I have a simple MATLAB function outputting multiple variables:
function [a,b] = MultipleOutputs()
a = 6;
b = 8;
end
I want to assign the two output variables to 2 certain elements in an existing vector:
x = ones(1,4);
x(2:3) = MultipleOutputs()
However, this gives me:
x =
1 6 6 1
Instead of:
x =
1 6 8 1
I have had this problem in multiple cases, was never able to find the solution.
You have 2 choices:
Concatenate the vectors after outputting them separately
[a,b] = MultipleOutputs();
x = ones(1,4);
x(2:3) = [a,b];
concatenate the vectors before outputting them
function a = MultipleOutputs()
a(1) = 6;
a(2) = 8;
end
x(2:3) = MultipleOutputs();
when you run MultipleOutputs() like that in another function, it only outputs only the first element, which in this case is a.
So eventually your statement x(2:3) = MultipleOutputs() is equivalent to x(2:3) = 6.
A simple fix would be to extract all the elements:
[a,b] = MultipleOutputs();
x(2:3) = [a b];
Related
I'm trying to call a numerical integration function (namely one that uses the trapazoidal method) to compute a definite integral. However, I want to pass more than one value of 'n' to the following function,
function I = traprule(f, a, b, n)
if ~isa(f, 'function_handle')
error('Your first argument was not a function handle')
end
h = (b-a)./ n;
x = a:h:b;
S = 0;
for j = 2:n
S = S + f(x(j));
end
I = (h/2)*(f(a) + 2*S + f(b)); %computes indefinite integral
end
I'm using; f = #(x) 1/x, a = 1 and b = 2. I'm trying to pass n = 10.^(1:10) too, however, I get the following output for I when I do so,
I =
Columns 1 through 3
0.693771403175428 0.069377140317543 0.006937714031754
Columns 4 through 6
0.000693771403175 0.000069377140318 0.000006937714032
Columns 7 through 9
0.000000693771403 0.000000069377140 0.000000006937714
Column 10
0.000000000693771
Any ideas on how to get the function to take n = 10.^(1:10) so I get an output something like,
I = 0.693771403175428, 0.693153430481824, 0.693147243059937 ... and so on for increasing powers of 10?
In the script where you are calling this from, simply iterate over n
k = 3;
f = #(x)1./x;
a = 1; b = 2;
I = zeros(k,1);
for n = 1:k
I(n) = traprule(f, a, b, 10^n);
end
% output: I = 0.693771403175428
% 0.693153430481824
% 0.693147243059937
Then I will contain all of the outputs. Alternatively you can adapt your function to use the same logic to loop over the elements of n if it is passed
as a vector.
Note, you can improve the efficiency of your traprule code by removing the for loop:
% This loop operates on every element of x individually, and is inefficient
S = 0;
for j = 2:n
S = S + f(x(j));
end
% If you ensure you use element-wise equations like f=#(x)1./x instead of f=#(x)1/x
% Then you can use this alternative:
S = sum(f(x(2:n)));
How Do i solve this summation in MATLAB without using for/while loop?
Here C is a vector(1*N matrix), n=length(c) and x is scalar.
c(1)*x^1+c(2)*x^2+c()*x^3+....+c(n)*x^n.
Or can i Create a matrix with all element equal to x but with increasing power, like x, x^2,x^3....?
There are several ways:
result = polyval(fliplr([0 c]), x);
result = sum(c.*x.^(1:numel(c)));
result = sum(c.*cumprod(repmat(x, 1, numel(c))));
As an example, for
c = [3 4 -5 2 3];
x = 9;
any of the above gives
result =
186975
Check:
>> c(1)*x^1+c(2)*x^2+c(3)*x^3+c(4)*x^4+c(5)*x^5
ans =
186975
In a previous question, a user asked about iterating over a cell array of anonymous functions. I am wondering whether there is a way to evaluate a set of functions without the explicit use of a for loop.
As an example, the following code creates an array of (simple) functions, evaluates them for a fixed value and stores the results:
fcnList = {#(x) (x+1), #(x) (x+2)};
a = 2;
for i = 1:numel(fcnList)
y(i) = fcnList{i}(a);
end
Is there a way to do this without looping?
