Lecture 15
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Log into Windows/ACENET. Use a web
browser to download today's exercise program.
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Go to course webpage
http://csserver.evansville.edu/~hwang/s11-courses/cs205.html
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Right-click on TestStatsProgram.zip link. Save
target/link to Documents -> Visual Studio 2010 ->
Projects folder.
Browse to TestStatsProgram.zip, right-click on it
and select Extract All... and extract project folder
Double-click into the TestStatsProgram folders to
the solution file, then double-click it to start up VS.
Monday, February 14
CS 205 Programming for the Sciences - Lecture 15
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Outline
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Programming Assignment 4 clarifications
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Reminder: Programming Assignment 4 due on
Wednesday by 4:30pm.
Questions?
Arrays
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Declaration and creation arrays
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Array element access
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Accessing all array elements using for-loops
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Array parameters
Monday, February 14
CS 205 Programming for the Sciences - Lecture 15
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Programming Assignment 4
∞
=4 ∑
k=0
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k
−1
1 1 1 1 1
=4  −  −  
2 k1
1 3 5 7 9
In the above summation, k (the loop counter)
starts at 0, rather than 1, so to sum
numberOfTerms terms (the number input by
the user), the final value of k in ComputePI
should be (numberOfTerms-1) rather than
numberOfTerms. This can be done by making
the loop counter test (k < numberOfTerms)
rather than (k <= numberOfTerms).
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CS 205 Programming for the Sciences - Lecture 15
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Programming Assignment 4
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The example output in the assignment sheet
counts 0 to numberOfTerms. (i.e.,
(numberOfTerms+1) terms). With only
numberOfTerms terms, the output would be:
Enter the number of terms for this approximation of PI: 10
10 terms give an approximate value for PI of 3.041840.
Enter the number of terms for this approximation of PI: 100
100 terms give an approximate value for PI of 3.131593.
Enter the number of terms for this approximation of PI: 1000
1000 terms give an approximate value for PI of 3.140593.
Enter the number of terms for this approximation of PI: 0
Good­bye!
Monday, February 14
CS 205 Programming for the Sciences - Lecture 15
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Programming Assignment 4
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Although you can use Math.Pow (-1, k) to compute the
numerator of each term, doing so is inefficient. All that
is needed is for the sign to alternate between
iterations. Here are a few ways to do so:
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Use an integer variable for the numerator of the
term that is initially 1.0 and negate it (using unary
minus) after each term is computed.
After computing a term, use an if-statement with a
condition so that the if-body adds the term and the
else-body subtracts the term. E.g., test whether k
is even or odd (evenly divisible by 2); or test a
boolean flag that is initially true and negated it
(using logical not) after the if-statement.
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TestStats Program
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Examine file TestStats.cs. So far, we have
dealt with variables that are a single value. But
sometimes it is inconvenient to have unique
names for every memory location.
For today's example, we want to compute the
average of a number of test scores and the
difference of each test score relative to the
average for a class of students. For CS 205,
we have 11 students, so we have variables
named score1, score2, score3, ...
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CS 205 Programming for the Sciences - Lecture 15
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TestStats Program
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To solve our problem, there are separate statements
to read in a test score from the console into each of
the variables. (Note that the GetTestScore method
returns the input value rather than passing it back.)
To compute the average, all the variables are summed
together using a very long addition expression
(score1+score2+score3+...).
Then there are separate statements to display all the
test scores and their differences.
This it tedious and error prone. And not very flexible:
what if the class has 50 students? 100?
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CS 205 Programming for the Sciences - Lecture 15
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Arrays
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What we need is a way to name a collection of
values and a way to access an individual item
in the collection using that name.
An array is a collection of items that can be
accessed by giving the position (or index) of
the value in the collection.
Formally, an array is a fixed-size ordered
collection of items (called the elements) of the
same type.
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Array Declaration and Creation
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Unlike the primitive types (int, double, etc.), an
array (in C#) must be created before it can be
used. Array variables are declared and array
objects are created using the following syntax:
array variable declaration
array creation statement
<type> [] <name> = new <type> [ <size> ];
always empty
where type is the element type and size is the
number of elements in the created array. Note
that the [ ]'s in the type name are empty.
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Array Declaration and Creation
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For our example, we could declare:
double [] score = new double [11];
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Even better would be to ask the user:
string userInput;
int numStudents;
Console.Write ("Enter the number of students: ");
userInput = Console.ReadLine();
numStudents = int.Parse(userInput);
double [] score = new double [numStudents];
score array's size is numStudents
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CS 205 Programming for the Sciences - Lecture 15
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Array Element Access
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To access an array element, you give the array
name and the index of the element.
Almost all programming languages (including
C#) use square brackets for indexing:
<array name>[<index>]
as the syntax for accessing an array element.
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This expression can be used on either the left
of an assignment (as a variable to set the array
element value) or the right side (as an
expression).
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Array Element Access
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Here is one way of looking at what an array is:
double [] score = new double [11];
score[0]
score
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score[1]
score[2]
score[10]
...
Note that indexing of the array elements starts
at 0 (not 1), so the 11th element has index 10.
So for all arrays, the last element has index
(size-1).
Monday, February 14
CS 205 Programming for the Sciences - Lecture 15
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Accessing All Array Elements
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Typically, we want to access each array
element and do something with it. As the
indexes are sequential, we can use a for-loop
for this. The indexes run from 0 to size-1, so
such a loop test uses < (rather than <=):
for (int index = 0; i < score.Length; i++)
{
// do something with score[i]
}
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Note: arrays have a property called Length,
that is the size of the array. Properties are
access using the dot ('.') notation.
Monday, February 14
CS 205 Programming for the Sciences - Lecture 15
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In-class Exercise
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Modify the Main method of the TestStats
program as follows:
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Replace the score variables with an array of 11
elements.
Replace the calls to GetTestScore with a for-loop
that calls GetTestScore for each array element.
Modify the computation of total to use the array
elements. (We will fix this later.)
Replace the calls to DisplayResultRow with a forloop that calls DisplayResultRow for each array
element.
Monday, February 14
CS 205 Programming for the Sciences - Lecture 15
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Array Parameters
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Just like other data items, arrays can be passed
to a method as a parameter. Unlike other data
items, arrays are always passed by reference,
and there is no special syntax for doing so.
The syntax for an array parameter is:
... <methodname> (<type> [ ] <arrayname>, ...)
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For example, a method ComputeAverage that
receives an array and returns the average of
the values in the array would have header:
static double ComputeAverage (double [ ] array)
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Array Arguments
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To call a method that has an array parameter,
the corresponding argument simply is the array
name. (That is, there are no square brackets or
size.)
In the example, to call ComputeAverage, we
would write:
average = ComputeAverage (score);
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CS 205 Programming for the Sciences - Lecture 15
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In-class Exercise
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Modify the TestStats program as follows:
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Add a method ComputeAverage that receives an
array of real numbers and returns the average of
the numbers in the array. The average should be
computed by using a for-loop to sum the elements
of the array together, then dividing by the number of
elements in the array.
At the beginnin of the Main method, write code to
get the number of test scores from the user; change
the array creation to use this number for its size.
Replace the average-computing code in the Main
method with a call to ComputeAverage.
Monday, February 14
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