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CIS162AD - C#
Arrays – Part 2
13_arrays_sorting.ppt
CIS162AD 2
Overview of Topics
Declaring Arrays Loading Arrays Processing Arrays Sorting Arrays Searching Arrays Parallel Arrays Multi-dimensional Arrays
CIS162AD 3
Array Processing
1. Declare Array2. Load Array
• After creating the array, data must be loaded.• Use arrayName.Length or arrayName.GetUpperBound(0) to
prevent out of range errors.• Note: Arrays can be declared and loaded with constant values.
3. Process Array• Use individual elements in calculations or as arguments.• Send entire arrays to procedures for processing.• Sort, Search, Display• Use a lot of For loops or For Each loops.
CIS162AD 4
Array Management If during execution, the subscript value referenced an element
past the end of the array, the program would throw a subscript out range exception (run-time error).
The programmer must make sure that the logic in the program does not allow the subscript to exceed the array size.
We can use some built in methods to manage arrays,arrayName.LengtharrayName.GetUpperBound(0)
For partially filled arrays we must count how many items are loaded and then use the count when processing the arrays.
CIS162AD 5
Array SubscriptdataType[ ] arrayName = new dataType[arraySize];
Arrays are allocated consecutive memory. Each element is referenced using a subscript. Subscript are integers. The number of elements that are created is arraySize. The first element in the array is referenced with a
value of zero [0]. The last element is referenced with a subscript value
of [arraySize – 1]. A subscript is also referred to as an index. Short variable names for subscripts are acceptable.
CIS162AD 6
Declare Array
//Arrays are declared at the class level, //so they can be referenced by all methods.
const int intARRAY_SIZE = 20;int[ ] cintTestScores = new int[intArraySize]; int cintNumberOfStudents;
//We can load up to 20 scores, but we will save //how many tests are actually loaded in
cintNumberOfStudents.
CIS162AD 7
Array Memory MapPosition Address Index Value
1 1010 [0] 0
2 1014 [1] 0
3 1018 [2] 0
4 1022 [3] 0
5 1026 [4] 0
6 1030 [5] 0
7 1034 [6] 0
… … … …
20 1086 [19] 0
CIS162AD 8
Loading Partially Filled Arrays//Loads Array with scores saved in a data file.private void btnLoadArray_Click( ){
FileStream studentFile = new FileStream("cs12ex.txt", FileMode.Open); StreamReader studentStreamReader = new StreamReader(studentFile);
int i = 0; //subscript initialized to zero while (studentStreamReader.Peek() != -1){ if (i < cintTestScores.Length) { cintTestScores[i] = int.Parse(studentStreamReader.ReadLine( ));
i ++; //Increment subscript by one } else { MessageBox.Show (“Array Size Exceeded”);
break; //Get of out of loop; Array is full. }}cintNumberOfStudents = i; //Save how many students were loadedstudentFile.Close( ); //Close file
}
cs12ex.txt
5040100301020
CIS162AD 9
Loaded ArraysPosition Address Index Value
1 1010 [0] 50
2 1014 [1] 40
3 1018 [2] 100
4 1022 [3] 30
5 1026 [4] 10
6 1030 [5] 20
7 1034 [6] 0
… … … …
20 1086 [19] 0
CIS162AD 10
Partially Filled Arrays
Up to 20 scores could be loaded, but only 6 scores are actually loaded.
When the array is not full, it is considered a partially filled array.
When processing arrays, we need to make sure we only process the number scores loaded.
The number of scores loaded were counted in the load routine, and the count was saved in cintNumberOfStudents.
CIS162AD 11
Sorting Arrays
Data is always being sorted. It is important that we understand how sorts work. There are various sort algorithms. We’ll only be looking at the simple selection sort. Selection Sort Algorithm:
– Find the lowest value in the array.
– Move it to the top of the array.
– Find the next lowest value, and move it to the 2nd position.
– Continue until the end of the array is reached.
CIS162AD 12
Sort Ascendingprivate void sortAscending(…){ int i, i2; int intMinScore, intMinSubscript, intHoldValue;
for (i = 0; i < cintNumberOfStudents - 1; i++) //-1 so we don’t all the way to the bottom {
intMinSubscript = i; intMinScore = cintTestScores[i]; for (i2 = i + 1; i2 < cintNumberOfStudents; i2++)
{ if {cintTestScore[i2] < intMinScore)
{ intMinSubscript = i2; intMinScore = intTestScores[i2]; } }
intHoldValue = cintTestScores[i]; //swap values cintTestScores[i] = cintTestScores[intMinSubscript]; cintTestScore[intMinSubscript] = intHoldValue; }}
CIS162AD 13
Sort Algorithm Demonstration
const int intARRAY_SIZE = 20;int[ ] cintTestScores = new int[intArraySize]; int cintNumberOfStudents;
CIS162AD 14
Declared Array - cintTestScoresAddress Subscript Value
1010 [0] 0
1014 [1] 0
1018 [2] 0
1022 [3] 0
1026 [4] 0
1030 [5] 0
1034 [6] 0
… … …
1086 [19] 0
CIS162AD 15
Load ArraySubscript Value
[0] 50
[1] 40
[2] 100
[3] 30
[4] 10
[5] 20
[6] 0
… …
[19] 0
Read 6 numbers from file and load them into array.
