Arrays

Learning objectives:

11.5.2.1 use the technical terms associated with arrays including upper and lower bounds 
11.5.2.3 write program code using 1D and 2D arrays 

I. Definition

A data structure is a collection of elementary data types such as integer, real, boolean, and char.

An array is a data structure that contains finite set of indexed elements of the same type, united by the same name.

1D array is similar to a table of objects or primitive types, keyed by an index.

Examples, 

1D Array: students

1D Array: marks

1D arrays in pseudocode:

students = ["Diana","Abai","Malika","Zhanna","Ruslan"]
marks = [34, 30, 37, 33, 31]
OUTPUT marks[3]              // output 33
OUTPUT students[0]           // output Diana
OUTPUT students[1], " - ", marks[1]    // output Abai - 30

II. Array processing

n - length of array 

arr - array

Input elements of the array from the keyboard:

FOR i = 0 TO n - 1

INPUT arr[i]

endFOR

Output elements of the array:

FOR i = 0 TO n - 1

OUTPUT arr[i]

endFOR

Determining the index of the minimum element of the array and its value:

index = 0 // index of minimal element

min = arr[index] // value of minimal element

FOR i = 1 TO n - 1

IF min > arr[i]

THEN min = arr[i] 

endFOR

OUTPUT index, min

Сalculating the sum of array elements:

sum = 0 // start sum

FOR i = 0 TO n - 1

sum = sum + arr[i]

endFOR

OUTPUT sum

Сounting the number of even elements of an array:

count = 0 // start count

FOR i = 0 TO n - 1

IF arr[i] MOD 2 = 0

THEN count = count + 1

endFOR

OUTPUT count

Outputting array elements with odd indices:

FOR i = 0 TO n - 1

IF i MOD 2 <> 0

THEN OUTPUT arr[i]

endFOR

III. PHP Indexed Array

1D arrays in PHP program:

<?php
$students = array("Diana","Abai","Malika","Zhanna","Ruslan");   // array of five elements
$marks = array(34, 30, 37, 33, 31);
echo $marks[3]."<br>";               // output 33
echo $students[0]."<br>";            // output Diana
echo $students[1]." - ".$marks[1];     // output Abai - 30
?>

Output all elements of an Indexed Array students.

<?php
$students = array("Diana","Abai","Malika","Zhanna","Ruslan");
$arrlength = count($students);   // count() - Counts the number of elements in an array
for($x = 0; $x < $arrlength; $x++)  // $arrlength = 5 (in array five elements)
{
  echo $students[$x]."<br>";
}
?>

IV. PHP Associative Arrays

Associative arrays are arrays that use named keys that you assign to them.

1D array result:

Example,

$result = array("Diana"=>34, "Abai"=>30, "Malika"=>37, "Zhanna"=>33, "Ruslan"=>31);

It means that

$result["Diana"] = 34

$result["Malika"] = 37

$result["Ruslan"] = 31

Output all elements of an Associative Array result.

<?php
$result = array("Diana"=>34, "Abai"=>30, "Malika"=>37, "Zhanna"=>33, "Ruslan"=>31);
foreach($result as $x => $x_value) // iterate each element of array by key
{
  echo "Key=" . $x . ", Value=" . $x_value;
  echo "<br>";
}
?>

Display:

Key=Diana, Value=34
Key=Abai, Value=30
Key=Malika, Value=37
Key=Zhanna, Value=33
Key=Ruslan, Value=31

Task. Determine who got 34 or more marks.

<?php
$result = array("Diana"=>34, "Abai"=>30, "Malika"=>37, "Zhanna"=>33, "Ruslan"=>31);
echo "Students who got 34 or more marks:<br>";
foreach($result as $x => $x_value) 
{
  if ($x_value >= 34)
      echo $x . "<br>";
}
?>

Some examples:

Questions:

1. What is an array, and how is it defined in programming?
2. Describe the process of inputting elements into an array from the keyboard.
3. Write a code snippet to calculate the sum of all elements in an array arr of length n.
4. How do you create an associative array?
5. Explain how to iterate over an associative array in PHP to display each key-value pair.
6. How to process associative arrays to print elements according to criteria?

Exercises:

Ex. 1 Define array elements

Ex. 2  

Tasks on https://www.w3resource.com/php-exercises/php-array-exercises.php

Ex. 3 Implement the Luhn algorithm

Task on https://www.101computing.net/is-my-credit-card-valid/

Exam questions: