2.1 Algorithms with Selection and Repetition

1. Example 1

   Same steps, different order

   A store's rule is supposed to be: the 10-dollar discount applies only if the order itself (before shipping) is at least 50 dollars. Which of these two segments follows the rule, and what does each print? Segment A:Javaint total = 45; if (total >= 50) { total = total - 10; } total = total + 8; System.out.println(total);Segment B:Javaint total = 45; total = total + 8; if (total >= 50) { total = total - 10; } System.out.println(total);

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   1. 1Step 1: Segment A starts with total at 45. The condition 45 >= 50 is false, so the discount is skipped. Then 8 is added: 53. It prints 53.
   2. 2Step 2: Segment B adds shipping first, so total becomes 53. Now 53 >= 50 is true, so 10 is taken off: 43. It prints 43.
   3. 3Step 3: The rule says the discount depends on the order before shipping, so the check has to happen before shipping is added. That's Segment A.

   Answer: Segment A prints 53 and Segment B prints 43. Segment A follows the rule.

2. Example 2

   Repetition until a goal

   A culture of bacteria doubles every hour. Starting from 3 bacteria, this code counts how many hours pass before there are more than 100. What does it print?Javaint bacteria = 3; int hours = 0; while (bacteria <= 100) { bacteria = bacteria * 2; hours++; } System.out.println(hours + " " + bacteria);

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   1. 1Step 1: The loop repeats while bacteria <= 100. Each pass doubles the count and adds 1 to hours.
   2. 2Step 2: Track both values after each pass: (6, 1), (12, 2), (24, 3), (48, 4), (96, 5), (192, 6).
   3. 3Step 3: After the sixth pass, bacteria is 192, so 192 <= 100 is false and the loop stops.

   Answer: It prints 6 192.

Common mistakes

• !Treating selection and repetition as the same thing. An if checks its condition once; a loop keeps checking and repeating.
• !Reading a description quickly and putting a check in the wrong place. Underline when each condition is supposed to be tested before you write the code.
• !Stopping a repetition one step too early or too late. Check the last pass by hand: what are the values right when the condition becomes false?

On the exam

• ✓Expect questions that describe a process in words and ask which code segment matches it. Check that the order of the steps matches the description, not just that the right steps are there.