COP 2360 – C# Programming

Chapter 7 & 8- Arrays and Strings

Review Test…..

Pseudo Code Exercise

Someone hands you a folder with student test records.

Each test record has the student’s name and five test scores (from 0 to 100)

Write the pseudo code to calculate the average of the three test scores for each student.

Arrays

Life without arrays– Suppose we needed to get the average grade for

five test scores and then print out all grades lower than that average.

– Since we don’t know which scores to output until AFTER all of the data has been input, we will need to hold data until the five test scores have been read in.

So we create five variables, input the information, and then test each one of them.

Life Without Arrays

//Program to find the average test score and output the average//test score and all the test scores that are less than //the average test score.

static void Main(string[] args) {int test0, test1, test2, test3, test4;double average;

Console.WriteLine("Enter five test scores. One on each line: ");

test0 = int.Parse(Console.ReadLine());test1 = int.Parse(Console.ReadLine());test2 = int.Parse(Console.ReadLine());test3 = int.Parse(Console.ReadLine());test4 = int.Parse(Console.ReadLine());

average = (test0 + test1 + test2 + test3 + test4) / 5.0;

Console.WriteLine("The average test score = {0}", average);

if (test0 < average) {Console.WriteLine("{0} is less that the average test score.", test0);}if (test1 < average) {Console.WriteLine("{0} is less that the average test score.", test1);}if (test2 < average) {Console.WriteLine("{0} is less that the average test score.", test2);}if (test3 < average) {Console.WriteLine("{0} is less that the average test score.", test3);}if (test4 < average) {Console.WriteLine("{0} is less that the average test score.", test4);}Console.ReadKey();return;}

What’s an Array And how can this help

An array is a collection of variables of the same data type.

The array is referenced by a single name suffixed with an index pointer.

Let’s redo our grading example

Life With an Array

int [] test = new int[5];double average;

Console.WriteLine("Enter five test scores. One on each line: ");

test[0] = int.Parse(Console.ReadLine());test[1] = int.Parse(Console.ReadLine());test[2] = int.Parse(Console.ReadLine());test[3] = int.Parse(Console.ReadLine());test[4] = int.Parse(Console.ReadLine());

int total = 0;for (int i =0;i<5;i++){total = total + test[i];}average = total / 5.0;

Console.WriteLine("The average test score = {0}", average);

for (int i =0;i<5;i++){ if (test[i] < average)Console.WriteLine("{0} is less that the

average test score.", test[i]);}

Console.ReadKey();return;

Array Basics

When declared, memory is set aside based on the variable type and number of elements in the array

– int[] nPrices = new int [100]Will allocate an array named nPrices with 100 elementsThe elements are indexed from 0 to 99:

(why not 1 to 100 – Which by the way is one of the things I hate with “C” Type Languages)

Each element can hold one integer value The above example is a “one dimensional” array. It may be easier to think that the computer is creating 100

separate variables named nPrices[0] through nPrices[99] (or maybe not)

Array Basics

Array variables can be used in any of our expressions just like any other variables. But they MUST include the suffix with the index.

In the example we just went through, we had:– Console.WriteLine("{0} is less that the average

test score.", test[i]); What do you think would happen if I

reference just test?– Console.WriteLine(test);

Array Basics

Normal arrays sizes are static.– That is, they cannot be changed once initialized.– You can use constants or variables in the definitions

const int nARRAY_SIZE = 20; int[] nMyArray = int[nARRAY_SIZE];

– Next Week we’ll touch or ArrayLists which can be dynamic

Arrays can be initialized when they are declared– char[] sLetterGrade = new char[5] {‘A’,’B’,’C’,’D’,’E’};– int[] nScore = new int[5] {0,0,0,0,0}; // initializes ALL

elements to 0– int[] nDemo = {3,6,9}; //Creates an array with three elements

Array Basics

Supposed we have two arrays– int[] nArrayOne = {1,2,3,4,5};– int[] nArrayTwo = new int[5];

We would like to copy nArrayOne to nArrayTwo– But we cannot do it directly!!– We must copy element by element.– Same is true for comparison, printing, etc.– Also, as parameters to methods, arrays are always By Ref

Standard Array Copy Routine

for (int i=0;i<5;i++){

nArraytwo[i] = nArrayone[i];}

how bout doing that in a function?static void copyArray(int[] pnArrayIn, int[] pnArrayOut, int pSize) { for (int i = 0; i < pSize; i++) { pnArrayOut[i] = pnArrayIn[i]; }

} What happens if I do say nArrayTwo = nArrayOne?

