07. Arrays

Arrays
Processing Sequences of Elements
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Software University
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Table of Contents
1. Defining, Initializing and Processing Arrays
2. Reading Arrays from the Console
 Using for Loop to Read Arrays
 Using String.Split(…)
3. Printing Arrays at the Console
 Using the foreach Loop
 Using String.Join(…)
4. Arrays – Exercises
2

Arrays
Working with Arrays of Elements

What are Arrays?
 In programming array is a sequence of elements
 Elements are numbered from 0 to Length-1
 Elements are of the same type (e.g. integers)
 Arrays have fixed size (Array.Length) – cannot be resized
4
0 1 2 3 4
Array of 5 elements
Element index
… … … … …
Element
of an array

5
 The days of week can be stored in array of strings:
Days of Week – Example
string[] days = {
"Monday",
"Tuesday",
"Wednesday",
"Thursday",
"Friday",
"Saturday",
"Sunday"
};
Expression Value
days[0] Monday
days[1] Tuesday
days[2] Wednesday
days[3] Thursday
days[4] Friday
days[5] Saturday
days[6] Sunday

6
 Enter a day number [1…7] and print the day name (in English) or
"Invalid day!"
Problem: Day of Week
string[] days = { "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday", "Sunday" };
int day = int.Parse(Console.ReadLine());
if (day >= 1 && day <= 7)
Console.WriteLine(days[day - 1]);
else
Console.WriteLine("Invalid day!");
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/172#0

7
 Allocating an array of 10 integers:
 Assigning values to the array elements:
 Accessing array elements by index:
Working with Arrays
int[] numbers = new int[10];
for (int i = 0; i < numbers.Length; i++)
numbers[i] = 1;
numbers[5] = numbers[2] + numbers[7];
numbers[10] = 1; // IndexOutOfRangeException
All elements are
initially == 0
The Length holds
the number of
array elements
The [] operator accesses
elements by index

8
 Write a program to find all prime numbers in range [0…n]
 Sieve of Eratosthenes algorithm:
1. primes[0…n] = true
2. primes[0] = primes[1] = false
3. Find the smallest p, which
holds primes[p] = true
4. primes[2*p] = primes[3*p] =
primes[4*p] = … = false
5. Repeat for the next smallest p
Problem: Sieve of Eratosthenes

9
Solution: Sieve of Eratosthenes
var n = int.Parse(Console.ReadLine());
bool[] primes = new bool[n + 1];
for (int i = 0; i <= n; i++) primes[i] = true;
primes[0] = primes[1] = false;
for (int p = 2; p <= n; p++)
if (primes[p]) FillPrimes(primes, p);
static void FillPrimes(bool[] primes, int step)
{
for (int i = 2 * step; i < primes.Length; i += step)
primes[i] = false;
}
TODO: print the
calculated prime
numbers at the end

10
 Enter two integers n and k
 Generate and print the
following sequence:
 The first element is: 1
 All other elements = sum of
the previous k elements
 Example: n = 9, k = 5
 120 = 4 + 8 + 16 + 31 + 61
Problem: Last K Numbers Sums
6
3
Sequence:
1 1 2 4 7 13
8
2
Sequence:
1 1 2 3 5 8 13 21
9
5
Sequence:
1 1 2 4 8 16 31 61 120
1 1 2 4 8 16 31 61 120
+

11
Solution: Last K Numbers Sums
var k = int.Parse(Console.ReadLine());
var seq = new long[n];
seq[0] = 1;
for (int current = 1; current < n; current++)
{
var start = Math.Max(0, current - k);
var end = current - 1;
long sum = // TODO: sum the values of seq[start … end]
seq[current] = sum;
}
// TODO: print the sequence seq[]

Working with Arrays
Live Exercises in Class (Lab)

Reading Arrays from the Console
Using String.Split() and Select()

14
 First, read from the console the length of the array:
 Next, create the array of given size n and read its elements:
Reading Arrays From the Console
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
 Write a program to read n integers and print their sum, min,
max, first, last and average values:
Problem: Sum, Min, Max, First, Last, Average
5
12
20
-5
37
8
Sum = 72
Min = -5
Max = 37
First = 12
Last = 8
Average = 14.4
4
50
20
25
40
Sum = 135
Min = 20
Max = 50
First = 50
Last = 40
Average = 33.75

16
Solution: Sum, Min, Max, First, Last, Average
using System.Linq;
…
var nums = new int[n];
for (int i = 0; i < n; i++)
nums[i] = int.Parse(Console.ReadLine());
Console.WriteLine("Sum = {0}", nums.Sum());
Console.WriteLine("Min = {0}", nums.Min());
// TODO: print also max, first, last and average values
Use System.Linq to enable aggregate
functions like .Max() and .Sum()

