Week 7: Arrays

Learning Goals

By the end of this week, you will be able to:

1. Make arrays of any data type
2. Learn how to make multidimensional arrays
3. Learn how to use arrays in functions
4. Be able to distinguish between pass by reference and pass by value Make arrays of any data type

CONTENTS:

• Learning Goals
• Readings
• Background
  • Making Arrays
  • How to Access Elements in an Array
  • Code to Play With
  • Passing Arrays By Reference
  • Multidimensional Arrays
• PreQuiz Assignment (due Monday Feb 24 at 9am)
• Recitation
  • Recitation Spot The Error - Problem 1
  • Recitation Spot The Error - Problem 2
  • Recitation Spot The Error - Problem 3
  • Recitation Spot The Error - Problem 4
  • Recitation Meal Combos - Problem 5
    • Recitation Meal Combos - Problem 5.a.: Algorithm
    • Recitation Meal Combos - Problem 5.b.: Implementation
• Recitation Submission Guidelines
• Homework

    

Readings

Please note the advised readings of “Brief C++ Late Objects” - Cay Horstmann:

1. Monday: 2.5, 6.1, 6.2
2. Wednesday: 6.4, 6.5, 6.6
3. Friday: 6.4, 6.5

    

Background

An array is a data structure which can store other data types like double, int, char, and boolean, and string. Arrays have both a type and a size.

   

Making Arrays

How to declare arrays

// data_type array_name[declared_size];
bool myBooleans[10]; 
string myStrings[15];
int myInts[7];

How to initialize arrays (method 1)

bool myBooleans[4] = {true, false, true, true};

If you do not declare the size inside the square brackets, the array size will be set to however many entries you provide on the right.

bool myBooleans[] = {true, false, true}; // size = 3

Note: the size specified in the brackets needs to match the number of elements you wrote in the curly brackets.

/**
 * When the specified size is larger than the actual number of elements, 
 * the elements provided in the curly brackets will be the first several
 * elements in the array, while the additional elements will be filled 
 * with default values. If it’s an integer/double array, the default 
 * values are zero, while if it’s a string array, the default values
 * are empty strings.
 */
#include <iostream>
using namespace std;
int main()
{
    int intArray[5] = {1,2,3};
    for (int i = 0; i < 5; i ++)
    {
        cout << intArray[i] << " ";
    }
}
// OUTPUT:
// 1 2 3 0 0

/**
 * When the specified size is smaller than the actual number of elements,
 * there will be a compilation error.
 */
#include <iostream>
using namespace std;
int main()
{
    int intArray[3] = {1,2,3,4,5};
}
// OUTPUT:
// error: excess elements in array initializer
// int intArray[3] = {1,2,3,4,5};
//                          ^
// 1 error generated.

How to Initialize Arrays (Method 2) You can also initialize elements one by using the arrays index. Note that the left most element has index 0, NOT index 1:

// method 1
int someInts[5] = { 2, 4, 6, 8, 10 };

// method 2
int alsoSomeInts[5];
alsoSomeInts[0] = 2;
alsoSomeInts[1] = 4;
alsoSomeInts[2] = 6;
alsoSomeInts[3] = 8;
alsoSomeInts[4] = 10;

How to Initialize Arrays (Method 3) Or course you can also initialize elements one by one using a for loop:

int yetAnotherVersionOfSomeInts[5];
// NOTE: i += 2 rather than i++ to match the arrays of method 2 above
int yetAnotherVersionOfSomeInts[5];
for (int i = 0; i < 5; i++ )
{   
    yetAnotherVersionOfSomeInts[i] = (i+1)*2;
} 
// { 2, 4, 6, 8, 10 }

   

How to Access Elements in an Array

We have essentially already had practice with accessing elements in an array, as in C++, string is basically an array of characters. You can access elements in arrays using the same syntax you used for strings:

string greetings[] = {"hello", "hi", "hey", "what’s up?"};
cout << greetings[3] << endl;

Arrays and strings can also be iterated through in the same way. See these two examples:

// Iterating through an array of many strings:
string greetings[] = {"hello", "hi", "hey", "what’s up?"};
int size = 4;
for (int i = 0; i < size; i++)
{
    cout << greetings[i] << endl;
}

// Iterating through a single string:
string greeting = "Hello world!";
for (int i = 0; i < greeting.length(); i++){
    cout << greeting[i] << ", " << endl;
}

   

Code to Play With

Here is some code to play with if you are interested. Note how it builds a printing function for us. Exercise: run this code to exectue the printing function with a larger array.

