arrays-120712074248-phpapp01

Arrays
Processing Sequences of Elements
Doncho Minkov
Telerik Software Academy
academy.telerik.com
TechnicalTrainer
http://minkov.it/

Table of Contents
1. Declaring and Creating Arrays
2. Accessing Array Elements
3. Console Input and Output of Arrays
4. Iterating Over Arrays Using for and foreach
5. Dynamic Arrays
 List<T>
 Copying Arrays
2

What are Arrays?
 An array is a sequence of elements
 All elements are of the same type
 The order of the elements is fixed
 Has fixed size (Array.Length)
0 1 2 3 4Array of 5
elements
Element
index
Element
of an array
… … … … …
4

Declaring Arrays
 Declaration defines the type of the elements
 Square brackets [] mean "array"
 Examples:
 Declaring array of integers:
 Declaring array of strings:
int[] myIntArray;
string[] myStringArray;
5

Creating Arrays
 Use the operator new
 Specify array length
 Example creating (allocating) array of 5
integers:
myIntArray = new int[5];
myIntArray
managed heap
(dynamic memory)
0 1 2 3 4
… … … … …
6

Creating and Initializing Arrays
 Creating and initializing can be done together:
 The new operator is not required when using
curly brackets initialization
myIntArray = {1, 2, 3, 4, 5};
myIntArray
managed heap
(dynamic memory)
0 1 2 3 4
1 2 3 4 5
7

Creating Array – Example
 Creating an array that contains the names of
the days of the week
string[] daysOfWeek =
{
"Monday",
"Tuesday",
"Wednesday",
"Thursday",
"Friday",
"Saturday",
"Sunday"
};
8

Accessing Array Elements
Read and Modify Elements by Index
10

How to Access Array Element?
 Array elements are accessed using the square
brackets operator [] (indexer)
 Array indexer takes element’s index as
parameter
 The first element has index 0
 The last element has index Length-1
 Array elements can be retrieved and changed
by the [] operator
11

Reversing an Array – Example
 Reversing the contents of an array
int[] array = new int[] {1, 2, 3, 4, 5};
// Get array size
int length = array.Length;
// Declare and create the reversed array
int[] reversed = new int[length];
// Initialize the reversed array
for (int index = 0; index < length; index++)
{
reversed[length-index-1] = array[index];
}
12

Arrays: Input and Output
Reading and Printing Arrays on the Console

Reading Arrays From the Console
 First, read from the console the length of the
array
 Next, create the array of given size and read
its elements in a for loop
int n = int.Parse(Console.ReadLine());
int[] arr = new int[n];
for (int i=0; i<n; i++)
{
arr[i] = int.Parse(Console.ReadLine());
}
15

Symmetry Check – Example
 Read int array from the console and
check if it is symmetric:
bool isSymmetric = true;
for (int i=0; i<array.Length/2; i++)
{
if (array[i] != array[n-i-1])
{
isSymmetric = false;
}
}
1 2 3 2 11 2 2 1 1 2 3 3 2 1
16

Printing Arrays on the Console
 Process all elements of the array
 Print each element to the console
 Separate elements with white space or a new line
string[] array = {"one", "two", "three"};
// Process all elements of the array
for (int index = 0; index < array.Length; index++)
{
// Print each element on a separate line
Console.WriteLine("element[{0}] = {1}",
index, array[index]);
}
18

Processing Array Elements
Using for and foreach

Processing Arrays: for Statement
 Use for loop to process an array when
 Need to keep track of the index
 Processing is not strictly sequential from the
first to the last element
 In the loop body use the element at the loop
index (array[index]):
{
squares[index] = array[index] * array[index];
}
21

Processing Arrays Using
for Loop – Examples
 Printing array of integers in reversed order:
 Initialize all array elements with their
corresponding index number:
Console.WriteLine("Reversed: ");
for (int i = array.Length-1; i >= 0; i--)
{
Console.Write(array[i] + " ");
}
// Result: 5 4 3 2 1
{
array[index] = index;
}
22

Processing Arrays: foreach
 How foreach loop works?
 type – the type of the element
 value – local name of variable
 array – processing array
 Used when no indexing is needed
 All elements are accessed one by one
 Elements can not be modified (read only)
foreach (type value in array)
23

Processing Arrays Using
foreach – Example
 Print all elements of a string[] array:
string[] capitals =
{
"Sofia",
"Washington",
"London",
"Paris"
};
foreach (string capital in capitals)
{
Console.WriteLine(capital);
}
24

Lists (Resizable Arrays)
 List<T> – array that can resize dynamically
 When adding or removing elements
 Also have indexers [] (like arrays)
 T is the type that the list will hold
 E.g. List<int> will hold integers
 List<object> will hold objects
 Basic methods and properties
 Add(T element) – adds new element to the end
 Remove(element) – removes the element
 Count – returns the current size of the list
27

List Example
List<int> intList = new List<int>();
for( int i=0; i<5; i++)
{
intList.Add(i);
}
28
int[] intArray = new int[5];
for( int i=0; i<5; i++)
{
intArray[i] = i;
}
 Is the same as:
 The main difference
 When using lists we don't have to know the
exact number of elements

Lists vs. Arrays
 Lets have an array with capacity of 5 elements
 If we want to add a sixth element (we have
already added 5) we have to manually resize
 With List<T> we simply call
29
int[] copyArray = intArray;
for (int i = 0; i < 5; i++)
{
intArray[i] = copyArray[i];
}
intArray[5] = newValue;
list.Add(newValue);

