There are two integer variables in our program, hours and days, which represents time. If in the program, we increase the number of hours by one, which of the following lines of code will correctly update days and hours?
我们的程序中有两个整型变量，hours 和 days，它们代表时间。如果在程序中，我们将小时数增加 1，以下哪一行代码可以正确更新 days 和 hours？

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Question restatement: There are two integer variables, hours and days, representing time. If we increase hours by 1, which line of code correctly updates days and hours?

Option 1: hours = hours % 24;
- Why this is not correct: This only reduces hours to the remainder after division by 24 and does not touch days at all. It fails to carry full days from hours into the days variable.

Option 2: days = days + hours / 24; 
hours = hours % 24;
- Why this is correct: This sequence first adds to days the number of full 24-hour blocks contained in hours (integer division hours / 24). Then it updates hours to the remainder after removing those full days (hours % 24). This preserves the correct time decomposition when hours crosses a day boundary.

Option 3: days = days + hours / 24;
- Why this is not fully correct: It updates days with the number of full days from hours, but it does not update hours to reflect the remaining hours after taking away those days. After this line, hours would remain unchanged, potentially leaving hours >= 24 and inconsistent with days.

Option 4: days = days + hours % 24; 
hours = hours / 24;
- Why this is incorrect: This reverses the intended decomposition. It adds the remainder of hours divided by 24 to days and assigns the quotient hours / 24 to hours. This results in hours being the number of days, which is not the intended meaning, and days being increased by the wrong value (the remainder rather than the number of full days).

Option 5: hours = hours + days % 24;
- Why this is not correct: This alters hours by adding a portion of days, but does not update days at all to reflect the passage of time. It also misuses the modulo operation, leading to an inconsistent representation of time between hours and days.

Summary: Among the options, the line that both transfers complete days from hours into days and reduces hours to the remaining hours is option 2, which uses days = days + hours / 24; followed by hours = hours % 24;. The other options either neglect updating days, misuse the modulo and division relationships, or invert the intended meaning of the operations, leading to incorrect time bookkeeping.

(862646) AP CS A Unit 1 Exam 第一单元考试

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