Numbers of Students = i
i = 6
CIS162AD 16
Sort Ascending 1st positionSubscript Value
[0] 50
[1] 40
[2] 100
[3] 30
[4] 10
[5] 20
[6] 0
… …
[19] 0
Move the value in the 1st position to holdValue = 50.
Find the lowest value and move it to the 1st position.
Move holdValue to the position that the lowest value was found, swap values.
CIS162AD 17
Sort Ascending 2nd positionSubscript Value
[0] 10
[1] 40
[2] 100
[3] 30
[4] 50
[5] 20
[6] 0
… …
[19] 0
Move the value in the 2nd position to holdValue = 40.
Find the next lowest value and move it to the 2nd position.
Move holdValue to the position that the lowest value was found.
CIS162AD 18
Sort Ascending 3rd positionSubscript Value
[0] 10
[1] 20
[2] 100
[3] 30
[4] 50
[5] 40
[6] 0
… …
[19] 0
Move the value in the 3rd position to holdValue = 100.
Find the next lowest value and move it to the 3rd position.
Move holdValue to the position that the lowest value was found.
CIS162AD 19
Sort Ascending 4th positionSubscript Value
[0] 10
[1] 20
[2] 30
[3] 100
[4] 50
[5] 40
[6] 0
… …
[19] 0
Move the value in the 4th position to holdValue = 100.
Find the next lowest value and move it to the 4th position.
Move holdValue to the position that the lowest value was found.
CIS162AD 20
Sort Ascending 5th positionSubscript Value
[0] 10
[1] 20
[2] 30
[3] 40
[4] 50
[5] 100
[6] 0
… …
[19] 0
Move the value in the 5th position to holdValue = 50.
A lower value than 50 will not be found, but the comparison must still be made.
CIS162AD 21
Sort Ascendingprivate void sortAscending(…){ int i, i2; int intMinScore, intMinSubscript, intHoldValue;
for (i = 0; i < cintNumberOfStudents - 1; i++) //-1 so we don’t all the way to the bottom {
intMinSubscript = i; intMinScore = cintTestScores[i]; for (i2 = i + 1; i2 < cintNumberOfStudents; i2++)
{ if {cintTestScore[i2] < intMinScore)
{ intMinSubscript = i2; intMinScore = intTestScores[i2]; } }
intHoldValue = cintTestScores[i]; //swap values cintTestScores[i] = cintTestScores[intMinSubscript]; cintTestScore[intMinSubscript] = intHoldValue; }}
CIS162AD 22
Sequential Search Algorithm
Before searching, the data is usually sorted first. Look through the array elements from the first to the
last looking for a match. After a match is found, we are going to do something
with item found later, so the index of where the item was found must be saved.
If the item was not found, we also need to record this using a flag or special value, so that the error can be reported.
CIS162AD 23
Search Array – part 1
private void searchArray(…) int i; int intSearchNumber; int intNumberLocation;
bool blnNumberFound; blnNumberFound = false; intSearchNumber = int.Parse(txtSearch.Text);
//see next slide for the rest of the method
CIS162AD 24
Search Array – part 2 for (i = 0; i < cintNumberOfStudents; i++) { if (intSearchNumber = = cintTestScores[i])
{ blnNumberFound = true; intNumberLocation = i;
lblOutput.Text = intSearchNumber.ToString(“N0”) + “ is located in array position “ + _
intNumberLocation. ToString(“N0”); } }
if (blnNumberFound = = false) lblOutput.Text = intSearchNumber.ToString(“N0”) +
“ is not in the array.“;}
CIS162AD 25
Search Array – Match FoundSubscript Value
[0] 10
[1] 20
[2] 30
[3] 40
[4] 50
[5] 100
[6] 0
… …
[19] 0
intSearchNumber = 40.
For loop walks through array checking if cintTestScores(i) = = intSearchNumber.
When a match is found, the subscript value is displayed (3).
CIS162AD 26
Search Array – Early ExitSubscript Value
[0] 10
[1] 20
[2] 30
[3] 40
[4] 50
[5] 100
[6] 0
… …
[19] 0
intSearchNumber = 25.
In the for loop we should also check if intSearchNumber < cintTestScores[i] .
If it is, then we know that we will not find the value. We can exit search.
Must be sorted.
CIS162AD 27
Search Array – Early Exit for (i = 0; i < cintNumberOfStudents; i++) { if (intSearchNumber = = cintTestScores[i])
{ blnNumberFound = true; intNumberLocation = i;
lblOutput.Text = intSearchNumber.ToString(“N0”) + “ is located in array position “ + _
intNumberLocation. ToString(“N0”); } else if (intSearchNumber < cintTestScores[i]) {
lblOutput.Text = "Match not found - Early Exit “;
break; // get out for loop } }
CIS162AD 28
Parallel Arrays An array is used to process a collection of data all of
which is of the same data type (Integer, Decimal, String, etc.).