HowThe List<T> Works?
 Why adding new elements is not slow?
 When adding n elements in List<T> it resizes
itself log(2)n times instead of n
 Initially a new List<T> has size of 0 elements
 Counter for total capacity (Capacity)
 Counter for number of used capacity (Count)
 When created, Capacity is 4both properties of
the list have values of 0
 When adding the first element Count becomes
1 and Capacity becomes 4
31

HowThe List<T> Works? (2)
 Initially the List<T> is empty
 When adding new element it is resized
 But not every time
 Only when it is needed
 Lets have a list with 3 elements
 It looks like this:
 When we add new element
it is appended to the end
 Adding a fifth element
doubles the Capacity of the list 32

Copying Arrays
The Array Class

Copying Arrays
 Sometimes we must copy the values from one
array to another one
 If we do it the intuitive way we would copy not
only the values but the reference to the array
 Changing some of the values in one array will
affect the other
 The way to avoid this is using Clone()
 This way only the values will be copied but not
the reference
int[] copyArray = (int[])array.Clone();
int[] copyArray = array;
35

Summary
 Arrays are a fixed-length sequences of
elements of the same type
 Array elements are accessible by index
 Can be read and modified
 Iteration over array elements can be done with
for and foreach loops
 List<T> holds resizable arrays
 Good when we don't know the number of
elements initially
36

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Arrays
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Exercises
1. Write a program that allocates array of 20 integers
and initializes each element by its index multiplied
by 5. Print the obtained array on the console.
2. Write a program that reads two arrays from the
console and compares them element by element.
3. Write a program that compares two char arrays
lexicographically (letter by letter).
4. Write a program that finds the maximal sequence of
equal elements in an array.
Example: {2, 1, 1, 2, 3, 3, 2, 2, 2, 1}  {2, 2, 2}.
38

Exercises (2)
5. Write a program that finds the maximal increasing
sequence in an array. Example:
{3, 2, 3, 4, 2, 2, 4}  {2, 3, 4}.
6. Write a program that reads two integer numbers N
and K and an array of N elements from the console.
Find in the array those K elements that have
maximal sum.
7. Sorting an array means to arrange its elements in
increasing order. Write a program to sort an array.
Use the "selection sort" algorithm: Find the smallest
element, move it at the first position, find the
smallest from the rest, move it at the second
position, etc.
39

Exercises (3)
8. Write a program that finds the sequence of maximal
sum in given array. Example:
{2, 3, -6, -1, 2, -1, 6, 4, -8, 8}  {2, -1, 6, 4}
Can you do it with only one loop (with single scan
through the elements of the array)?
9. Write a program that finds the most frequent
number in an array. Example:
{4, 1, 1, 4, 2, 3, 4, 4, 1, 2, 4, 9, 3}  4 (5 times)
10. Write a program that finds in given array of integers
a sequence of given sum S (if present). Example:
{4, 3, 1, 4, 2, 5, 8}, S=11  {4, 2, 5}
40

Exercises (6)
11. Write a program that finds the index of given
element in a sorted array of integers by using the
binary search algorithm (find it inWikipedia).
12. Write a program that creates an array containing all
letters from the alphabet (A-Z). Read a word from
the console and print the index of each of its letters
in the array.
13. * Write a program that sorts an array of integers
using the merge sort algorithm (find it in Wikipedia).
14. Write a program that sorts an array of strings using
the quick sort algorithm (find it in Wikipedia).
41

15. Write a program that finds all prime numbers in the
range [1...10 000 000]. Use the sieve of Eratosthenes
algorithm (find it in Wikipedia).
16. *We are given an array of integers and a number S.
Write a program to find if there exists a subset of
the elements of the array that has a sum S.
Example:
arr={2, 1, 2, 4, 3, 5, 2, 6}, S=14  yes (1+2+5+6)
17. *Write a program that reads three integer numbers
N, K and S and an array of N elements from the
console. Find in the array a subset of K elements
that have sum S or indicate about its absence.
Exercises (7)
42

Exercises (8)
18. *Write a program that reads an array of integers
and removes from it a minimal number of elements
in such way that the remaining array is sorted in
increasing order. Print the remaining sorted array.
Example:
{6, 1, 4, 3, 0, 3, 6, 4, 5}  {1, 3, 3, 4, 5}
19. *Write a program that reads a number N and
generates and prints all the permutations of the
numbers [1 … N]. Example:
n = 3  {1, 2, 3}, {1, 3, 2}, {2, 1, 3}, {2, 3, 1}, {3, 1, 2},
{3, 2, 1}
43

Exercises (9)
20. Write a program that reads two numbers N and K
and generates all the variations of K elements from
the set [1..N]. Example:
N = 3, K = 2  {1, 1}, {1, 2}, {1, 3}, {2, 1}, {2, 2}, {2, 3},
{3, 1}, {3, 2}, {3, 3}
21. Write a program that reads two numbers N and K
and generates all the combinations of K distinct
elements from the set [1..N]. Example:
N = 5, K = 2  {1, 2}, {1, 3}, {1, 4}, {1, 5}, {2, 3}, {2, 4},
{2, 5}, {3, 4}, {3, 5}, {4, 5}
44

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