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Activity 4.2 — Using .length

Introduction

Data Structures with Arrays

Lesson 2 — Access + Update + .length

Topics

Using .length to measure an array
Finding the last index with length - 1
Accessing first and last elements safely
Updating elements using index expressions
Using array data inside if/else

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Objectives

Use .length to determine how many elements are in an array.
Compute the last index using array.length - 1.
Access and update the first and last elements without hard-coding the last index.
Use an if/else statement based on array data.

Activity Tasks

Create a new project named arrays-length.
Complete each task individually.

Task 1: Class Score Snapshot

Create an array, print .length, and safely display the first and last values.
Do not hard-code the last index; use lastIndex = scores.length - 1.

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// Task 1 — "Class Score Snapshot" (Create + Access + .length)
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//
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// Goal: Use .length to safely access the last element and print a short report.
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// Step 1: Create an array named 'scores' with at least 6 quiz scores.
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let scores = [84, 91, 76, 88, 95, 82];
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// Step 2: Print how many scores are stored.
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console.log("Number of scores:", scores.length);
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// Step 3: Create a variable named 'lastIndex' equal to the last valid index.
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let lastIndex = scores.length - 1;
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// Step 4: Print the first and last score (do NOT hard-code the last index).
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console.log("First score:", scores[0]);
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console.log("Last score:", scores[lastIndex]);
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// Step 5: Print ONE summary line exactly like this format:
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// We have X scores. First: A, Last: B
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console.log("We have " + scores.length + " scores. First: " + scores[0] + ", Last: " + scores[lastIndex]);

Task 2: Score Fix + Pass Check

Update the first and last elements safely, then print the updated array.
Use if/else to print whether the last score is passing.

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// Task 2 — "Score Fix + Pass Check" (Update + Decision)
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//
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// Scenario: The first and last scores were entered incorrectly.
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// - The first score should be 90
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// - The last score should be 87
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//
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// Step 1: Update the FIRST score to 90.
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scores[0] = 90;
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// Step 2: Update the LAST score to 87 using lastIndex (do NOT hard-code).
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scores[lastIndex] = 87;
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// Step 3: Print the updated scores array.
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console.log("Updated scores:", scores);
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// Step 4: Print whether the LAST score is passing (70 or higher).
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// Print exactly ONE of these lines:
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// "Last score is passing."
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// "Last score is not passing."
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if (scores[lastIndex] >= 70) {
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  console.log("Last score is passing.");
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} else {
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  console.log("Last score is not passing.");
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}
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// Step 5 (Optional Challenge):
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// Print: Last index is X (array length is Y).
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// Use lastIndex and scores.length

Sample Output

Your program output should something similar to the sample output below.

Number of scores: 6
First score: 84
Last score: 82
We have 6 scores. First: 84, Last: 82
Updated scores: [90, 91, 76, 88, 95, 87]
Last score is passing.

Reflection Questions

You may write your reflection answers as comments at the bottom of your code.

Why is array.length - 1 the last valid index?
What is wrong with scores[scores.length]?
Why is it better to use lastIndex instead of hard-coding a number?

Submission

Submit your activity and reflection answers to the appropriate dropbox.