AP CSA 2.1 Exercise 1: Choosing Without an if

Unit 2: Selection and Iteration · Lesson 2.1 · Exercise 1

Choosing Without an if

Algorithms with Selection and Repetition. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 2.1 page All Unit 2 lessons

Why this one is worth doing

Selection and repetition are ideas before they are keywords. This exercise makes a real choice and does a real repetition without using an if or a loop, so the ideas arrive before the syntax does.

What to write

Read three integers: a, b and c.
Print the largest of the three using Math.max, which selects without an if statement.
Print the smallest using Math.min.
Print the middle value, which is the total minus the largest minus the smallest.
Read one more integer n, then print the sum of every whole number from 1 to n using the closed form n * (n + 1) / 2, which does a repetition without a loop.

Your program reads

Four integers on separate lines: a, b, c, then n.

Your program prints

Four lines: the largest, the smallest, the middle value and the sum from 1 to n.

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

Example 1 output

Example 2 input

Example 2 output

Your answer

Main.java

import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int a = input.nextInt();
    int b = input.nextInt();
    int c = input.nextInt();
    int n = input.nextInt();

    // Four lines, and not an if or a loop anywhere.
  }
}

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

Math.max takes two arguments, so three values need two calls: Math.max(a, Math.max(b, c)).

Hint 2

The middle value trick works even when two of the three are equal. Try it on 5, 5, 2 by hand and see.

Hint 3

n * (n + 1) is always even, so the / 2 is exact and integer division loses nothing here.

Where to go next

Rework the Lesson 2.1 page Pick another Unit 2 lesson

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