# AP Computer Science A : Identifying & Correcting Errors

## Example Questions

### Example Question #1 : Identifying & Correcting Errors

Consider the following code:

int num = 1;

for(int i = 0; i < 10; i++) {

num *= i;

}

The code above is intended to provide the continuous product from num to 10.  (That is, it provides the factorial value of 10!.)  What is the error in the code?

You cannot use *= in a loop.

The i initialization is wrong.

Both the i intialization and the loop control value are incorrect.

The loop control, the initialization, and the initialization for num are all wrong.

The loop control is incorrect.

Both the i intialization and the loop control value are incorrect.

Explanation:

This loop will indeed run 10 times.  However, notice that it begins on 0 and ends on 10.  Thus, it will begin by executing:

num *= 0

Which is the same as:

num = num * 0 = 0

Thus, you need to start on 1 for i.  However, notice also that you need to go from 1 to 10 inclusive.  Therefore, you need to change i < 10 to i <= 10.

### Example Question #2 : Identifying & Correcting Errors

Consider the code below:

public class Clock {

     private int seconds;



     public Clock(int s) {

          seconds = s;

     }



     public void setTime(int s) {

          seconds = s;

     }



     public void setSeconds(int s) {

          int hoursMinutes = seconds - seconds % 60;

          seconds = hoursMinutes + s;

     }



     public void setMinutes(int min) {

          int hours = seconds / 3600;

          int currentSeconds = seconds % 60;

          seconds = hours + min * 60 + currentSeconds;

     }

}

Which of the following issues could be raised regarding the code?

The setMinutes method should return an integer value.

There are no issues.

There is a potential null pointer exception in one of the methods.

There is insufficient data verification for the mutator methods.

There is an error in the setMinutes calculation.

There is insufficient data verification for the mutator methods.

Explanation:

Recall that a "mutator" is a method that allows you to change the value of fields in a class. Now, for the setTimesetMinutes, and setSeconds methods, you do not check several cases of errors. For instance, you could give negative values to all of these methods without causing any errors to be noted. Likewise, you could assign a minute or second value that is greater than 60, which could cause errors as well. Finally, you could even make the clock have a value that is greater than "23:59:59", which does not make much sense at all!

### Example Question #3 : Identifying & Correcting Errors

What is wrong with the following code?

1. int main()
2. {
3.    bool logic;
4.    double sum=0;
5.    for(j=0;j<3;j++)
6.     {
7.       sum=sum+j;
8.     }
9.  if (sum>10)
10.   {
11.     logic=1;
12.    }
13.  else
14.   {
15.     logic=0;
16.    }
17.   }
18. }

You cannot define the variable logic as a bool.

Variable j is not defined.

There is an opening/closing bracket missing.

You must define the sum variable as an integer, not a double.

The code has no errors.

Variable j is not defined.

Explanation:

If you notice, in our for loop, the integer j is used as the iteration variable. However, no where in the code is that variable defined. To fix this issue this could have been done.

for (int j=0;j<3,j++)

Note the bold here is inserted. We need to define j here. We could have also defined j as a double.

# FILE INPUT/OUTPUT

Consider the following C++ code:

1. #include <iostream>

2. #include <fstream>

3. using namespace std;

4. int main() {

5.      ifstream outputFile;

6.      inputFile.open("TestFile.txt");

7.      outputFile << "I am writing to a file right now." << endl;

8.      outputFile.close();

9.      //outputFile << "I'm writting on the file again" << endl;

10.      return 0;

11. }

What is wrong with the code?

There is nothing wrong with the code.

Testfile.txt shouldn't have double quotations around it.

The outputFile should be of type "ofstream", not "ifstream".

The ifstream library should be included.

The outputFile should be of type "ofstream", not "ifstream".

Explanation:

Type ifstream objects are used to read from files, NOT to write to a file. To write to a file you can use ofstream. You can also you fstream to both read and write to and from a file.

### Example Question #24 : Program Analysis

a. public void draw() {
b.   int i = 0;
c.   while(i < 15){
d.     system.out.println(i);
e.     i++
f.   }
g. }

Which lines of code have errors?

e

d e

cb

b

d c