MATLAB Answers

Alfonso
0

Sampling with vectors at the same time

Asked by Alfonso
on 24 Feb 2013
Hello,
its a simple question, but i havent managed to solve it:
I have to sample a vector and the matlab doesn't understandme and prompts:
??? Error using ==> mtimes
Inner matrix dimensions must agree.
I wanted to sample a SUM (cos(2*pi*d*A) )where A is a vector of many degrees value, so I have:
function sample(d)
A = [1,2,3,....]
SUM (cos(2*pi*d*A) )
end
and the input is:
d=[1:1:20]
How can I make understand first vector is a sampling and second is a vector of values? (The idea is plotting (sample,d) )
The fact is that the expresion of the vector is a real for each value of d so I should expect something returning results of the SUM of different d's like:
Columns 1 through 11
1 2 3 4 5 6 7 8 9 10 11
Columns 12 through 20
12 13 14 15 16 17 18 19 20
But it doesn't understand each
Thank you

  7 Comments

d is a scalar, i want to test my function with 20 different d values evolving from 1 to 20 so:
SUM (cos(2*pi*1*A) )
SUM (cos(2*pi*2*A) )
SUM (cos(2*pi*3*A) )
.....
so in the end i'll receive
Columns 1 through 11
a b c ..
Columns 12 through 20
...
scalar results of a sumatory of cosinus of de values in vector
PEXAMPLE: cos(33,3)+cos(45,3)+....(the sum as long as the lenght of the vector A). so should return an scalar.
d will be 1 -> cos(33,3)+cos(45,3)
then
d will be 2 -> cos(66,6)+cos(90,6)
then ...
See the code in my comment.
yes but A is NOT of twenty values, in my code A is 8128 and i just want 100 experiments anyway.. No more resolution needed, it has nothing to do because d is a scalar. in the end d will be a 1 or a 2 or a 3, it will be performed in 20 experiments or (100 in my code) (as 20 inputs are passed) and then returned this way:
Columns 1 through 11
a b c ..
Columns 12 through 20
...

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5 Answers

Answer by Dr. Seis
on 25 Feb 2013
Edited by Dr. Seis
on 25 Feb 2013
 Accepted Answer

Try this:
d = 1:20;
A = [24,34,56,66,86];
plot(d,arrayfun(@(x)sum(cos(2*pi*A*x)),d))
Obviously, in this simple case, the answer will always be 5 for all d since we are dealing with integer multiples of 2*pi (and there are only 5 elements in A), but I think this is what you are looking for.

  2 Comments

That was PRO!! That's what i'm searching for, Thank you!!
I'll search with the arrayfunc to draw my function, one thousand thanks to all.
To close the topic, you have to accept the answer which helped you

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Answer by Image Analyst
on 24 Feb 2013
Edited by Image Analyst
on 24 Feb 2013

What is SUM? Is it a custom function you wrote, or an array of yours, because it's not the built-in MATLAB sum() function because MATLAB is case sensitive.
Perhaps you meant something like this:
A = 1:20
d=[1:1:20]
cosA = cos(2*pi*d.*A)
theSum = sum(cosA)
Note the .* instead of * between d and A to do an element by element multiplication, not a matrix multiplication like you were trying to do.

  8 Comments

Hi again, with your code in a new script file, it prompted:
??? Error: File: prueba.m Line: 8 Column: 1
Function definitions are not permitted in this context.
in the LINE: function theSum = sample(d)
I've created an script file with your code since the for is supposed to be inside a function. How do yo do to have 2 functions without calling a for at the main. This tool is expected to be used in my project so it would be cool that it was inside a function.
Thank you.
It worked typing 60% of your code on the command window but i need something more elegant, Matlab is quite stranger that i though.
Need help to write some code equivalent to this almost all in a function
Very grateful anyway.
Alfonso, you need to read the getting started guide. You can't just type all that code into a single m-file because you can't have a script followed by a function in the same file. The main program can be in one m-file, and the other function in another m-file, OR if you want them to be in the same file, then you must have a function name at the beginning of the file with the same name as the m-file. For example if it's test.m, then your code would be
function test()
dMain = [1:1:20]
for k = 1 : length(dMain)
% Call sample() once for each value of d
theSumMain(k) = sample(dMain(k));
end
theSumMain % Print to command window
function theSum = sample(d)
A = 1:20;
cosA = cos(2*pi*d.*A);
theSum = sum(cosA);
If it's like that they can all be in the same m-file. I guess I assumed that you knew this.

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Answer by Azzi Abdelmalek
on 24 Feb 2013
Edited by Azzi Abdelmalek
on 24 Feb 2013

Maybe you mean indexing
v=[2 3 5 4 10 20]
d=[2 3 1 4 5 6]
new_v=v(d)

  1 Comment

I meant to perform multiple experiments due to an entry of different values the d only affects for each experiment the d is first 1, the sum is done, then the d is two, the sum is done, then the d is three....
and then the result is returned of the function with 20 different d's as 20 different inputs to the function, the matlab wants to understand is a vector to multiply and is only 20 experiments so D is an scalar.

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Answer by Alfonso
on 24 Feb 2013
Edited by Alfonso
on 24 Feb 2013

<< d=[1:1:20]
<< sample(d)
no more code needed.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function sample(d)
A = [24,34,56,....]
SUM (cos(2*pi*d*A) )
end

  0 Comments

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Answer by Alfonso
on 24 Feb 2013
Edited by Alfonso
on 24 Feb 2013

MAYBE Im complicating myself, forget about the 20 (Or 100)entries, how would you plot:
function sample(d)
A = [24,34,56,....]
SUM (cos(2*pi*d*A) )
end
I used to plot
plot (d,sample(d))
I want function to evolve non like an integer (0,1:0,1:20) instead, so i want increases of the X axis of 0,1 from 1 to 20 HOW WOULD YOU PLOT IT? I simplified in your example, but i don't want a deffault plotting progress, so i use multiple entry (d=[0,1:0,1:20]).

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