Problems in using map

Hi all, I have some problems using map in matlab. I don't understand why I get the error messages ""The specified key is not present in this container" after the 251th entry.

clear all;
k = -pi : 2*pi/500 : pi

k = 1×501

-3.1416 -3.1290 -3.1165 -3.1039 -3.0913 -3.0788 -3.0662 -3.0536 -3.0411 -3.0285 -3.0159 -3.0034 -2.9908 -2.9782 -2.9657 -2.9531 -2.9405 -2.9280 -2.9154 -2.9028 -2.8903 -2.8777 -2.8651 -2.8526 -2.8400 -2.8274 -2.8149 -2.8023 -2.7897 -2.7772

delta_omega = -0.4: 0.008 : 0.4;

%Want to create a map
valueset_k = 1:1:(length(k));
valuesset_deltaomega= 1:1:length(delta_omega);
M_k = containers.Map(k,valueset_k);
M_deltaomega = containers.Map(delta_omega, valuesset_deltaomega);

%Want to construct an array to store the values
xi = [];
for p = -pi : 2*pi/500 : pi
    xi(M_k(p)) = p^2;
end

Error using indexing
The specified key is not present in this container.

 
%Want to construct a matrix to store the values
Matrix = [];
for k = -pi : 2*pi/500 : pi
    for delta_omega = -0.4: 0.008 : 0.4
      Matrix(M_k(k),M_deltaomega(delta_omega)) = k*delta_omega;
    end
end

[Stephen23]

You incorrectly assume that two different COLON calculations using binary floating point numbers will give results that are bit identical, but in fact they don't:

• the COLON in the FOR loop does not generate the entire vector (it basically generates the loop iterator using the step).
• the colon assigned to k generates the entire vector in memory using an algorithm that tries to ensure that the vector's first and last values match the COLON start and end values.

If you loop over k then you will avoid this error (but still have complex, fragile code).

It looks like you are trying to write MATLAB code using paradigms from some other language, because that code is just very obfuscated indexing: MATLAB already has indexing built in.

Note that your complex first loop with fragile dictionary simply reduces down to this:

xj = k.^2;

[Tsz Tsun]

Thank you very much for pointing out the error, I loop over k and the problem is solved now.

Answer 1

[Star Strider]

You are seeing floating-point approximation error.

Changing the first loop slightly to check that:

kount = 0;

format long

for p = -pi : 2*pi/500 : pi

kount = kount+1;

if k(kount)-p ~= 0

Check = k(kount)-p

end

xi(M_k(p)) = p^2;

end

demonstrates this. Apparently MATLAB does not calculate ‘k’ and ‘p’ exactly the same way.

If you are not familiar with this chgaracteristic of floating-point numbers, see: Floating-Point Numbers for details.

clear all;
k = -pi : 2*pi/500 : pi;
delta_omega = -0.4: 0.008 : 0.4;

%Want to create a map
valueset_k = 1:1:(length(k));
valuesset_deltaomega= 1:1:length(delta_omega);
M_k = containers.Map(k,valueset_k);
M_deltaomega = containers.Map(delta_omega, valuesset_deltaomega);

%Want to construct an array to store the values
xi = [];
kount = 0;
format long
for p = -pi : 2*pi/500 : pi
    kount = kount+1;
    if k(kount)-p ~= 0
        Check = k(kount)-p
    end
    xi(M_k(p)) = p^2;
end

Check = 
    -4.440892098500626e-16

Error using indexing
The specified key is not present in this container.

 
%Want to construct a matrix to store the values
Matrix = [];
for k = -pi : 2*pi/500 : pi
    for delta_omega = -0.4: 0.008 : 0.4
      Matrix(M_k(k),M_deltaomega(delta_omega)) = k*delta_omega;
    end
end

EDIT — (23 Sep 2023 at 14:46)

The easiest solution to this would likely be:

%Want to construct an array to store the values

xi = [];

for h = 1:numel(k)

p = k(h);

xi(M_k(p)) = p^2;

end

providing ‘k’ is still in your workspace (or stored somewhere that you can refer to it easily).

.

[Tsz Tsun]

Thank you very much for spotting the error detailing the break down of the problem, my problem is solved now using for loop again to define the k array.

[Star Strider]

As always, my pleasure!