Math2220 Lecture 12

Array

An array is a collection of data of same type.
It can be used for list with like items. examples: test scores, temperatures, names, etc.
avoids declaring multiple simple variables
can manipulates 'list' as one entity
Example, you can do something like
- Do this to i-th indexed variable
- display all elements of an array
- fill elements of array score from user

Example

#include <iostream>
using namespace std;

// the program asks user to input 5 numbers
// then output the average
void main() {
    // declare an array with 5 numbers
    // sum is the total sum of all the scores
    double score[5], sum = 0;
    cout << "Please input 5 numbers. "
         << "Seperate the numbers by spaces: \n";
   
    int i;
    for(i = 0; i < 5; i++) {
        cin >> score[i]; //ask the user to input the i-th score
        sum+=score[i];
    }

    // magic formula
    cout.setf(ios::fixed);
    cout.setf(ios::showpoint);
    cout.precision(2);
    cout << "The scores are : \n";
   
    for(i = 0; i < 5; i++) {
        cout << score[i] << " ";
    }

    cout << endl;
    cout << "The average is " << sum/5 <<endl;

    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

Output

Please input 5 numbers. Seperate the numbers by spaces:
92.25 33 45.75 88.8 77.77
The scores are :
92.25 33.00 45.75 88.80 77.77
The average is 67.51

`score[0]`	`score[1]`	`score[2]`	`score[3]`	`score[4]`
92.25	33	45.75	88.8	77.77

Explanation of the program

double score[5] declares an array of 5 doubles names scores
score[i] is the i-th element.

Array consists of 2 parts

name of the array
size of the array

Syntax

Declare an array
Type arrayName[size];

size should be a number or a const variable. It shouldn't be other type of variables.

Example:

int year[50];
int is called the base type of the array, 50 is the size. year is the name.
You can declare arrays and regular variables together. example:
```
double taxRate, income[99];
```
The order doesn't matter.

You can use constant to declare an array size.

#include <iostream>

using namespace std;

// the program asks user to input 5 numbers
// then output the average
void main() {
    const int NUM_OF_STUDENTS = 5;
    // declare an array with NUM_OF_STUDENTS numbers
    // sum is the total sum of all the scores
    double score[NUM_OF_STUDENTS], sum = 0;
    cout << "Please input "<< NUM_OF_STUDENTS << " numbers. "
         << "Seperate the numbers by spaces: \n";
   
    int i;
    for(i = 0; i < NUM_OF_STUDENTS; i++) {
        cin >> score[i]; //ask the user to input the i-th score
        sum+=score[i];
    }

    // magic formula
    cout.setf(ios::fixed);
    cout.setf(ios::showpoint);
    cout.precision(2);
    cout << "The scores are : \n";
   
    for(i = 0; i < NUM_OF_STUDENTS; i++) {
        cout << score[i] << " ";
    }

    cout << endl;
    cout << "The average is " << sum/NUM_OF_STUDENTS <<endl;

    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

Advantage

Increase readability
You can easily change the number of students by
```
const int NUM_OF_STUDENTS = 100;
```

You can't use variables to declare an array size.
Compiling error

#include <iostream>
using namespace std;

void main() {
    int numOfStudents;
    cout << "Please enter the number of students: ";
    cin >> numOfStudents;
    double score[numOfStudents], sum = 0;
  
    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

Access elements in an array
Individual parts called many things

Indexed or subscripted variables
Elements of the array
Value in brackets called index or subscript, it is a number from 0 to size-1

score[0],score[1],score[2],score[3],score[4] represent the first, second, third, forth and fifth element in the array score
Array indexes always start with zero and ends with the size-1
you can use score[i] as variables. i.e. you can use it in assignments, arithmetic expression etc etc
You can put any arithmetic expression inside the bracket, like score[i], or income[n+1].

Note the 2 uses of brakets []

In declaration, specifies SIZE of array
Anywhere else, specifies a subscript

Natural counting loop
Naturally works well ‘counting thru’ elements of an array.

    for(i = 0; i < NUM_OF_STUDENTS; i++) {
        cin >> score[i]; //ask the user to input the i-th score
        sum+=score[i];
    }

Example

#include<iostream>
#include<cmath>

using namespace std;

void main() {
    // declare an integer array and an integer x
    double A[10], x=2.5;

    // array index starts with 0
    for(int i=0; i < 10; i++) {
        // access A[i]
        A[i] = x*i;
    }

    for(int i=0; i < 10; i++) {
        cout << A[i] << " ";
    }
    cout << endl;

    // A[i] can be used like a variable;
    A[0]++;
    cout << "A[0] = " << A[0] << endl;
    
    A[1] = A[0]*A[2];
    cout << "A[1] = " << A[1] << endl;

    cout << "sqrt(A[3]) = " << sqrt(A[3]) << endl;

    x+=A[5];
    cout << "x = " << x << endl;


    char line[100];
    cin.getline(line, 100);
}

Sample output

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5
A[0] = 1
A[1] = 5
sqrt(A[3]) = 2.73861
x = 15

Major array pitfall

Array indexes always start with zero!
C++ will ‘let’ you go beyond range
- Unpredictable results
- Compiler will not detect these errors!

Indexes range from 0 to size – 1
Example:
double temperature[24]; // 24 is array size
Declares array of 24 double values called temperature
They are indexed as:

temperature[0], temperature[1] …
temperature[23]

Common mistake
temperature[24] = 5;

Index 24 is ‘out of range’!
No warning, possibly disastrous results

#include<iostream>
using namespace std;

void main(){
    int temperature[24];
    int i;
    for(i = 0; i < 24; i++) {
        temperature[i] = i + 60;
    }

    cout << temperature[24] << endl;

   char line[100];
   cin.getline(line,100);
}

Output on my PC
The output is a meaningless number. Your answer may be different.

Initializing array

Array can be initialized like this

int children[3] = {2, 12, 1};

It is equivalent to the following code

int children[3];
children[0]=2;
children[1]=12;
children[2]=1;

if you list fewer values than there are indexed variables, those values will be used to initalize the first few indexed variables.
The remaining indexed variables will be initialized to a zero of the array base type.

If you initialize an array when it is declared, you can omit the size of the array, and the array will automatically be declared to have the minmum size needed for the initialization values.

int b[] = {5, 12, 11};

is equivalent to

int b[3] = {5, 12, 11};

Example

#include<iostream>

using namespace std;


void main(){
    int array[5] = {1,3,5};
    int i;
    for(i = 0; i < 5; i++) {
        cout << array[i] << " ";
    }
    cout << endl;
    char line[100];
    cin.getline(line,100);
}

Output

1 3 5 0 0