Translating MQL4 ibands() to Matlab

Question

I'm trying to translate an indicator from MQL4 (Metatrader language) to Matlab. The Bollinger bands code is as follows:

for(int i=Bars;i>=0;i--)
{
    BANDS=iBands(Symbol(),0,20,2,1,0,1,i+1);
}

the iBands() documentation lists the 8 inputs as:

symbol 
timeframe   
period    
deviation   
bands_shift 
applied_price   
mode     
shift

I understand all these except bands_shift and shift. Question: If i = Bars is the entire range of the data, why does the i+1 not create an out of range error? As far as I can tell, this is code for a 20 period, 2 standard deviation Bollinger band. For a given time interval, are the associated Bollinger band values the values calculated for the previous time interval (hence the 1 after the fourth comma)? What does the i+1 do then? Given this code, how would I implement in matlab? My attempt, using this moving standard deviation and this moving average:

moving_average = movemean(EURUSD_closes(1:end-1),20); %end-1 in order to shift by 1 
moving_average = [NaN; moving_average]; %adding NaN to make BANDS the length of price

moving_std = movestd(EURUSD_closes(1:end-1),20,'backward');
moving_std = [NaN; moving_std1];

BANDS = moving_average + 2*moving_std;

I don't think this gives the same output as the MQL4 code. Any hints would definitely be appreciated!

Answer 1

How to understand iBars+1 and a "Missing" Out of Range Error

MQL4 works in a "reversed-TimeDOMAIN-indexing" space. Thus the iBar shows the "depth" of historical TimeSeriesDataSET, while the most recent (live) bar has an index of [0].

Always [0].

This means, that for a calculation of any technical indicator, the coder must arrange the processing in this manner.

This also means, that for any new bar, the internal representation of the data-storage-layer has to somehow "shift" all DataCELLs by one to the "left" ( backwards in a TimeDOMAIN direction towards History ) to make a "space" for a new bar having still index of [0] ( a Now moment in a TimeDOMAIN ).

While "physically" moving all the current "depth" of the DataSTORE would be an absolute vaste of resources ( both time, CPU, .. ), the data-storage-layer works smarter, adjusts the indexing-head on each new bar event plus uses some form of elastic DataSTORE capacity planning/re-size on-demand, so as to minimise the mem-alloc(s) during the continuous growth of the DataSTORE.

This means, that testing for an Out of Range Error does not have support in user-code namespace of the MQL4 language.

How to understand bands_shift and shift.

Calling iBands() has to state for which Bar one asks the function to calculate a result.

shift provides input for this. The index obeys the rules above.

Once the Bollinger Bands calculations are done, one may wish to offset the curves by some amount of Bars -- transposing the graph in TimeDOMAIN { +N << left | -N >> right } -- so that the visualised graphics meets one's expectations or pleasure.

bands_shift provides input for this graph ad-hoc shifting.

Also note, that the observed differences between Google, Y!Finance, MATLAB and MQL4 graphs simply have to appear and account for additional ( not known ) details one can hardly decode from the lines just shown on the screen.

applied_price := { PRICE_CLOSE | ... | ..._TYPICAL | ..._WEIGHTED } provides an input for selecting appropriate type of price entering the Bollinger calculus.

mode := { MODE_UPPER | MODE_MAIN | MODE_LOWER } provides input for receiving either an { upper_band | Bollinger Bands' axis | lower_band } PriceDOMAIN value. Thus a "Lazy approach" is to call the iBands() thrice to get the tree-line-Bollinger, or many times for a spectrum-coloured Bollinger Band heat-maps.