How to initialize a variable only once in an embedded matlab function?

I'm working on a smart metering system simulation.I have a billing algorithm matlab function inserted in my simulation.The inputs are:

1.No of units of energy consumed

2.Clock

3.Price signal from utility

Outputs are:

1.Remaining balance

2.zero balance indicator switch

I'm trying to use the following algorithm Store the current units consumed in a variable which will become prev units in the next clock cycle.Then i perform current balance-prev balance in next cycle. Multiply the answer with the current price and add it to a variable y0 which will be subtracted from initial balance to get the current balance.

I want prev_bal=0 and y0=0 in the start.But if i write the 2 statements at the start of the code both variables become 0 at the start of every sampling.As a result the algorithm fails.Please guide me how to set the two variables 0 only for the first sampling or suggest an alternate algorithm.

Thanks Nikhil

[MAYANK JHA]

hi Nikhil,

did you find a better solution??... try the "persistent" functionality in Matlab function block to initialize. refer here:http://fr.mathworks.com/help/simulink/ug/defining-and-initializing-persistent-variables.html

use isempty command to check and therefore assign the intial value at first instant.

your welcome. :)

Answer 1

[Ryan G]

You can try the exist function. This will check to see if they exist. If they do, then you can skip that line of code.

[Nikhil]

I get the following error on using the exist function. "Function 'exist' implicitly resolved in the MATLAB workspace. Implicit evaluation in MATLAB is not supported. Please declare this function extrinsic using coder.extrinsic('exist'), or call it using feval. Here is my code after making the function call extrinsic.

function [y0,y1,y2,x1] = fcn(u1,u2)
coder.extrinsic('exist');
a= exist('x1','var');
if a == 0
    x1=0;
    y0=0;
else
end
y0= y0+(u1-x1)*u2;
x1 = u1;
y1=2000-y0;
if y1 > 0
    y2 = 1;
else
    y2=0;
end
end

Now i get the following error: Expected either a logical, char, int, fi, single, or double. Just for clarification i'm using an embedded matlab function inside a simulink model.What to do now?

[Ryan G]

Will you be generating code for this function? If so, you will need to find another method of doing this.

If not, I think you need to pre-define a before you call exist.

a=0;
a= exist('x1','var');

When you call a function extrinsically it uses the mxarray datatype from MATLAB. MATLAB Function Blocks generate code and it can be a bit confusing as to where you need to pre-define variables if you've never done it before.

The line above essentially declares 'a' as a double type so when you call the exist function it will maintain that datatype.

Answer 2

[harjot]

Hi Nikhil .. I am facing similar issue . Are you able to initialize the variable inside a function block for just once. I tried the exist function but it seems it dioesn't work inside a function block