MATLAB: I have a problem with plotting a summation equation, Help me!

#plotting #summationequation

The question: For the fuction:

I tried as described below, but failed. I hope to help from you. Thanks.

k=6; %scalar, example

def f(k,x): return sum( sqrt((2*k-3)/((3+i)*(2*k-4-i))) for i in range(x+1))

plot(f(k,x) for x in range (2*k-8) )

Best Answer

Hi.

Editing - I had a (3-i) instead of (3+i) in the denominator - it's fixed now:

k = 6;
f = @(x)sum(sqrt((2*k-3)./((3 + [0:(x+1)]).*(2*k-4-[0:(x+1)]))));
x = 0:(2*k-8);
y = zeros(size(x));
for j = 1:length(x)
y(j) = f(x(j));
end
plot(x,y,':+');
hold on;
title(['f(k,x) where k = ' num2str(k)]);
ylabel('f(k,x)'); xlabel('x');
legend('location','southwest');

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MATLAB: Using 3×3 matrix to create 21×21 matrix

It is not clear the overlap values. I assumed the following:

k33+k11
k33+k12
K33+k13
K33+k23
..., Until the last is k33+k33

The other values is just copy from the given location. Then, you might use only for loop for this. Check in the following:

k19 = reshape(k(1:3,1:3),1,9); % Reshape for [k11 k12 k12 k21 k22 k23 k31 k32 k33]
j = 1;
for i=4:2:21
    k(i,i-1) = k(2,1);
    k(i-1,i) = k(2,1);
    
    k(i+1,i-1) = k(3,1);
    k(i-1,i+1) = k(3,1);
    
    k(i+1,i) = k(3,2);
    k(i,i+1) = k(3,2);
    
    k(i,i) = k(2,2);
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MATLAB: Array indices must be positive integers or logical values.

Here is a minimal working example of your problem

x = rand(1,15);
ind = 2*0.14/0.02+1; % occurs when s = 8
x(ind) 
Error: Array indices must be positive integers or logical values

It would seem that the value of ind is 15, but it is not.

ind - 15
% ans = 1.77635683940025e-15

So you are trying to access x at a non-integer position. The easiest solution is to round (in some manner) to an integer.

x(round(ind))
% and this works!

Also see fix, ceil, and floor for other ways to round.

Best Answer

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MATLAB: Using 3×3 matrix to create 21×21 matrix

MATLAB: Array indices must be positive integers or logical values.

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