Values, types, and variables

What is a program?

A program is a set of instructions a computer follows to work with data. A variable gives a name to a value, a condition chooses what to do, a loop repeats work, and a function groups instructions you can reuse. We will introduce each of these before using them in an algorithm.

Example: To add three marks, store [70, 80, 90], visit each number, and add it to a running total. The final total is 240.

Understand the idea

A value is data; a type determines which operations make sense. A variable is a name bound to an object in Python. Distinguish integers, booleans, strings, and missing values before writing an algorithm.

Work through an example

In largest([4,1,7,2]), values names a list, value names an integer on each visit, and best changes from 4 to 7. None represents no maximum for empty input; it is different from zero.

Time and space complexity

Reading an integer is constant work in the usual bounded-number model; very large Python integers have digit-dependent costs.

Why it works

Every assignment gives the name its latest meaning; it does not retroactively change previous integers.

Common mistakes

Do not mix text '7' with numeric 7 in comparisons.
An empty collection needs an explicit return contract.

Chapter lab · Python implementation

Our running task is to find the largest value in a sequence. A program turns a precise input/output contract into small state changes. Read the example from the top, predict each update, and run its assertions. Python examples use ordinary built-in types; statements inside an indented block belong to that block.

This shared lab connects the chapter's topics. Follow the current lesson's worked example first, then trace the complete implementation. Additional algorithms described in topic exercises are variations you can implement independently.

def largest(values):
    if not values:
        return None
    best = values[0]
    for value in values[1:]:
        if value > best:
            best = value
    return best

def largest_without_slice(values):
    if not values:
        return None
    best = values[0]
    for i in range(1, len(values)):
        best = max(best, values[i])
    return best

Run and test the lab

Copy the implementation and these assertions into a Python 3 file and run it. An assertion failure means the actual result differs from the expectation. Add the current topic's exercise and boundary tests to deepen your understanding.

assert largest([]) is None
assert largest([-8, -3, -5]) == -3
assert largest_without_slice([4, 1, 7, 2]) == 7
assert largest_without_slice([0]) == 0

CHECK YOUR UNDERSTANDING

What is a program?

Start with a real element, not an invented zero.