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Some basic questions about MATLAB operation

Hi, I have recently started using MATLAB for a DSP and a
Communication Theory class. I have some questions about how MATLAB
does some things.

1. To plot a graph, say a sinusoid, I always do this:
t=0:.1:10;
a=sin(t);
plot(t,a)
What does MATLAB do to generate the data for the points that are not
in increments of .1? for example sin(1.13)? Does it interpolate to
generate the graph? Is this sinusoid only valid for .1 increments? In
fact, a function in MATLAB is inherently discrete, but we use it to
model continuous functions (its 'domain' is actually only 101
samples)?

2. I'm having trouble mixing up the indices of the array (1..101 for
t matrix) and the actual values (t(1)=0 t(1)=0.1, etc.)I am doing a
lot of shifting of functions (such as f(x) = x -> f(x-2) = x-2)
and this is messing me up, I usually get the error, "indices must be
positive or logicals.." Mathematically, it makes sense to compute
f(x(1)-2) for x(1)=1, but it doesn't make sense in MATLAB to ask for
f(-1). This brings me to the question of what is a reasonable matrix
size for x. -100:.1:100 ? Does it depend on the precision you need?
Is there a 'convention' used to model functions?

3. I had a misconception that MATLAB was very close to mathematical
notation. In fact, you have to interpret a lot when using it, often
going back to the theory to make sure computations make sense. So am
I using it the 'right' way. Usually only crunching huge amounts of
numbers?

I know it's a lot to ask, but it would help me a lot to clear up my
mind so I can use the program more efficiently.

Thanks,

Luis.
0
10/17/2004 4:11:24 AM
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Hi Luis,

1/ for any curve, Matlab use an interpolation to plot points that are
not in the abscissa vector ...

2/ to use a function, u have to put this in an M file, using this
syntax :

function result = f(x)

result = sin(x-pi/2);

and for x, you can have a double, a vector, a matrix ...
When u use the syntax f(-1) in the workspace MATLAB, without having
written your function in an M-File, f is a vector and -1 is the index
of the vector, and that's also impossible, that's the reason why it
gives u the message : "indices must be positive or logicals..".

With the M-File, u can now use in the workspace that :

v = [-pi:.1:pi];
y = f(v) and u get the values for each value of v with the function f
....

Yes, it depends on the precision u want for the plot !!!

Good luck, and have a good day ...

Jeje.
0
10/17/2004 11:28:25 AM
In article <eeed41d.-1@webx.raydaftYaTP>,
 "luis a" <l_alvergue@NOCAPITALShotmail.com> wrote:

> Hi, I have recently started using MATLAB for a DSP and a
> Communication Theory class. I have some questions about how MATLAB
> does some things.
> 
> 1. To plot a graph, say a sinusoid, I always do this:
> t=0:.1:10;
> a=sin(t);
> plot(t,a)
> What does MATLAB do to generate the data for the points that are not
> in increments of .1? for example sin(1.13)? Does it interpolate to
> generate the graph? Is this sinusoid only valid for .1 increments? In
> fact, a function in MATLAB is inherently discrete, but we use it to
> model continuous functions (its 'domain' is actually only 101
> samples)?

If you sample a function at 101 points, then all you have
is 101 points. Its no longer a "function". When you plot
those points, you have just 101 points.

If you want to connect the dots, then try doing 

  help plot

You will see that if you add a third argument to the plot
call

  plot(t,a,'-')

then you get a curve composed of line segments between
the dots.


> 2. I'm having trouble mixing up the indices of the array (1..101 for
> t matrix) and the actual values (t(1)=0 t(1)=0.1, etc.)I am doing a
> lot of shifting of functions (such as f(x) = x -> f(x-2) = x-2)
> and this is messing me up, I usually get the error, "indices must be
> positive or logicals.." Mathematically, it makes sense to compute
> f(x(1)-2) for x(1)=1, but it doesn't make sense in MATLAB to ask for
> f(-1). This brings me to the question of what is a reasonable matrix
> size for x. -100:.1:100 ? Does it depend on the precision you need?
> Is there a 'convention' used to model functions?

Matlab does not support indexing arrays with zero or
negative integers.


> 3. I had a misconception that MATLAB was very close to mathematical
> notation. In fact, you have to interpret a lot when using it, often
> going back to the theory to make sure computations make sense. So am
> I using it the 'right' way. Usually only crunching huge amounts of
> numbers?

Matlab really is quite close to mathematical notation.
Is it exact? No, this is not possible. How close is
close?

Do you need to think about what you are doing? Yes.
Use of any tool with no thought employed is a fools
game, regardless of the tool.

Should you check to see if your computations make
sense? Always. ALWAYS. I'll tell you a story.

Long ago, in a land far, far away, I was once a
student of mathematics. My professor assigned a
problem to compute the steady-state temperature
along the length of a bar, given known conditions
at the ends of the bar. I did the computations,
handing in my results with no thought to check my
results for sensibility. After all, I was his best
student. How could I have made a mistake? Should
I have asked myself if a temperature of -5000
degrees made any physical sense? Perhaps. As it
was, I merely gave my professor a good laugh at
my expense, and had the opportunity to redo my
flawed results.

The moral of the story? Think about what you do. 
Check your results. Its worth a few extra seconds.

HTH,
John D'Errico



-- 
There are no questions "?" about my real address.

The best material model of a cat is another, or
preferably the same, cat.
A. Rosenblueth, Philosophy of Science, 1945
0
woodchips (7944)
10/17/2004 1:17:59 PM
Reply:

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