Today is the 822th day of learning programming with a slightly cold rain!
Hello, everyone! This is the CSP 2024 Group J Preliminary Exam Questions.
Day 822
CSP 2024 Group J Preliminary Exam Questions
Read the Program
2. Read the Program (The program input does not exceed the defined range of arrays or strings; for true/false questions, fill in √ for correct and ⨉ for incorrect; unless otherwise specified, true/false questions are worth 1.5 points, multiple-choice questions are worth 3 points, totaling 40 points)
Question 1
True/False Questions
1. When the input is 10, the first output of the program is 44, and the second output is 17. ( )
2. If the condition in the isPrime(i) function is changed to i<=n/2, when inputting 20, the output of countPrimes(20) will change to 6. ( )
3. The sumPrimes function calculates the sum of all prime numbers between 2 and n.
Multiple Choice Questions
1. When the input is 50, the output of sumPrimes(50) is ( ).
A. 1060
B. 328
C. 381
D. 275
2. If the for (int i = 2; i * i <= n; i++) is changed to for (int i = 2; i <= n; i++), when inputting 10, the program’s output is ( ).
A. It will not correctly calculate the number of primes and their sum below 10
B. Still outputs 4 and 17
C. Outputs 3 and 10
D. The output remains unchanged, but the runtime is shorter
Answer: B
START OF SPRING
Solution: This question tests the logic of prime number determination in C++.
True/False Questions
1. True
Explanation: The prime numbers between 2 and 10 are 4 in total: 2, 3, 5, 7. Their sum is 17. Therefore, the first question is correct.
2. False
There are 8 prime numbers between 2 and 10: 2, 3, 5, 7, 11, 13, 17, 19. So the judgment is incorrect.
3. True
The function of sumPrimes is indeed to calculate the sum of all prime numbers between 2 and n.
Multiple Choice Questions
1. B
The prime numbers between 2 and 50 are:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47.
The total sum is 328. So the answer is B.
2. A
Explanation: Because changing to <=n means that every judgment will return false, every number will be determined as non-prime. So the answer is A.
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