17
 Arrays can be read from a single line of space separated values:
 Or even write the above as a single line of code:
Reading Array Values from a Single Line
string values = Console.ReadLine();
string[] items = values.Split(' ');
int[] arr = new int[items.Length];
for (int i = 0; i < items.Length; i++)
arr[i] = int.Parse(items[i]);
int[] arr = Console.ReadLine().Split(' ')
.Select(int.Parse).ToArray();
2 8 30 25 40 72 -2 44 56
string.Split(' ')
splits string by space
and produces string[]
Use System.Linq to
enable .Select()

18
 Write a program to read an array of integers and find all triples of
elements a, b and c, such that a + b == c (a stays left from b)
Problem: Triple Sum (a + b == c)
1 1 1 1
No
4 2 8 6
4 + 2 == 6
2 + 6 == 8
3 1 5 6 1 2
3 + 2 == 5
1 + 5 == 6
1 + 1 == 2
1 + 2 == 3
5 + 1 == 6
1 + 2 == 3
Check your solution here:
https://judge.softuni.bg/Contests/Practice/Index/172#4
2 7 5 0
2 + 5 == 7
2 + 0 == 2
7 + 0 == 7
5 + 0 == 5

19
Solution: Triple Sum (a + b == c)
int[] nums = Console.ReadLine().Split(' ')
.Select(int.Parse).ToArray();
for (int i = 0; i < nums.Length; i++)
for (int j = i + 1; j < nums.Length; j++)
{
int a = nums[i];
int b = nums[j];
int sum = a + b;
if (nums.Contains(sum))
Console.WriteLine($"{a} + {b} == {sum}");
}
TODO: print "No"
when no triples
are found

Printing Arrays at the Console
Using foreach and String.Join()

Printing Arrays on the Console
 To print all array elements, a for-loop can be used
 Separate elements with white space or a new line
 Example:
string[] arr = {"one", "two", "three", "four", "five"};
// Process all array elements
for (int index = 0; index < arr.Length; index++)
{
// Print each element on a separate line
Console.WriteLine("arr[{0}] = {1}", index, arr[index]);
}
21

22
Problem: Rounding Numbers
 Read an array of real numbers (space separated values), round them
in "away from 0" style and print the output as in the examples:
0.9 1.5 2.4 2.5 3.14
0.9 => 1
1.5 => 2
2.4 => 2
2.5 => 3
3.14 => 3
-5.01 -1.599 -2.5 -1.50 0
-5.01 => -5
-1.599 => -2
-2.5 => -3
-1.50 => -2
0 => 0

23
Solution: Rounding Numbers
 Rounding turns each value to the nearest integer
 What to do when the value is exactly between two integers?
double[] nums = Console.ReadLine()
.Split(' ').Select(double.Parse).ToArray();
int[] roundedNums = new int[nums.Length];
roundedNums[i] = (int) Math.Round(nums[i],
MidpointRounding.AwayFromZero);
Console.WriteLine($"{nums[i]} -> {roundedNums[i]}");

Printing with foreach / String.Join(…)
 Use foreach-loop:
 Use string.Join(separator, array):
int[] arr = { 1, 2, 3 };
Console.WriteLine(string.Join(", ", arr)); // 1, 2, 3
string[] strings = { "one", "two", "three", "four" };
Console.WriteLine(string.Join(" - ", strings));
// one - two - three - four
24
int[] arr = { 10, 20, 30, 40, 50};
foreach (var element in arr)
Console.WriteLine(element)

25
 Read an array of strings (space separated values), reverse it and
print its elements:
 Reversing array elements:
Problem: Reverse Array
1 2 3 4 5 5 4 3 2 1 -1 20 99 5 5 99 20 -1
1 2 3 4 5
exchange

26
Solution: Reverse Array
var nums = Console.ReadLine()
.Split(' ').Select(int.Parse).ToArray();
for (int i = 0; i < nums.Length / 2; i++)
SwapElements(nums, i, nums.Length - 1 - i);
Console.WriteLine(string.Join(" ", nums));
static void SwapElements(int[] arr, int i, int j)
{
var oldElement = arr[i];
arr[i] = arr[j];
arr[j] = oldElement;
}

27
 Another solution to the "reverse array" problem:
 Or even shorter (without parsing strings to numbers):
Solution: Reverse Array – Functional Style
Console.WriteLine(string.Join(" ",
Console.ReadLine().Split(' ').Select(int.Parse)
.Reverse().ToArray()));
Console.WriteLine(string.Join(" ",
Console.ReadLine().Split(' ').Reverse()));