Let’s do some fun Array stuff

int[] nSomeFun = {5, 4, 7, 2, 3, 5, 1, 8, 9, 0};nSomeFun[5] = nSomeFun[5] + nSomeFun[9];nSomeFun[1] = nSomeFun[2] * nSomeFun[6];nSomeFun[0] = nSomeFun[0] + nSomeFun[9];nSomeFun[4] = nSomeFun[1] + nSomeFun[2];for (int i=0;i<10;i++)

{nSomeFun[9] = nSomeFun[9] + nSomeFun[i];

} What are the values of each element of nSomeFun?

– It may help to create a grid to hold the initial values like an excel sheet.

Strings – Stringsand even more Strings

In the beginning, c had no string manipulation abilities.

– Instead all they had were character arrays.– They had to manipulate text using the same type of array

manipulation stuff we just looked at for other simple variables.

– c was used a lot for scientific calculations and as a replacement for assembly language (and when compared to assembly language, the programmers were just as happy as they could be!!)

Strings – Stringsand even more Strings

But, as we have seen above, arrays have limitations that make sense for a series of numbers, but not a series of letters.– You can’t compare two arrays without using a for

loop– You can’t easily move the contents of one array to

another

The ‘c-string’

But, because c used functions and libraries in a very similar manner as c#, some industious programmers came up with the ‘c-string’

– Basically they wrote a library (cstring) that included functions to make it easier to manipulate character arrays

– At first, there were many, many libraries until they were standardized into the cstring library available today.

They defined a ‘c-string’ as an array of type char where the last character of the string contain the “null” character (0)

The string library

Because c-strings sucked so badly, many more businesses with many more programmers wrote a new, new library called “string” which makes string values look and act like regular variable types.

These also had a bunch of different varieties, and these were again standardized into what we now know as the “string” data type.

The String Class

C# carried the String Library further into the String Class Stores a collection of Unicode characters Immutable series of characters (What does that mean)

– Making a change isn’t making a change, it’s making a new string!!

Reference type – Normally equality operators, == and !=, compare the object’s

references, but operators function differently with string than with other reference objects

Equality operators are defined to compare the contents or values

Includes large number of predefined methods

The String Class

Can process variables of string type as a group of characters– Can also access individual characters in string

using an index with [ ] First character is indexed by zero

string sValue = "C# Programming";object sObj;string s = "C#";

Some String Functions

More Useful String Functions

More Useful String Functions

Let’s play with some string functions

Write a program that uses Console.ReadLine() to store a line of text into a string variable.

Then, go into a loop and ask the user to enter a character.

Find the character in the string. If found, tell the user where the character is and replace it with the letter “X”, then display the text back. If not found, say that it wasn’t found.

If the user enters a “#”, stop the program.

A now for Assignment 3

Let’s Play Hangman!!

And now for a very bad segue

Let’s go back to our talk about arrays….

Let’s Search for some stuff

Let’s suppose we have an array of integers We want to have the user enter an integer

and then search through the array for that value

How would we do this?– How would we do this by hand?– What programming language basic process would

we need to use to facilitate this?

Let’s Sort an Array

Let’s suppose we have the same array of integers

We want to sort the array from smallest to largest integer value

How would we do this?– How would we do this by hand?

Let’s Do It Now Let’s Program It

Let’s Sort an Array

Write a program that creates a 10 item array of type int.

The program should then ask the user to enter 10 integers.

Once stored in the array, the program should use the sort algorithm we just worked out to sort the numbers, and then output the sorted array.

Let’s talk about our Find algorithm now that the data is sorted.

What are some of the ways we can improve our find algorithm if the array is sorted?

How does each change potentially change how long it takes to search for something?– ut oh, we may be talking logs here

Parallel Arrays

Sometimes you may have two or more arrays that are linked in someway– State abbreviations to State Names for example.

String[] sStateAbbrevs = new String[50]; String[] sStateNames = new String[50];

– Now, we can have a user enter the two digit state abbreviation, use our look-up logic from a couple of slides ago, and find the abbreviation. Once we have the abbreviation, we are also pointing to the name.

Two Dimensional Array

double[,] fSalesArray = new double [10] [5];– initializes a two dimensional array with 10 rows and 5 columns– What could be some examples where we might want to use a two

dimensional array?– What other “things” look like the example below?

Jagged Arrays

Really an Array of Arrays where the the first brackets identifies the number of arrays. The second the item within that array– int[][] n7 = new int[2][] { new int[] {2,4,6}, new int[]

{1,3,5,7,9} }; – int[][] n8 = new int[][] { new int[] {2,4,6}, new int[]

{1,3,5,7,9} }; – int[][] n9 = { new int[] {2,4,6}, new int[]

{1,3,5,7,9} };

Multidimensional Arrays

Just like we did with two dimensional arrays, one can define as many dimensions as you would like when an array is created.