.g., O(n), O(n log n), O(n²)).

Testing

Unit tests check expected inputs/outputs. Add edge cases to catch “broken” behavior early.

Practice prompts

Write a function that validates an email address format.
Write a function that returns the median of a list of numbers (handle even and odd lengths).
Write a function that parses a date string and rejects invalid dates.
Improve a function’s performance by changing an O(n²) loop to O(n) using a hash map.