# Is there a quicker algorithm for calculating 'drifted' portfolio weights? (R, Pandas/NumPy, MATLAB)

'Sup, QuantSX.

BLOT (Bottom Line On Top): Is there a nice clean algorithm for rapidly calculating portfolio weight drift? In R, Python or MATLAB - I'm not fussed which.

Details

I'm in the final stages of extending a stress testing script (in MATLAB) that I've used for a while, and I've hit a performance wall on what should be a pretty easy updating function.

The guts of the script is a medium-sized Monte Carlo - 1000 iterations of 240 x n[i] for ~70 portfolios, where n[i] is the number of portfolio components in the ith portfolio (n[i] ranges from ~5 to ~25). Gaussian copula, so generates the pseudo-data super fast.

In the past the output has just been presented under a monthly-rebalance assumption, so the pseudo-observations for the portfolio itself are just

$$w\cdot R = \sum_i\bar{w}_i \times R_{i,t}$$

where $$\bar{w}_i$$ are the original-and-target portfolio weights. And of course that's also super-quick - O(msec). In fact it's the quickest module of the whole stress-testing framework, which completes in ~35 seconds per portfolio.

Part of the extension is to calculate the portfolio path under a range of alternative rebalancing assumptions - full-drift (i.e., no rebalancing, buy-and-hold); quarterly; annual; and 'threshold' by asset class.

In testing the full-drift version, it's taking 2 seconds per iteration to calculate the drifted weights; that seems a very long time for 240 rows of data.

And 70 lots of 1000 × ~2 seconds is a long time.

The weight transition equation is really straightforward -

$$w_{i,t+1} = w_{i,t}\cdot\left(\frac{1+R_{i,t}}{1+R_{p,t}}\right)$$

$$w_{i,0} = \bar{w}_i$$

The drifted weight depends on the portfolio return, which is the weighted sum of the component returns in period t -

$$R_{p,t} = \sum_i(w_{i,t}\cdot R_{i,t}) = w'̌\cdot R$$

So far, no rocket surgery.

To do this in MATLAB, the only way I could think of was to initialise an aggregator, and to accumulate the recalculated weights (and as a byproduct, the 'drifted' portfolio returns).

So my function is presented below (although my function's properly indented)

function[outPVals, outWts, wtDiff] = DriftWts(inTable, inWts, outName)
%% Calculates Portfolio Value and Component Weights over time if weights drift.
xT = inTable;
xP = inTable;
xT.Properties.DimensionNames{2} = 'Weights';
xT.Weights = zeros(size(xT)); %% pre-allocate weights table
xD = xT;                      %% pre-allocate weight-delta table
tBeg = min(xT.MthYr);
xT(tBeg,:).Weights = transpose(inWts); %% initialises weights at current AA
xR = inTable(tBeg,:).Returns/100;
xP.(outName)(tBeg,:) = 100*(xR * inWts);
xT = sortrows(xT,'MthYr','ascend');
for mmm = 2:length(xT.MthYr)
m = xT.MthYr(mmm);
m_l = eomdate(m - calmonths(1));
w_l = xT(m_l,:).Weights;
R_l = inTable(m_l,:).Returns/100;
xR = inTable(m,:).Returns/100;
Rp_l = w_l * transpose(R_l);
w_0 = w_l .* (1 + R_l)/( 1 + Rp_l);
xT(m,:).Weights = w_0;
xD(m,:).Weights = w_0 - w_l;
xP.(outName)(m,:) = 100*(w_0 * transpose(xR));
end
outWts = xT;
outPVals = xP(:,outName);
wtDiff = xD;
end


I considered an alternative - i.e., generating cumprod(1 +$$R_{i,t}$$ /100) of the component returns, weighting them by $$\bar{w}_i$$ at t=0, summing them, then dividing everything through by the row sum.

It's not obvious to me that this alternative would be quicker (but I have no idea what goes on under the hood in cumprod(), so I'll try it today)

OK, so the 'alternative' mechanism (cumprod, weight, divide by row sum) is O(msec) and produces identical results.

• Old stupid method: 1.705sec for a 200×9 table.
• New sexy method: 0.004sec for the same table.

And it's only 4 lines of code.

T01 = cumprod(1+inTable/100,'reverse');
T02 = inWts' .* T01;
R01 = sum(T02, 2);
W01 = T02 ./ R01;

• Makes sense, I was about to say “find a way to vectorise”, but that’s what cumprod is doing. It’s often the answer to slow matlab code.
– Ivan
Oct 29 '19 at 13:30