Weight Log.

Chapter 2 — Data. In Foundations you already walked a list with for. Now we go deeper: length, index access, and appending a new item.

A list holds a sequence under one name. Square brackets [] mark the start and end. Items are separated by commas: weights = [70.0, 72.0, 74.0, 75.0]. Order matters — the first number is index 0; the last is the “newest” measurement in our example.

len(weights) returns the item count. That helps when the list grows: you do not count by hand. print(len(weights)) shows 4 for the lesson data.

Index 0 selects the first item, while -1 selects the last. append(value) adds a value at the end and changes the list:

weights.append(76.0)
print(weights[0])
print(weights[-1])

These operations matter for logs that grow. for remains a familiar useful tool, but this chapter does not teach it again from scratch.

Mini example:

nums = [1.0, 2.0]
nums.append(3.0)
print(len(nums))
print(nums[0])
print(nums[-1])

Common mistake: weights = 70.0 (one number instead of a list); or for without indentation; or using a comma as the decimal mark (70,0) — in Python it is a dot.

Today the log is the foundation: visible numbers, len, indexes, and append. Later classes will structure and transform the data.