I have a simple model written in minizinc and I use gecode to solve it by compiling it into flat-zinc first. As an input, the model takes some constants, arrays, and matrices (in the form of 2-dimensional arrays). The output of the model is another 2d matrix that has to satisfy some constraints.
Target optimization is to minimize the value of "target" which is a particular function of the output matrix and defined as following:
var float: target = sum(i in 1..nodes, j in 1..nodes) (F(i, j) * output_matrix[i, j]);
solve minimize target;
When I execute this model as follows:
mzn2fzn model.mzn model.dzn
fzn-gecode -a model.fzn
I can see a stream of possible solutions with the last one in the list to be the optimal. However, if I add an output statement into the model to print the value of the "target" variable - gecode hangs for hours without finding any solutions at all and prints ==UNKNOWN== if interrupted.
output [
"target: ", show(target), "\n"
];
Is this is an expected behavior, if so, could you explain why?
Cheers
This happens because output statement has influence on variables order during the solve phase.
In your case you output "target", so solver will try to assign value first to "target", then assign values to F matrix (i assume that F is your decision variables?), and this order of solving might take forever.
2 options:
Try to output your matrix F first, then target variable
output [show(F),show(target)];
Directs solver to assign values to F first during solve
solve ::int_search(array1d(F),input_order, indomain, complete) minimize target;
output [ "target: " ++ show(target) ++ "\n"];
Related
I am trying to replicate this formula:
I have gathered all variables in my workspace. However estimating vec(Theta') does not seem to work and so I am a little bit stuck.
Theta = A*B-C;
vTheta = vec(Theta');
A, B and C are defined. The problem is that MATLAB does not seem to know the function vec to do what I would like to do with Theta as in the formula.
How to fix this?
I don't know where you got that equation from, but vec is a function in R, maybe it's related to that? If you want to convert a matrix Theta into a vector, do
Theta(:)
Edit: If you need to transpose the matrix first, MATLAB might not let you do Theta'(:). Instead do it in two steps:
tmp = Theta'; tmp(:)
As written above the Colon Operator is the way vectorize defined variable.
Yet, sometime we want to vectorize a sub set of a variable.
Let's say we have a matrix - mA and we'd like to vectorize a sub section of it - mA(2:3, 4:7).
One way is to define a new variable and vectorize it:
vA = mA(2:3, 4:7);
vA = vA(:);
Yet, what if we only wanted to use this inside another expression and only once?
Could we escape the need to generate explicit variable?
Well, unfortunately MATLAB doesn't have the view() functionality like in Julia.
Yet if you want to avoid explicitly naming new variable (I'm not sure if MATLAB's JIT Engine can also void the memory allocation as Julia) you can do:
reshape(mA(2:3, 4:7), [], 1)
This will always yield a column vector.
You can also use:
reshape(mA(2:3, 4:7), 1, [])
To generate row vector.
For instance you can do:
reshape(mA(2:3, 4:7), 1, []) * reshape(mA(2:3, 4:7), [], 1, )
This will be the sum of squared values of those elements.
Code
clear;clc
T=800;
Pc=48.45;
Tc=375;
w=0.153;
R=82.06;
a=((0.45724)*(R^2)*(Tc^2))/Pc;
b=((0.07780)*R*Tc)/Pc;
B=(0.37464+(1.54226*w)-(0.26992*(w^2)));
Tr=T/Tc;
s=(1+(B*(1-sqrt(Tr))))^2;
for Vm=90:5:1000
P=((R*T)/(Vm-b))-((a*s)/((Vm)^2+(2*b*Vm)-b^2));
end
plot(Vm, P)
Problem
Every time I run this code, it comes out with a completely empty plot with just numbers on both axes as the image shown below. I've checked my code a few times, but I still can't find the problem, especially since the code runs with no errors. The result I am supposed to be getting on this plot is the behavior of P as the value of Vm increases.
The result of the code
Additional information about the source of the question
Here's the original question if you're interested (Exercise 1).
The original question (Exercise 1)
Try displaying your variables. You'll see Vm is not an array, rather it's a single-valued scalar. When you loop over Vm it takes one value at a time; it doesn't build an array.
MATLAB can do calculations on multiple values at once, so if you define Vm to be an array and drop the loop I'm guessing it'll work...
Try something like this (replace the for-loop with these lines):
Vm = 90:5:1000
P=((R*T)./(Vm-b))-((a*s)./((Vm).^2+(2*b.*Vm)-b^2));
P will then be an array. Notice we use .* rather than * when multiplying by the array Vm since we want to do element-wise multiplication, rather than matrix multiplication. Similarly we use ./ rather than / and .^ rather than ^.
EDIT: If you need to use a for-loop then you could define both P and Vm as arrays, and then work on each element separately within a loop:
Vm = 90:5:1000;
P = NaN(size(Vm));
for i=1:numel(Vm)
P(i)=((R*T)./(Vm(i)-b))-((a*s)./((Vm(i)).^2+(2*b.*Vm(i))-b^2));
end
Since the above is working on scalar values, it doesn't matter if you use .* or *...