When there is related data of different data types, then the data must be loaded into separate but parallel Arrays.
This means that related values are loaded into different arrays but are connected to each by the subscript value.
All of the related values will have the same subscript value.
CIS162AD 29
Parallel Arrays – Simplified Example
Names and scores stored in a file
string[ ] cstrName = new string[19]; int[ ] cintScore = new int[19];
for (i = 0; i < 20; i++){
cstrName[i] = studentStreamReader.ReadLine( );cintScore[i] =
int.Parse(studentStreamReader.ReadLine( ));}
The related student data in different arrays must be kept together, especially when sorting.
CIS162AD 30
Sorting Parallel Arrays
If sorting by Name, John would be moved to the first position. The data in the related array (Score of 70) must also be swapped. The sort routine would require additional swap statements.
Subscript Name
[0] Todd
[1] Mary
[2] John
[3]
Subscript Score
[0] 90
[1] 50
[2] 70
[3] 0
Comparing Strings The only comparison operators defined for a
string are == and !=. To find less than or greater than use CompareTo method.
if (cstrName[i2].CompareTo(strMinName) < 0)
The CompareTo method returns:-1 if current element is less than the compare element 0 if current element is equal to the compare element 1 if currernt element is greater than the compare element
CIS162AD 31
CIS162AD 32
Array Class Sort & Search Methods
Along with the Length property and GetUpperBound method, the array class also has a Sort and BinarySearch method.
Array.Sort(arrayName) sorts the elements in a one-dimensional array into ascending order.
Array.Sort will not work with parallel arrays because associated data are stored in different arrays.
Array.BinarySearch(arrayName, searchValue) searches a one-dimensional array that is sorted in ascending order for an element with a specified value. It returns the index of the element if found.
CIS162AD 33
Two Dimensional Array
Single Dimensional Arrays are thought of as one column and many rows.
Single Dimensional Arrays require one subscript to reference an individual element.
Two Dimensional Arrays are many columns and many rows.
Two Dimensional Arrays require two subscripts to reference an individual element.
CIS162AD 34
Two Dimensional Array - Example Sum sales data for 10 departments and 4 quarters.
decimal[,] decSales = new decimal[10, 4];
for (i = 0; i < 10; i++) // 0 – 9 {
for (j = 0; j < 4; j++) // 0 – 3 {
decTotal += decSales[i, j];}
}
Each individual element requires two subscripts.
CIS162AD 35
Two Dimensional Array - Data
Subscript i is going down the rows.
Subscript j is going across the columns.
Subscripts [0] [1] [2] [3]
[0] 10 15 25 30
[1] 20 22 32 42
[2] 30 33 43 15
[3] 0 0 0 0
[etc.] 0 0 0 0
10, 15, 25, 3020, 22, 32, 4230, 33, 43, 15
CIS162AD 36
Name Array – 1D Need an array
to store the names of 5 people that are head of the household.
string[ ] strName = new string[5]
Juan Marquez
John Smith
Sue Bradley
Pat West
Mark Jones
CIS162AD 37
Name Array – 2D Need an array
to store 5 names, but first and last name separately.
string[ , ] strName
= new string[5,2]
Juan Marquez
John Smith
Sue Bradley
Pat West
Mark Jones
CIS162AD 38
Name Array – 3D Need an array to
store the first and last names of up to 5 household members of 4 households.
string[ ,, ] strName = new string[4,5,2]
West is in strName[0,3,1].
household, row, col
Juan Marquez
John Smith
Sue Bradley
Pat West
Mark Jones
CIS162AD 39
Name Array – 4D Need an array to store the first and last names of up
to 5 household members of 4 house holds for two different years.
string[ ,,, ] strName = new string[2,4,5,2] See next slide. The first dimension is the cube (year). West is in strName[0,0,3,1] and [1,0,3,1].
CIS162AD 40
Name Array – 4D picture
Juan Marquez
John Smith
Sue Bradley
Pat West
Mark Jones
Juan Marquez
John Smith
Sue Bradley
Pat West
Mark Jones
CIS162AD 41
Multi-Dimensional Arrays Arrays are actually stored consecutively in memory, not
in a form represented in the drawings in the prior slides. We draw pictures so that we can visually see the data we
are trying to manipulate. The computer can handle as many dimensions as you
would like to add. 3 or 4 dimensions should be our maximum. Keep in mind that eventually you or someone will need
to maintain the program. If the array is to complex, it will take a while to get reacquainted with the logic.
CIS162AD 42
Summary Declaring Arrays Loading Arrays Processing Arrays Single Dimensional Multi Dimensional The best way to understand these slides is to work
through the examples using your own values. Desk check the logic to see how values are sorted.
