AP CSA 4.3 Exercise 1: Build It, Index It

Unit 4: Data Collections · Lesson 4.3 · Exercise 1

Build It, Index It

Array Creation and Access. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 4.3 pageAll Unit 4 lessons

Why this one is worth doing

An array of length n has indexes 0 through n minus 1, and asking for index n is the single most common runtime error in this unit. Two of the lines here are about the ends of the array for that reason.

What to write

  1. Read an integer count, then create an int array of that length.
  2. Read that many values into the array, in order.
  3. Print the length of the array.
  4. Print the FIRST element, then the LAST element, each on its own line.
  5. Print the element at the middle index, which is the length divided by 2 using integer division.
  6. Print the sum of the first and last elements.

Your program reads

An integer count of at least 1, then that many integers.

Your program prints

Five lines: the length, the first element, the last element, the middle element and the sum of the ends.

Worked examples

These are the cases you can see. There are more you cannot, and they use different values, so an answer that prints these numbers as constants will fail.

Example 1 input
5
10 20 30 40 50
Example 1 output
5
10
50
30
60
Example 2 input
1
7
Example 2 output
1
7
7
7
14

Your answer

Main.java

Input for the Run button

Run sends whatever is in the input box below. Submitting runs your program against every test case, including hidden ones with different input.


  

  
Stuck?

    
Hint 1
The last index is length - 1. data[data.length] is one past the end and throws ArrayIndexOutOfBoundsException.

    
Hint 2
new int[count] fills the array with zeros. Your loop is what puts the real values in.

    
Hint 3
For an array of length 5 the middle index is 2, which has two elements on each side. For length 4 it is 2, which is the right hand middle.


  
Where to go next

  

    Rework the Lesson 4.3 page
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