Final (AY18/19)

Problems

Question paper without answers:

109KB

final-1819-s1.pdf

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Question without Answers

Question paper with answers:

192KB

final-1819-s1-with-comments.pdf

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Question with Answers

Answer sheet:

48KB

final-1819-s1-sheet.pdf

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Answer sheet

3. String Manipulation*

In this question, what we should pay attention to is that since string has a null character ('\0') at the end. So, when you calloc a string of length len, the len-1-th element will be '\0'. Hence, when you manipulation each of its character using a for-loop like below, you should stop at the len-2-th element, which means the terminating condition should be i < len - 1.

char *str = calloc(len, sizeof(char));
for (long i = 0; i < len - 1; i += 1)
{
  str[i] = 'a';
}
cs1010_println_string(str);

4. Uninitialized Variable*

Uninitialized variables may cause strange behavior, and you cannot tell what will be going.

5. Macro*

It was taught in Macro. Pay attention to the operating sequence using macro. If not braket is included, we just insert the expression into the macro and follows the +,-,*,/ operation sequence.

7. Time Complexity*

For a loop like below:

for (long i = 0; i < n/2; i += 2)
{
  for (long j = i/2; j < n; j += 1)
  {
  }
}

To judge its time complexity quickly, we can only pay attention to the terminating condition and the loop changing condition to decide the time complexity of the loop. In this problem, the time complexity for the outer loop is just $O(n/2/2)=O(n/4)=O(n)$. And for inner loop, it's just $O(n/1)=O(n)$. So, the overall time complexity will be $O(n^2)$.

Note that this method of only look at terminating condition and loop changing condition works at most of the cases except for

1. the edge cases. e.g. for (i=0;i<n;i*=2), here it's an infinite loop and

2. when the initialization involves terms of input n. e.g. for(i=n;i<n+1;i+=1).

8. Time Complexity

$$T(n)=n+\frac{n}{2}+\frac{n}{4}+\frac{n}{8}+\cdots+1=O(n)$$

This is the same with:

$$T(n)=1+2+4+\cdots+n=O(n)$$

15. Call Stack Diagram

In CS1010, the convention for the drawing of the call stack diagram is shown below (including how to draw arrays, pointers and variables):

Call Stack Diagram

This kind of diagram is very useful when doing pointer questions! Remember to draw it every time you deal with pointer questions!

Tips

1. When working with strings in C, always account for the null character by stopping the loop at len - 1 (excluding) to avoid overwriting it.

2. Uninitialized variables can cause unpredictable behavior and lead to errors that are difficult to diagnose.

3. Include the Pitfall into Cheatsheet.
4. Include the operator precedence in C in the cheatsheet, e.g. ->, &, which one is first operated.

5. Include the 15. Call Stack Diagram in the cheatsheet. (Use the AY22/23 Version)