surf, plotting, axis

Hi

Simple question....

I have an output 'P' which is a 10X10 matrix. I start with

for i=1:10
  for j=1:10
    P(i,j) = %some operation that gives the answer at each i,j
  end
end

I use 'surfl' to plot P.

Now I have

for i=1:0.5:10
  for j=1:0.5:10
    P(i,j) = %some operation that gives the answer at each i,j
  end
end

Then, P will still be a 20X20 matrix, but its now plotting on surf, because I think it takes only integers.

So, I used

c1=1;
for i=1:0.5:10
  c2=1; 
  for j=1:0.5:10
    P(c1,c2) = %some operation that gives the answer at each i,j
    c2=c2+1;
  end
  c1=c1+1;
end
surf(P)

This gives me a figure for a 20X20 matrix I want, but its mentioned as 1:20 and 1:20 on x & y-axes, while I want it to depict 1:10 and 1:10....

Please advice what I need to change to get that!!!

[Walter Roberson]

http://www.mathworks.com/matlabcentral/answers/13205-tutorial-how-to-format-your-question-with-markup

[PChoppala]

Whoops, I never knew about this. thanks Walter

Answer 1

[Walter Roberson]

xv = 1 : 0.5 : 10;
yv = 1 : 0.5 : 10;

for i = 1 : length(xv)
  for j = 1 : length(yv)
    P(i,j) = %some operation that gives the answer at each xv(i), yv(j)
  end
end
surf(xv, yv, P)

[PChoppala]

I am sorry, I assumed it would work and accepted your answer, but 'no'.

I get the error

*Subscript indices must either be real positive integers or logicals.*

*Error in MUSIC_ifft (line 166)

P(xv,yv) = (a_r * a_r');*

I think that is because P takes only integer values of its locations.

Can you suggest another method???

I used

_c1=1;

_for i = 1:0.5:10

_ c2=1;

_ for j = 1:0.5:10

_ %statements

_ P(xv,yv) = (a_r * a_r');

_ c2=c2+1;

_ end

_ c1=c1+1;

_ end

_ surfl(1:0.5:10,1:0.5:10,20*log10(P));

But I think there should be another simpler way. Some other form of surf or plot that makes assignment and plotting simpler!!!

Please advice

Cheer

[PChoppala]

I used the underscore and asterisk, but letter did not bold or go italic!!!

[Walter Roberson]

Sorry, markup cannot be used in comments. Markup in comments is the highest-rated item on the wish-list !

Notice in my version of the code, I assigned to P(i,j), whereas you assigned to P(xv,yv) . Assigning to (i,j) is correct. Notice too that I wrote in the comment that the operation should give the answer at xv(i) and yv(j) -- the indexed values are the "real" values to use, whereas i and j are the counters.

[PChoppala]

Oh, but I need each of 1,1.5,2,2.5...10 in side my loop.

[Walter Roberson]

The sample code you posted does not need the complete list of values inside the loop: only the _current_ values. The current x is xv(i) and the current y is yv(j)

[PChoppala]

Whoops, sorry then, I need them.

for i=1:0.5:10

for j=1:0.5:10

d_r = sqrt(sum(([1 8 ; 0 7]-repmat([i11;j11],1,10)).^2));

% other statements that process d_r and give a_r

P(i,j) = (a_r * a_r');

end

end

So, P is now a 20X20 matrix but has calculated the algorithm in the range space of 10X10. So i want the surf command to show me only 10X10 on its x and y axes.

Please advice

Cheers

[Walter Roberson]

If you are going to insist on using for i=1:0.5:10 then just use your code from your first comment, above, where you use the c1 and c2 counters. It isn't ideal code, but you are stuck on the idea that i and j must be the values instead of being allowed to index the values, so you might as well stick with what you understand.

-----

Perhaps this would make things more clear:

xv = 1 : 0.5 : 10;

yv = 1 : 0.5 : 10;

for xidx = 1 : length(xv)

i = xv(xidx);

for yidx = 1 : length(yv)

j = yv(yidx);

P(xidx,yidx) = %some operation that gives the answer at each i, j

end

end

surf(xv, yv, P)

Then you can have clear indexes _and_ you can still use "i" and "j" as the names of your variables that hold the current values.

-----

In your code, you might want to clarify what i11 and j11 are. Your d_r appears to be calculating the same value for all i and j combinations; if so then it would be more efficient to calculate it once outside the loop.