For your example, you could do the following using the cellfun function:
fcnList = {#(x) (x+1), #(x) (x+2)};
a = 2;
cellfun(#(func) func(a),fcnList)
ans =
3 4
Where I have created a handle called func which accepts as input a function from the fcnList variable. Each function is then evaluated for a.
If you need to pass a vector instead of a scalar, for instance b, you will need to set the 'UniformOutput' option to false:
b=[3 4]
fcnList = {#(x) (x+1), #(x) (x+2)};
cellfun(#(func) func(b),fcnList,'UniformOutput',false)
ans =
{
[1,1] =
4 5
[1,2] =
5 6
}
To avoid a for loop or cellfun (which is more or less the same as a loop), you can define a single function with vector output:
fcn = #(x) [x+1, x+2];
Then fcn(a) gives you a vector cointaining the results:
>> fcn = #(x) [x+1, x+2];
>> a = 2;
>> fcn(a)
ans =
3 4
If the results of each original function have different sizes you can define a single function with cell array output:
>> fcn = #(x) {x+1, [x+2; x+3]};
>> a = 2;
>> x = fcn(a)
x =
[3] [2x1 double]
>> celldisp(x)
x{1} =
3
x{2} =
4
5
I want to create a matrix of functions, however I'd like to dynamically generate it. For example:
myMatrix = zeros(3);
test = #(x) x*y;
for ii = 1:3
myMatrix(ii) = test(ii);
end
something like that to generate: #(y) [y, 2*y, 3*y]
I do not have access to the sym library.
You can't make a matrix of functions, but you can make cell of function handles, e.g.
cellOfFunctions = {};
for i = 1:3
cellOfFunctions{end + 1} = #(y) y*i;
end
Then you can get each handle as follows (for the first function handle):
fh1 = cellOfFunctions{1};
Then execute it with y = 3:
result = fh1(3);
Depending on your purposes, you can make a single function which generates the matrix you have in your example:
>> f = #(y) bsxfun(#times, 1:3, y(:));
>> f(2:5)
ans =
2 4 6
3 6 9
4 8 12
5 10 15
Hey there, I've been having difficulty writing the matlab equivalent of the conv(x,y) function. I cant figure out why this gives the incorrect output. For the arrays
x1 = [1 2 1] and x2 = [3 1 1].
Here's what I have
x1 = [1 2 1];
x2 = [3 1 1];
x1len = leng(x1);
x2len = leng(x2);
len = x1len + x2len - 1;
x1 = zeros(1,len);
x2 = zeros(1,len);
buffer = zeros(1,len);
answer = zeros(1,len);
for n = 1:len
buffer(n) = x(n);
answer(n) = 0;
for i = 1:len
answer(n) = answer(n) + x(i) * buffer(i);
end
end
The matlab conv(x1,x2) gives 3 7 6 3 1 as the output but this is giving me 3 5 6 6 6 for answer.
Where have I gone wrong?
Also, sorry for the formatting I am on opera mini.
Aside from not having x defined, and having all zeroes for your variables x1, x2, buffer, and answer, I'm not certain why you have your nested loops set up like they are. I don't know why you need to reproduce the behavior of CONV this way, but here's how I would set up a nested for-loop solution:
X = [1 2 1];
Y = [3 1 1];
nX = length(X);
nY = length(Y);
nOutput = nX+nY-1;
output = zeros(1,nOutput);
for indexY = 1:nY
for indexX = 1:nX
indexOutput = indexY+indexX-1;
output(indexOutput) = output(indexOutput) + X(indexX)*Y(indexY);
end
end
However, since this is MATLAB, there are vectorized alternatives to looping in this way. One such solution is the following, which uses the functions SUM, SPDIAGS, and FLIPUD:
output = sum(spdiags(flipud(X(:))*Y));
In the code as given, all vectors are zeroed out before you start, except for x which is never defined. So it's hard to see exactly what you're getting at. But a couple of things to note:
In your inner for loop you are using values of buffer which have not yet been set by your outer loop.
The inner loop always covers the full range 1:len rather than shifting one vector relative to the other.
You might also want to think about "vectorizing" some of this rather than nesting for loops -- eg, your inner loop is just calculating a dot product, for which a perfectly good Matlab function already exists.
(Of course the same can be said for conv -- but I guess you're reimplementing either as homework or to understand how it works?)