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COMP 1200Matlab Course Notes Set 5 - 1 of 59Auburn UniversityComputer Science and Software Engineering

Course Notes Set 5a:

COMP1200

Introduction to Computing for Engineers and ScientistsMATLAB Programming

Arrays: Vectors and Matrices

Computer Science and Software EngineeringAuburn University

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Arrays: IntroductionWhat if we had a problem where we needed to average all thegrades in a COMP1200 class? How would we store all thegrades entered by the user?

Using knowledge we have now we could approach this in acouple ways:

1. We could put all the grades in a data file. Then, open and readthe data file each time we want to work with the grades.Advantage: Code is relatively simple to write

Disadvantage: File I/O is very slow

2. We could declare a variable for each student in the class.

Wed then just write an input statement to load each variable.Advantage: All the variables are in memory, so its fast

Disadvantage: Thats a LOT of variables in a class with 100students!

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% Read in grades in a variable for each gradeg1 = input(Enter 1st grade: );

g2 = input(Enter 2nd grade: );

. . .g100 = input(Enter 100th grade: );

% Average gradesavg = (g1 + g2 + g3 +...+ g100)/100.0;printf(Average is: %f,avg);

Lets imagine what a program using technique #2would look like:

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Arrays

Now what happens when we add 20 students to the class? Wehave to add all those new variables! This is not good.

92 72 87 61 89 55 99 . 86 76

g1 g2 g3 g4 g5 g6 g7 . g99 g100

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Arrays

A program with this many variables can be long and frustrating touse. What if there was a shorthand notation for the many relatedvariables. It would be nice if we could write:

%Read in grades

disp(Enter 100 grades: );

for i=1:100

gi = input(Enter a grade: );

This will not work because MATLAB would treat gi as a variable

and reassign 100 grades to it.

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Arrays

This is where a MATLAB data structure called an arraycomes in tohelp us out.

You can visualize the compiler giving you 100 variables arranged in arow:

92 72 87 61 89 55 99 86 76

g1 g2 g3 g4 g5 g6 g7 g99 g100

g(1) g(2) g(3) g(4) g(5) g(6) g(7) g(99) g(100)

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7/28COMP 1200Matlab Course Notes Set 5 - 7 of 59

Auburn UniversityComputer Science and Software Engineering

Array elementArray name Subscript

g(0)

We call each individual variable in an array an elementof thearray. We give the collection of elements a name, the array

name. An array can be given any valid variable name.

To get at the individual elements of an array, we number themstarting at one, incrementing by one. These numbersare called the subscripts.

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8/28COMP 1200Matlab Course Notes Set 5 - 8 of 59Auburn UniversityComputer Science and Software Engineering

Arrays: Initializing with values>> t = [5 4 -2 1 3 9 7 0 -6] % Spaces or commas

t = 5 4 -2 1 3 9 7 0 -6

>> t(4)

ans =

1

>> t(3:2:9) % Colon Notation to access blocks

ans =

-2 3 7 -6

>> t(1:5) % Default step is 1

ans =5 4 -2 1 3

>> t(6:end)

ans =

9 7 0 -6

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Arrays: Initializing using expressions

>> x = [0 .1*pi .2*pi .3*pi .4*pi .5*pi]

x =0 0.3142 0.6283 0.9425 1.2566 1.5708

>> x = (0:.1:.5)*pi

x =

0 0.3142 0.6283 0.9425 1.2566 1.5708

>> x = x * 10

x =

0 3.1416 6.2832 9.4248 12.5664 15.7080

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Standard Arrays

>> ones(1,5)

ans =1 1 1 1 1

>> count=zeros(1,4)

count =

0 0 0 0

>> count(1)

ans =

0

>> par(1:5)=4

par =

4 4 4 4 4

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Arrays: Initializing

We can also initialize arrays from within our program, this is usually

done with a loop:

for k = 1:10

g(k) = k*0.5;

This program segment takes each element of an array and sets it tothe current value of k. After running, the array looks like:

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

g(1) g(2) g(3) g(4) g(5) g(6) g(7) g(8) g(9) g(10)

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Arrays: Modifying

>> t = [5 4 -2 1 3 9 7 0 -6]

t =5 4 -2 1 3 9 7 0 -6

>> t(4) = 22

t =5 4 -2 22 3 9 7 0 -6

>> g = zeros(1,5)

g =

0 0 0 0 0

>> g = [10 20 30 40]

g =

10 20 30 40

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Arrays: Using In Programs

Using arrays in a program is no different than using normalvariables, you just have to remember the array subscript.