Reading and Printing Arrays

30
 Write a program that reads two arrays of integers and sums them
 When elements are less, duplicate the smaller array a few times
Problem: Sum Arrays
1 2 3 4
2 3 4 5
3 5 7 9
1 2 3 4 5
2 3
3 5 5 7 7
1 2 3 4 5
2 3 2 3 2
5 4 3
2 3 1 4
7 7 4 9
5 4 3 5
2 3 1 4

31
Solution: Sum Arrays
var arr1 = Console.ReadLine()
var arr2 = // TODO: read the second array of integers
var n = Math.Max(arr1.Length, arr2.Length);
var sumArr = new int[n];
for (int i = 0; i < n; i++)
sumArr[i] =
arr1[i % arr1.Length] +
arr2[i % arr2.Length];
Console.WriteLine(string.Join(" ", sumArr));
Loop the array: end  start
arr[len]  arr[0]
arr[len+1]  arr[1]
…

32
 Reads an array of integers and condense them by summing
adjacent couples of elements until a single integer is obtained:
Problem: Condense Array to Number
2 4 1 2
6 5 3
11 8
19
2 10 3
12 13
25
5 0 4 1 2
5 4 5 3
9 9 8
18 17
35

33
Solution: Condense Array to Number
int[] nums = Console.ReadLine()
while (nums.Length > 1)
{
int[] condensed = new int[nums.Length - 1];
for (int i = 0; i < nums.Length - 1; i++)
condensed[i] = // TODO: sum nums
nums = condensed;
}
Console.WriteLine(nums[0]);
2 10 3
12 13
0 1 2
nums[] :
condensed[] :

34
 Write a method to extract the middle 1, 2 or 3 elements from
array of n integers
 n = 1  1 element
 even n  2 elements
 odd n  3 elements
 Read n integers from the console and print the middle elements
Problem: Extract Middle 1, 2 or 3 Elements
1 2 3 4 5 6 7 { 3, 4, 5 }
2 3 8 1 7 4 { 8, 1 }
5 { 5 }
2 8 12 7 3 4 5 33 -2 8 22 4 { 4, 5 }

35
Solution: Extract Middle 1, 2 or 3 Elements
static int[] ExtractMiddleElements(int[] arr)
{
int start = arr.Length / 2 - 1;
int end = start + 2;
if (arr.Length == 1) start = end = 0;
else if (arr.Length % 2 == 0) end--;
int[] mid = new int[end - start + 1];
// Copy arr[start … end]  mid[]
return mid;
}
1 2 3 4 5 6 7
start end

36
 Read two arrays of words and find the length of the largest
common end (left or right)
Problem: Largest Common End
hi php java csharp sql html css js
hi php java js softuni nakov java learn
3
hi php java xml csharp sql html css js
nakov java sql html css js
4
I love programming
Learn Java or C#?
0

37
Solution: Largest Common End
static int LargestCommonEnd(
string[] words1, string[] words2)
{
var rightCount = 0;
while (rightCount < words1.Length &&
rightCount < words2.Length)
if (words1[words1.Length - rightCount - 1] ==
words2[words2.Length - rightCount - 1])
rightCount++;
else break;
return rightCount;
}

Arrays – Exercises

39
 Read an array of 4*k integers, fold it like shown below, and print
the sum of the upper and lower rows (2*k integers):
Homework: Fold and Sum
1 2 3 4 5 6 7 8
2 1 8 7
3 4 5 6
5 5 13 13
4 3 -1 2 5 0 1 9 8 6 7 -2
-1 3 4 -2 7 6
2 5 0 1 9 8
1 8 4 -1 16 14
5 2 3 6
5 6
2 3
7 9
3 4 5 6
3 4 5 6

40
 Read an array of n integers and an integer k. Rotate the array
right k times and sum the obtained arrays as shown below:
Homework: Rotate and Sum
3 2 4 -1
2
-1 3 2 4
4 -1 3 2
3 2 5 6
1 2 3
1
3 1 2 3 1 2
1 2 3 4 5
3
5 1 2 3 4
4 5 1 2 3
3 4 5 1 2
12 10 8 6 9

41
 Arrays hold sequence of elements
 Elements are numbered from 0 to length-1
 Creating (allocating) an array:
 Accessing array elements by index:
 Printing array elements:
Summary
int[] numbers = new int[10];
numbers[5] = 10;
Console.Write(string.Join(" ", arr));

?
Arrays
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is licensed under the "Creative Commons Attribution-
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 "Fundamentals of Computer Programming with C#" book by Svetlin Nakov & Co. under CC-BY-SA license
43

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