# AP Computer Science A : Choosing Appropriate Data Structures

## Example Questions

### Example Question #301 : Computer Science

What is the difference between declaring a function as void and declaring it as int?

A void declaration does not expect an output to that function, whereas an int function declarations expects an intger value as the output to the function.

An int function needs to declare an integer variable inside the function and a void function doesn't.

A void function can have any return type whereas an int function can only have one return type.

A void function has no input and an int function has at least one integer input.

There is no difference.

A void declaration does not expect an output to that function, whereas an int function declarations expects an intger value as the output to the function.

Explanation:

Function delcarations are very significant, as you must define what the return type of the function is. A void function doesn't need a return type, whereas the int function must have an integer value as its return type.

### Example Question #1 : Choosing Appropriate Data Structures

What's wrong with the following code?

void printsometext()
{
cout<<"printing text\n";
for(int i=0;i<3;i=i+1)
{
cout<<i<<"\n"
}
return 2;
}

The loop is formatted incorrectly.

You need semicolons after the function declaration.

Cout cannot output a number.

The character \n does not exist.

A void function cannot return an output.

A void function cannot return an output.

Explanation:

It is very important when creating a function to select the return type. If you want your function to return an integer, you would place "int" before your function to signify that the function expects an integer output. In this case, the function is declared as void, meaning there is no output. However, the code indicated a return value of 2, which conflicts with the void keyword. To fix this problem, either remove the return statement or change void to int.

## CHOOSING LOOPS

Imagine that I want to write a code for a bank that asks a user whether he/she wants to make a withdrawal, make a deposit, or quit. What type of loop would be best to use in order to make sure the user is able to do as many transactions as they want until they press quit to end the program?

A while loop.

A do-while loop.

A for loop.

An infinite loop.

A do-while loop.

Explanation:

In our problem, the program has to run AT LEAST ONCE in order to present all of the options to the user. Because of this reason, the best option is a do-while loop because the statements within that loop execute at least one time and the condition to get out of the loop is at the end. In this case, the exit condition would be if the user presses quit (after the options have been shown at least once).

A while loop might work in this case; however it is not necessarily the smartest option. This is because the condition is tested before anything is executed within the loop. This means that whatever is inside the while loop does not necessarily have to be executed at all.

A for loop is also not a good option for this problem because a for loop is executed a set amount of times. In this case, we don't know if the user is going to want to do 1 transaction or 10. Therefore, a for loop is not the best choice.

An infinite loop is also not what we want because it runs forever. In our case, we want the code to stop prompting the user and end after "quit."

## USING POINTERS

Study the following pseudocode.

int * var1;

int foo = 63;

var1 = &foo;

var2 = *var1;

What are the values of var1 and var2?

var1 is assigned the memory address value of foo.

var2 is assigned a value of 63

var1  is assigned a value of 63

var2  is assigned a value off 63

var1 is assigned the memory address value of foo

var2 is assigned the memory address value of var1

var1  is assigned a value of 63

var2 is assigned the memory address value of var1

var1 is assigned the memory address value of foo.

var2 is assigned a value of 63

Explanation:

Pointers store the address of another variable. Pointers are declared by naming the type, then an asterisk, followed by the name of the variable. In our example, var1 was declared a pointer that points to an integer:

int * var1;

Next in the code, we see that an integer variable named "foo" is created and is assigned a value of 63.

The address-of operator (&) is then used to get the address of a variable. Now lets take a look at the next line of the code:

var1 = &foo;

Here the ampersand is used to get the address of foo within memory. This means that var1 contains the address value of where foo is stored in memory.

Next in the code, we have the following statement:

var2 = *var1;

Here the dereference operator (*) is being used. This operator is used whenever we want to get the value (not the address) of the variable that a pointer is pointing to. In this case, var1 is storing the address of foo. However, by using the dereference operator we can get the actual value of the variable whose address is being stored in var1. In this case, dereferencing var1, we get the actual value of foo which is 63. Therefore, var2 = 63.

# TWO DIMENSIONAL ARRAYS

Given the following initialized array:

int fourth;

int[][] myArray = { {1, 2, 3},

{4, 5, 6},

{7, 8, 9} };

Using myArray, how can I store in variable "fourth", the number 4?

fourth = myArray;

fourth = myArray;

fourth = myArray;

fourth = myArray;

fourth = myArray;

Explanation:

When a two dimensional array is created and initialized, the way to access the items inside the matrix is by calling the array with the row and column (i.e. myArray[ROW][COLUMN]). Keeping in mind that arrays start at 0, the number four would be in row 1, column 0. Therefore to save that number into the variable "fourth" we'll do the following:

fourth = myArray;

*Note: myArray is not the same as myArray.

myArray =4 because it is the item located at row=1 and column = 0.

myArray =12 because it is the item located at row=0 and column = 1.

### Example Question #5 : Program Design

What would be the best data structure for a library? The data is in the form of a title and a number of copies of the title.

Static values

ArrayList

Hash map

Array

Hash map

Explanation:

Hash map - key being title and value being the number of copies

Hash maps are a collection of (key, value) pairs.

Hash maps have O(1) access, so this would be the quickest and best way to store the data.

### Example Question #1 : Arrays

True or False.

The best data structure to represent a set of keys and values is an array.

True

False

False

Explanation:

Arrays can be two-dimensional. However, when trying to keep track of keys and values it can become complicated when using an array. HashMaps are the best way to represent data containing keys and values.

### Example Question #1 : Choosing Appropriate Data Structures

What is not a feature of the Java programming language?

Ability to write functions without making them class methods

Garbage Collection

Compile time error checking

primitive data types such as int, boolean and double

Object Oriented Programming 