#include<iostream>
using namespace std;

void formattedDisplayArray( string title, int arr[], int len )
{
    cout << title << "\n";
    for ( int index = 0 ; index < len ; index++ )
    {
        cout << "\tarr[" << index << "] == " << arr[index] << "\n";
    }
    return;
}

int main()
{
    // method 1
    int someInts[5] = { 2, 4, 6, 8, 10 };

    // method 2
    int alsoSomeInts[5];
    alsoSomeInts[0] = 2;
    alsoSomeInts[1] = 4;
    alsoSomeInts[2] = 6;
    alsoSomeInts[3] = 8;
    alsoSomeInts[4] = 10; 

    // method 3
    int yetAnotherVersionOfSomeInts[5];
    for (int i = 0; i < 5; i++ )
    {   
        yetAnotherVersionOfSomeInts[i] = (i+1)*2;
    }   

    formattedDisplayArray( "METHOD 1", someInts, 5 );
    formattedDisplayArray( "METHOD 2", alsoSomeInts, 5 );
    formattedDisplayArray( "METHOD 3", yetAnotherVersionOfSomeInts, 5 );

    // TODO: call formattedDisplayArray on an array of your own creation
    //       with length 10

    return 0;
}

   

Passing Arrays By Reference

Up until now, when calling functions, we have always passed by value. When a parameter is passed in a function call, a new variable is declared and initialized to the value passed in the function call. This has likely led to many questions about why our function calls are not changing the values of variables in main.

Observe that the variable x in main and variable x in AddOne are separate variables in memory. When AddOne is called with x on line 10, it is the value of 5 that is passed to the function. This value is used to initialize a new variable x that exists only in AddOne’s scope. Thus the value of the variable x in main’s scope remains unchanged even after the function AddOne has been called.

// Pass by value:
void AddOne(int x){
    x = x + 1;
    cout << "AddOne" << x << endl;
}

int main(){
    int x = 5;
    cout << "main: " << x << endl;
    AddOne(x);  /** LINE 10 */
    cout << "main: " << x << endl;
    cout << "Ask yourself, why doesn't the above line print main: 6?" << endl;
}

// OUTPUT:
// main: 5
// AddOne: 6
// main: 5
// Ask yourself, why doesn't the above line print main: 6?

Passing By Reference: Arrays, on the other hand, are passed by reference (a reference to the original array’s location in the computer’s memory). So, when an array is passed as a parameter, the original array is used by the function. Observe that there is only one array X in memory for the following example. When the function AddOne is called on line 10, a reference to the original array X is passed to AddOne. Because the array X is passed by reference, any modifications done to X in AddOne are done to the original array. These modifications persist and are visible even after the flow of control has exited the function and we return to main.

// Pass by reference example:
void AddOne(int X[]){
   X[0] = X[0] + 1;
   cout << "AddOne: " << X[0] << endl;
}

int main(){
    int X[4] = {7, 5, 3, 2};
    cout << "main: " << X[0] << endl;
    AddOne(X);  /** LINE 10 */
    cout << "main: " << X[0] << endl;
    cout << "Ask yourself, why doesn't the above line print main: 7?" << endl;
}

// OUTPUT:
// main: 7
// AddOne: 8
// main: 8
// Ask yourself, why doesn't the above line print main: 7?

When we pass a one-dimensional array as an argument to a function we also provide its length. For two-dimensional arrays, in addition to providing the length (or number of rows), we will also assume that we know the length of each of the subarrays (or the number of columns). A function taking a two-dimensional array (somtimes called a matrix) with 3 rows and 10 columns as an paremeter might look something like this:

void TwoDimensionalFunction( int matrix[3][10], int rows )
{
    // ...
}

   

Multidimensional Arrays

In C++ we can declare an array of arrays known as a multidimensional array. Multidimensional arrays store data in tabular form.

How to Declare Multidimensional Arrays

// data_type array_name[dimension_1][dimension_2]....;
int myInts[7][5];  // 7 rows, 5 columns
bool myBooleans[10][15][12];  // 10 rows, 15 columns, 12 depths
string myStrings[15][10];  // 15 rows, 10 columns

How to Initialize Multidimensional arrays (Method 1)

int myInts[2][2] = {1, 2, 3, 4};

The 2D array in this case will be filled from left to right from top to bottom.

int myInts[2][2] = { {1, 2}, {3, 4} };

You can also initialize a 2D array by explicitly separating the rows as shown above.

How to Initialize Multidimensional arrays (Method 2) You can also initialize elements using nested loops:

int myInts[2][2];
for(int i=0; i < 2; i++)
{
    for(int j=0; j < 2; j++)
    {
        myInts[i][j] = i + j;
    }
}

The above code will create the following 2D array: { {0,1}, {1,2} }.

