I have an array in Matlab "Numbers" that is stored in the workspace as a 400x1 double. I know how to calculate the mean of this data, but the problem I have is actually writing the code to do this. I know there are functions built-in which I could use, but I want to try and calculate this using only low-level IO commands and I'm not sure how to go about doing this. I was thinking the correct way to do this would be to create a for loop and a variable containing the total that adds each element in the array until it reaches the end of the array. With that, I could simply divide the variable 'Total' by the number of elements '400' to get the mean. The main problem I have is not knowing how to get a for loop to search through each element of my array, any help in figuring that part out is much appreciated. Thank you.
mean(Numbers) will do it for you. If not,
sum(Numbers)/length(Numbers)
or, if you insist on not using built-in functions,
sums = 0;
counter = 0;
for val = Numbers
sums = sums + val;
counter = counter + 1;
end
Numbers_mean = sums/counter;
although this will almost always be slower than just calling mean.
Related
Following to the question published in How expressive can we be with arrays in Z3(Py)? An example, I expressed the following formula in Z3Py:
Exists i::Integer s.t. (0<=i<|arr|) & (avg(arr)+t<arr[i])
This means: whether there is a position i::0<i<|arr| in the array whose value a[i] is greater than the average of the array avg(arr) plus a given threshold t.
The solution in Z3Py:
t = Int('t')
avg_arr = Int('avg_arr')
len_arr = Int('len_arr')
arr = Array('arr', IntSort(), IntSort())
phi_1 = And(0 <= i, i< len_arr)
phi_2 = (t+avg_arr<arr[i])
phi = Exists(i, And(phi_1, phi_2))
s = Solver()
s.add(phi)
print(s.check())
print(s.model())
Note that, (1) the formula is satisfiable and (2) each time I execute it, I get a different model. For instance, I just got: [avg_a = 0, t = 7718, len_arr = 1, arr = K(Int, 7719)].
I have three questions now:
What does arr = K(Int, 7719)] mean? Does this mean the array contains one Int element with value 7719? In that case, what does the K mean?
Of course, this implementation is wrong in the sense that the average and length values are independent from the array itself. How can I implement simple avg and len functions?
Where is the i index in the model given by the solver?
Also, in which sense would this implementation be different using sequences instead of arrays?
(1) arr = K(Int, 7719) means that it's a constant array. That is, at every location it has the value 7719. Note that this is truly "at every location," i.e., at every integer value. There's no "size" of the array in SMTLib parlance. For that, use sequences.
(2) Indeed, your average/length etc are not related at all to the array. There are ways of modeling this using quantifiers, but I'd recommend staying away from that. They are brittle, hard to code and maintain, and furthermore any interesting theorem you want to prove will get an unknown as answer.
(3) The i you declared and the i you used as the existential is completely independent of each other. (Latter is just a trick so z3 can recognize it as a value.) But I guess you removed that now.
The proper way to model such problems is using sequences. (Although, you shouldn't expect much proof performance there either.) Start here: https://microsoft.github.io/z3guide/docs/theories/Sequences/ and see how much you can push it through. Functions like avg will need a recursive definition most likely, for that you can use RecAddDefinition, for an example see: https://stackoverflow.com/a/68457868/936310
Stack-overflow works the best when you try to code these yourself and ask very specific questions about how to proceed, as opposed to overarching questions. (But you already knew that!) Best of luck..
Working in MATLAB R2017a. I'm trying to optimise a piece of code I'm working on. It uses arrays to store field values on a grid.
In order to create a specific function in a field array I originally used the straight forward method of two for loops iterating over all the array elements. But i know for loops are slow so since then I came back and tried my best to remove them. However I could only manage to remove one of the loops; leaving me with this:
for n = 1:1:K
%%% define initial pertubation
t=n*dt;
% create array for source Ez field.
xtemps = (1:Ng)*dX;
for k = 1:Ng
ztemp = k*dX;
Ez0(k,:) = THzamp * (1/(1+exp(-(t-stepuppos)))) * exp(-((xtemps-...
THzstartx).^2)./(bx^2)) .* (t-((ztemp-THzstartz)/vg))*exp(-((t-((ztemp-...
THzstartz)/vg))^2)/(bt^2));
end
The important bit here is the last 5 lines, but I figured the stuff before might be important for context. I've removed the for loop looping over the x coordinates. I want to vectorize the z/k for loop but I can't figure out how to distinguish between the dimensions with the array oporators.
Edit: THzamp, stepuppos, bx, bt, THzstartz, THzstartx are all just scalars, they control the function (Ez0) I'm trying to create. dX and t are also just scalars. Ez0 is a square array of size Ng.
What I want to achieve is to remove the for loop that loops over k, so that that the values of ztemp are defined in a vector (like xtemps already is), rather than individually in the loop. However, I don't know how I'd write the definition of Ez0 in that case.
First time posting here, if I'm doing it wrong let me know. If you need more info just ask.
It isn't clear if n is used in the other headers and as stated in the comments your sizes aren't properly defined so you'll have to ensure the sizes are correct.
However, you can give this vectorize code a try.
n = 1:K
%%% define initial pertubation
t=n*dt;
% create array for source Ez field.
xtemps = (1:Ng)*dX;
for k = 1:Ng
ztemp = k*dX;
Ez0(k,:) = THzamp .* (1./(1+exp(-(t-stepuppos)))) .* exp(-((xtemps-...