This is how the compiler will know which array element youwant to work with.

Lets take an example program that averages the elements of

an array:

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y = [76.0, 35.4, 99.2, 81.0,65.5];count = numel(y); % returns number of elements

sum = 0;

% Total each array elementfor n = 1:count

sum = sum + y(n);

end

% Compute the averageaverage = sum / count ;

fprintf('Average = %.2f\n',average);

76.0 35.4 99.2 81.0 65.5

y(1) y(2) y(3) y(4) y(5)


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Lets suppose we wanted to print all the values in the array:

for n = 1:count

fprintf(%.1f\n',y(n));

end

This prints each element of the array, one per line.

76.0

35.4

99.281.0

65.5

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Similarly, if we wanted to print the numbers on one line:

for n = 1:count

fprintf(%.1f ',y(n));

end% Add a next line after all numbers

fprintf(\n);

76.0 35.4 99.2 81.0 65.5

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Similarly, if we wanted to print them to a data file :

for n = 1:count

fprintf(myfile, '%.1f\n', y(n));

end

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Arrays: Passing To Functions

We have to consider a couple cases when dealing witharrays and functions.

Passing A Single ElementPassing An Entire Array

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Arrays: Passing A Single ElementTo Functions

function sum = sumFun( a, b )

... function bodyend

Now lets suppose in my main program I want to use sumFun,and I want it to operate on a couple of my array elements.

Heres how I would call the function:

x=[3 2 5 6 2]; i=4; j=1;result = sumFun(x(4), x(1));

or

result = sumFun(x(i), x(j));

So heres the rule: When the arguments are non-arrayvalues, the actual arguments sent must be subscripted arrayelements.

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Arrays: Passing An Entire Array To Functions

When passing an entire array to a function, we usually have to

supply two parameters: the name of the array being passed andthe number of values that will be used.

Consider the following initialization:

g = [10, 20, 30, 40, 0, 0, 0, 0, 0, 0];

Memory will look like this:

10 20 30 40 0 0 0 0 0 0

g(1) g(2) g(3) g(4) g(5) g(6) g(7) g(8) g(9) g(10)

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Now only the first 4 elements of this 10 element array holdvalues that we care about. What we want to do is tell a

function to only consider those values when it operates on thearray.

Heres how we define the formal parameters for the function:

function sum = sumFun( x, n)...for j = 1:n

sum = sum + x(j);end

end

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To call the function,

function sum = sumFun( x, n)

we could do this:

g = [10, 20, 30, 40, 0, 0, 0, 0, 0, 0];

...

result = sumFun(g, 4); % NO subscript

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[tempfile,msg] = fopen('temperature.txt','r');

if tempfile < 0 disp(msg);

else

numTemps = fscanf(tempfile, '%d',1);

% EOF determined by the count

for i = 1 : numTemps

temp(i) = fscanf(tempfile, '%f', 1);

end

fprintf('Maximum value: %.1f \n',

maximum(temp,numTemps));

fclose(tempfile);

end

This program reads values from a data file anddetermines the maximum value with a function.

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[tempfile,msg] = fopen('temperature.txt','r');

if tempfile < 0 disp(msg);

else % NO blank line at end of values

numTemps = 0;

while ~feof(tempfile) % Looks for EOF

numTemps = numTemps + 1;

temp(numTemps) = fscanf(tempfile, '%f', 1);

end



fclose(tempfile);

end

This program reads values from a data file anddetermines the maximum value with a function.

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% This function returns the maximum% value in the array x with n elements.

functionmax_x = maximum( x, n )

max_x = x(1);for k = 1:n

if x(k) > max_x

max_x = x(k);

endend

end

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maxTemp = maximum( temp,numTemps )

functionmax_x = maximum( x, n)

fprintf('Max temp: %.1f \n',


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The load command reads values

Saved with the save command Saved in text (.txt)

All at once and assigns to the given variable name

clc, clear all

temp = load('temperature.txt');


maximum(temp, numel(temp)));

Reading all values using load()

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Auburn University

To be continued

05a arrays m0

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