How to Access Elements in a Multidimensional array You can use myInts[i][j] to access the ith row and jth column of a 2D array

Multidimensional arrays can be iterated using nested loops as shown below:

int myInts[2][2] = { {0,1}, {1,2} };
int res = 0;
for(int i=0; i < 2; i++)
{
    for(int j=0; j < 2; j++)
    {
        res = res + myInts[i][j];
    }
}
cout << "Result is " << res << endl;

// OUTPUT:
// Result is 4

    

PreQuiz Assignment (due Monday Feb 24 at 9am)

Please complete your "pre-quiz" for the week on Canvas. This is due Monday morning at 9am. Here is a link to the quiz for Spring 2025: Recitation 7 Pre-quiz

         

Recitation

    

Recitation Spot The Error - Problem 1

Given two positive integers x and y, this program prints all the integer points (i, j) in the rectangle formed by (0, 0) and (x, y). Identify the error(s) in the code below, and write the correct line(s).

#include <iostream>
using namespace std;

int main() {

    int x = 3, y = 4;

    for (int i = y; i >= 0; i--) {
        for (int j = x; j >= 0; j--) {
            cout << "(" << i << ", " << j << ")  ";
        }
        cout << endl;
    }
}

    

Recitation Spot The Error - Problem 2

The program prints the contents of an array and then calculates the sum of all the elements. Identify the error(s) in the code below, and write the correct line(s).

#include <iostream>
using namespace std;

int main() {

    int numbers[5] = {10, 20, 30, 40, 50};

    cout << "The contents of the array are: ";

    for (int i = 0; i <= 5; i++) {
        cout << numbers << " ";
    }

    cout << endl;

    for (int i = 0; i <= 5; i++) {
        int sum = 0;
        sum += numbers;
    }

    cout << "Sum = " << sum << endl;
    return 0;
}

    

Recitation Spot The Error - Problem 3

The following program determines if a given number num is a prime number. Identify the error(s) in the code and provide the corrected line(s).

#include <iostream>
#include <cmath>
using namespace std;

bool isPrime(int num) {

    if (num <= 1) 
        return false;

    for (int i = 2; i < sqrt(num); i++) {
        if (num % i = 0) {
            return false;
        }
    }

    return true;
}

int main(){

    int num = 36;
    if (isPrime(num)) {
        cout << num << " is a prime number." << endl;
    } else {
        cout << num << " is not a prime number." << endl;
    }

    return 0;
}

    

Recitation Spot The Error - Problem 4

The program prints the product of the lengths of the contents of a string array. Identify the error(s) in the code below, and write the correct line(s).

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string languages[6] = {"C++", "Python", "Java", "Matlab", "Julia"};
    int product = 0;
    int total = 5;

    for (int i = 0; i <= total; i++){
        product *= languages[i].length;
    }

    cout << "Product of lengths = " << product << endl;
    return 0;
}

    

Recitation Meal Combos - Problem 5

Problem:
Create two integer arrays, set1 and set2, in the main(). The length of set1 should be 3, representing different main dishes, and set2 should be 2, representing available drinks. Prompt the user to enter 3 integers for the main dishes (as their codes) and 2 integers for the drink options. Use these arrays to print all the possible meal combinations.

Example output :

Please enter 3 codes for the main dishes:
1 2 3
Please enter 2 codes for the drinks:
10 20
Here are the available meal combinations:
Dish 1 with Drink 10
Dish 1 with Drink 20
Dish 2 with Drink 10
Dish 2 with Drink 20
Dish 3 with Drink 10
Dish 3 with Drink 20

    

Recitation Meal Combos - Problem 5.a.: Algorithm

Write out the steps you would use to solve this problem by hand as pseudocode.

   

Recitation Meal Combos - Problem 5.b.: Implementation

Translate your pseudocode into a C++ program to solve the above code, verify that your program works as expected.

    

Recitation Submission Guidelines

Important: Follow these instructions carefully when preparing your recitation assignments. Your final submission should be in a single document, and the only action required on Canvas is uploading that document.

1. Documentation:
   • Create a pdf that includes your submission for all recitation questions. This is the pdf you will upload to your canvas assignment. Feel free to use Word/Google doc to create the pdf.
   • Clearly label each question with its corresponding number and include content as applicable (see #2).
2. Content to Include:
   • Screenshots of Your Code:
     • For each question, include a screenshot of your code.(corrected code in case of spot the errors)
   • Screenshots of Code Output (if applicable):
     • For some longer questions, it might be required to take a screenshot of the code’s output. Include these screenshots as part of your submission.
   • Longer Recitation Questions (Multiple Parts):
     • Option A:
       • Comment your answers directly within your code file.(Spot the errors)
       • Take screenshots of the commented code and paste them into your document.
     • Option B:
       • Take screenshots of the unmodified code.
       • Write your answers (Free Response/Pseudocode/Edge case identifictation) to the subquestions in the pdf document next to the corresponding screenshots.
3. Submission:
   • Upload the final pdf document to Canvas. This is the only action required on Canvas for your submission.

By following these steps, your submission will be clear, organized, and standardized across all recitation assignments.

         

Homework

Please continue working on project 1. It is due Friday February 28th.