I have 5 different structure and I want to calculate some variables for all of them. To do that, I wrote the following code:
for i=1:5
[StructureI(i), ReqTab(i), jt(i), B(i)]=Checkall(E);
end
The values StructureI, ReqTab, jt and B are calculated in another function and they are
StructureI= 1X4 matrix,
ReqTab= 4X2 matrix,
jt=2x1 matrix,
B=4x4 matrix
When I run the code it calculates all the varibles in the function Checkall. However, when it turns to the parent code, it gives and error "Conversion to double from struct is not possible."
How can I solve this problem?
Thanks in advance.
You cannot assign directly from double to struct, instead you have to write the specific field field_name to assign to:
[StructureI(i).field_name, ReqTab(i), jt(i), B(i)] = Checkall(E);
If all of these variables (i.e. also ReqTab, jt, B) are structures, then off course you need to specify the field in each one of them, using the . notation.
However, as mentioned in the comments, all iterations of your loop are just the same (no usage of i within it), so why do you need this loop? just to make 5 copies?
I would like to export the answers of an equation with the order of 2 into a vector. The input is R01and the variable is n.
The problem is where I want to "double" the symbol for each step of i, I get the following error:
In an assignment A(I)=B, the number of elements in B and I must be the
same.
There will be no error if I do not use a for loop. What is my mistake and how can I modify it that I could get the data as a matrix or vector.
R01=[0.07941 0.07942 0.07952 0.07946 0.07951 0.07947]
syms n
for i=1:length(R01)
eq3=((1+n)^2)*R01(i)-(n-1)^2
sol1=solve([eq3]);
nsol(i)=double(sol1);
end
The efficient way to solve the problem (by Daniel):
syms n
for i=1:length(R01)
eq3=((1+n)^2)*R01(i)-(n-1)^2
sol1=solve([eq3]);
nsol(i,:)=double(sol1);
end
i have two problems in mathematica and want to do them in matlab:
measure := RandomReal[] - 0.5
m = 10000;
data = Table[measure, {m}];
fig1 = ListPlot[data, PlotStyle -> {PointSize[0.015]}]
Histogram[data]
matlab:
measure =# (m) rand(1,m)-0.5
m=10000;
for i=1:m
data(:,i)=measure(:,i);
end
figure(1)
plot(data,'b.','MarkerSize',0.015)
figure(2)
hist(data)
And it gives me :
??? The following error occurred
converting from function_handle to
double: Error using ==> double
If i do :
measure =rand()-0.5
m=10000;
data=rand(1,m)-0.5
then, i get the right results in plot1 but in plot 2 the y=axis is wrong.
Also, if i have this in mathematica :
steps[m_] := Table[2 RandomInteger[] - 1, {m}]
steps[20]
Walk1D[n_] := FoldList[Plus, 0, steps[n]]
LastPoint1D[n_] := Fold[Plus, 0, steps[n]]
ListPlot[Walk1D[10^4]]
I did this :
steps = # (m) 2*randint(1,m,2)-1;
steps(20)
Walk1D =# (n) cumsum(0:steps(n)) --> this is ok i think
LastPointold1D= # (n) cumsum(0:steps(n))
LastPoint1D= # (n) LastPointold1D(end)-->but here i now i must take the last "folding"
Walk1D(10)
LastPoint1D(10000)
plot(Walk1D(10000),'b')
and i get an empty matrix and no plot..
Since #Itamar essentially answered your first question, here is a comment on the second one. You did it almost right. You need to define
Walk1D = # (n) cumsum(steps(n));
since cumsum is a direct analog of FoldList[Plus,0,your-list]. Then, the plot in your code works fine. Also, notice that, either in your Mathematica or Matlab code, it is not necessary to define LastPoint1D separately - in both cases, it is the last point of your generated list (vector) steps.
EDIT:
Expanding a bit on LastPoint1D: my guess is that you want it to be a last point of the walk computed by Walk1D. Therefore, it would IMO make sense to just make it a function of a generated walk (vector), that returns its last point. For example:
lastPoint1D = #(walk) (walk(end));
Then, you use it as:
walk = Walk1D(10000);
lastPoint1D(walk)
HTH
You have a few errors/mistakes translating your code to Matlab:
If I am not wrong, the line data = Table[measure, {m}]; creates m copies of measure, which in your case will create a random vector of size (1,m). If that is true, in Matlab it would simply be data = measure(m);
The function you define gets a single argument m, therefor it makes no sense using a matrix notation (the :) when calling it.
Just as a side-note, if you insert data into a matrix inside a for loop, it will run much faster if you allocate the matrix in advance, otherwise Matlab will re-allocate memory to resize the matrix in each iteration. You do this by data = zeros(1,m);.
What do you mean by "in plot 2 the y=axis is wrong"? What do you expect it to be?
EDIT
Regarding your 2nd question, it would be easier to help you if you describe in words what you want to achieve, rather than trying to read your (error producing) code. One thing which is clearly wrong is using expression like 0:steps(n), since you use m:n with two scalars m and n to produce a vector, but steps(n) produces a vector, not a scalar. You probably get an empty matrix since the first value in the vector returned by steps(n) might be -1, and 0:-1 produces an empty vector.