THzstartx).^2)./(bx^2)) .* (t-((ztemp-THzstartz)/vg)).*exp(-((t-((ztemp-...
THzstartz)/vg)).^2)/(bt.^2));
end
So now t has the size K you'll need to ensure stepupposand (ztemp-THzstartz)/vg) have the same size K. Also you can take a look at vectors vs array operators here.
I have a dozens of arrays with different array names and I would like to do some mathematical calculations in to for loop array by array. I srucked in calling these array into for loop. Is there anybody can help me with this problem? text1 array contains array names. My "s" struct has all these arrays with the same name content of text1 array.
text1=['s.CustomerArray.DistanceDriven','s.CustomerArray.TimeDriven'];
for i=1:3
parameter=str2num(text1(i));
k=size(parameter,2);
a=100;
y=zeros(a,k);
end
After this part my some other calculations should start using "parameter"
Regards,
Eren
I think you are doing several things wrong, here are some pointers.
Rather than listing them manually, consider looping over the fieldnames which can be obtained automatically.
If you are looping over strings, make sure to use a cell array with , rather than a matrix.
If you have a constant, declare it outside the loop, rather than inside the loop. This won't break the code but just makes for obsolete evaluations.
If you want to store results obtained inside a loop, make sure to add an index to the variable that you loop over.
That being said, here is a guess at what you are trying to do:
f = fieldnames(s.CustomerArray);
y = cell(numel(f),1);
parameter = NaN(numel(f),1);
for t = 1:numel(f)
parameter(t) = s.CustomerArray.(f{t});
y{t} = zeros(100,numel(f{t}));
end
Thanks! I just had to cast the right side of the assignment to Term.
I have to make a dynamic array of polynomials that each have a dynamic array of terms. When giving the term a exponent and coefficient, I get an error "expected expression before '{' token". What am I doing incorrectly when assigning the values?
Also, is there an easy way of keeping the dynamic array of terms ordered by their exponent? I was just planning on looping through, printing the max value but would prefer to store them in order.
Thanks!
polynomialArray[index].polynomialTerm[0] = {exponent, coefficient}; // ISSUE HERE
change to
polynomialArray[index].polynomialTerm[0] = (Term){exponent, coefficient};
polynomialArray[index].polynomialTerm[0]->exponent = exponent;
polynomialArray[index].polynomialTerm[0]->coefficient = coefficient;
There's an efficiency problem here in your code:
if(index > (sizeof(polynomialArray)/sizeof(Polynomial)))
polynomialArray = (Polynomial*)realloc(polynomialArray, index * sizeof(Polynomial));
as polynomialArray is a pointer, I think sizeof(polynomialArray) would always be 4 or 8(64-bit system). So the above if statement will always true as long as index is greater than 0.
If this is C99, I think you need
polynomialArray[index].polynomialTerm[0] = (Term){exponent, coefficient};
You cannot attribute values like that (only during declaration).
You should assign like this:
polynomialArray[index].polynomialTerm[0].exponent = exponent;
polynomialArray[index].polynomialTerm[0].coefficient = coefficient;
About the other question, you really don't need assert here. The pointer will not be NULL if it has a value malloc allocated to it. If not, it is better to be NULL, so you can test if malloc failed.
To have it ordered, you will need to order using some sort algorithm. I think that if you are looking for an easy way, the way you are doing is fine. If it is critical to be ordered (like real time applications), than you need to rethink the approach. If not, keep it and go forward!
Take care,
Beco
Here's the code I am using now, where decimal1 is an array of decimal values, and B is the number of bits in binary for each value:
for (i = 0:1:length(decimal1)-1)
out = dec2binvec(decimal1(i+1),B);
for (j = 0:B-1)
bit_stream(B*i+j+1) = out(B-j);
end
end
The code works, but it takes a long time if the length of the decimal array is large. Is there a more efficient way to do this?
bitstream = zeros(nelem * B,1);
for i = 1:nelem
bitstream((i-1)*B+1:i*B) = fliplr(dec2binvec(decimal1(i),B));
end
I think that should be correct and a lot faster (hope so :) ).
edit:
I think your main problem is that you probably don't preallocate the bit_stream matrix.
I tested both codes for speed and I see that yours is faster than mine (not very much tho), if we both preallocate bitstream, even though I (kinda) vectorized my code.
If we DONT preallocate the bitstream my code is A LOT faster. That happens because your code reallocates the matrix more often than mine.
So, if you know the B upfront, use your code, else use mine (of course both have to be modified a little bit to determine the length at runtime, which is no problem since dec2binvec can be called without the B parameter).
The function DEC2BINVEC from the Data Acquisition Toolbox is very similar to the built-in function DEC2BIN, so some of the alternatives discussed in this question may be of use to you. Here's one option to try, using the function BITGET:
decimal1 = ...; %# Your array of decimal values
B = ...; %# The number of bits to get for each value
nValues = numel(decimal1); %# Number of values in decimal1
bit_stream = zeros(1,nValues*B); %# Initialize bit stream
for iBit = 1:B %# Loop over the bits
bit_stream(iBit:B:end) = bitget(decimal1,B-iBit+1); %# Get the bit values
end
This should give the same results as your sample code, but should be significantly faster.