07 Arrays

Arrays
Processing Sequences of Elements
Svetlin Nakov
Telerik Corporation
www.telerik.com

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. Matrices and Multidimensional Arrays
6. Dynamic Arrays
 Lists<T>
 Copying Arrays

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
… … … … …

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;

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
… … … … …

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
… … … … …

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"
};

Accessing Array Elements
Read and Modify Elements by Index

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

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];
}

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());
}

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+1)/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

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]);
}

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]):
for (int index = 0; index < array.Length; index++)
{
squares[index] = array[index] * array[index];
}

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
for (int index = 0; index < array.Length-1; index++)
{
array[index] = index;
}

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)

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);
}

Multidimensional Arrays
Using Array of Arrays, Matrices and Cubes

What is Multidimensional Array?
Multidimensional arrays have more than one
dimension (2, 3, …)
The most important multidimensional arrays
are the 2-dimensional
 Known as matrices or tables
Example of matrix of integers with 2 rows and
4 columns:
5 0 -2 4
5 6 7 8
0 1 2 3
0
1

Declaring and Creating
Declaring multidimensional arrays:
Creating a multidimensional array
Use new keyword
Must specify the size of each dimension
int[,] intMatrix;
float[,] floatMatrix;
string[,,] strCube;
int[,] intMatrix = new int[3, 4];
float[,] floatMatrix = new float[8, 2];
string[,,] stringCube = new string[5, 5, 5];

Initializing Multidimensional
Arrays withValues
Creating and initializing with values
multidimensional array:
Matrices are represented by a list of rows
 Rows consist of list of values
The first dimension comes first, the second
comes next (inside the first)
int[,] matrix =
{
{1, 2, 3, 4}, // row 0 values
{5, 6, 7, 8}, // row 1 values
}; // The matrix size is 2 x 4 (2 rows, 4 cols)

AccessingThe Elements of
Accessing N-dimensional array element:
Getting element value example:
Setting element value example:
nDimensionalArray[index1, … , indexn]
int[,] array = {{1, 2}, {3, 4}}
int element11 = array[1, 1]; //element11 = 4
int[,] array = new int[3, 4];
for (int row=0; row<array.GetLength(0); row++)
for (int col=0; col<array.GetLength(1); col++)
array[row, col] = row + col;
Number
of rows
Number of
columns

Reading Matrix – Example
Reading a matrix from the console
int rows = int.Parse(Console.ReadLine());
int columns = int.Parse(Console.ReadLine());
int[,] matrix = new int[rows, columns];
String inputNumber;
for (int row=0; row<rows; row++)
{
for (int column=0; column<cols; column++)
{
Console.Write("matrix[{0},{1}] = ", row, column);
inputNumber = Console.ReadLine();
matrix[row, column] = int.Parse(inputNumber);
}
}

Printing Matrix – Example
Printing a matrix on the console:
for (int row=0; row<matrix.GetLength(0); row++)
{
for (int col=0; col<matrix.GetLength(1); col++)
{
Console.Write("{0} ", matrix[row, col]);
}
Console.WriteLine();
}

Reading and
Printing Matrices
Live Demo

Finding a 2 x 2 platform in a matrix with a
maximal sum of its elements
int[,] matrix = {
{7, 1, 3, 3, 2, 1},
{1, 3, 9, 8, 5, 6},
{4, 6, 7, 9, 1, 0}
};
int bestSum = int.MinValue;
for (int row=0; row<matrix.GetLength(0)-1; row++)
for (int col=0; col<matrix.GetLength(1)-1; col++)
{
int sum = matrix[row, col] + matrix[row, col+1]
+ matrix[row+1, col] + matrix[row+1, col+1];
if (sum > bestSum)
bestSum = sum;
}
Maximal Platform – Example

Lists
Lists are arrays that resize dynamically
When adding or removing elements
Also have indexers ( like Array)
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

List Example
List<int> intList=new List<int>();
for( int i=0; i<5; i++)
{
intList.Add(i);
}
38
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 do
With List we simply do
39
int[] intArray=new int[5];
int[] copyArray = intArray;
intArray = new int[6];
for (int i = 0; i < 5; i++)
{
intArray[i] = copyArray[i];
}
intArray[5]=newValue;
list.Add(newValue);

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 Array.Copy()
 This way only the values will be copied but not
the reference
Array.Copy(sourceArray, copyArray);
int[] copyArray=array;

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
Matrices (2-dimensional arrays) are very useful
for presenting tabular data

Questions?Questions?
Arrays
http://academy.telerik.com

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}.

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.

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}

Exercises (4)
11. Write a program that fills and prints a matrix of size
(n, n) as shown below: (examples for n = 4)
1 5 9 13
2 6 10 14
3 7 11 15
4 8 12 16
7 11 14 16
4 8 12 15
2 5 9 13
1 3 6 10
1 8 9 16
2 7 10 15
3 6 11 14
4 5 12 13
1 12 11 10
2 13 16 9
3 14 15 8
4 5 6 7
a) b)
c) d)

Exercises (5)
12. Write a program that reads a rectangular matrix of
size N x M and finds in it the square 3 x 3 that has
maximal sum of its elements.
13. We are given a matrix of strings of size N x M.
Sequences in the matrix we define as sets of several
neighbor elements located on the same line, column
or diagonal. Write a program that finds the longest
sequence of equal strings in the matrix. Examples:
ha fifi ho hi
fo ha hi xx
xxx ho ha xx
s qq s
pp pp s
pp qq s
ha, ha, ha s, s, s

Exercises (6)
14. Write a program that finds the index of given
element in a sorted array of integers by using the
binary search algorithm (find it in Wikipedia).
15. 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.
16. Write a program that sorts an array of integers using
the merge sort algorithm (find it in Wikipedia).
17. Write a program that sorts an array of strings using
the quick sort algorithm (find it in Wikipedia).

Exercises (7)
18. 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).
19. *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)
20. *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 (8)
21. *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}
22. *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}

Exercises (9)
23. 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}
24. 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}

Exercises (10)
25. Write a program that fills a matrix of size (N, N) as
shown in the examples (for N=4):
16 15 13 10
14 12 9 6
11 8 5 3
7 4 2 1
7 11 14 16
4 8 12 15
2 5 9 13
1 3 6 10
10 11 12 13
9 2 3 14
8 1 4 15
7 6 5 16
1 12 11 10
2 13 16 9
3 14 15 8
4 5 6 7
a) b)
*c) *d)

Exercises (11)
26. *Write a program that finds the largest area of
equal neighbor elements in a rectangular matrix and
prints its size. Example:
Hint: you can use the algorithm "Depth-first search"
or "Breadth-first search" (find them in Wikipedia).
1 3 2 2 2 4
3 3 3 2 4 4
4 3 1 2 3 3
4 3 1 3 3 1
4 3 3 3 1 1
13

07 Arrays

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