MQL4 Reference 
Timeseries and Indicators Access
Access to Timeseries and Indicator DataThese are functions for working with timeseries and indicators. A timeseries differs from the usual data array by its reverse ordering - elements of timeseries are indexed from the end of an array to its begin (from the most recent data to the oldest ones). To copy the time-series values and indicator data, it's recommended to use dynamic arrays only, because copying functions are designed to allocate the necessary size of arrays that receive values. There is an important exception to this rule: if timeseries and indicator values need to be copied often, for example at each call of OnTick() in Expert Advisors or at each call of OnCalculate() in indicators, in this case one should better use statically distributed arrays, because operations of memory allocation for dynamic arrays require additional time, and this will have effect during testing and optimization. When using functions accessing timeseries and indicator values, indexing direction should be taken into account. This is described in the Indexing Direction in Arrays, Buffers and Timeseries section. Access to indicator and timeseries data is implemented irrespective of the fact whether the requested data are ready (the so called asynchronous access). This is critically important for the calculation of custom indicator, so if there are no data, functions of Copy...() type immediately return an error. However, when accessing form Expert Advisors and scripts, several attempts to receive data are made in a small pause, which is aimed at providing some time necessary to download required timeseries or to calculate indicator values. If data (symbol name and/or timeframe differ from the current ones) are requested from another chart, the situation is possible that the corresponding chart was not opened in the client terminal and the necessary data must be requested from the server. In this case, error ERR_HISTORY_WILL_UPDATED (4066 - the requested history data are under updating) will be placed in the last_error variable, and one will has to re-request (see example of ArrayCopySeries()).
It is historically accepted that an access to the price data in an array is performed from the end of the data. Physically, the new data are always written at the array end, but the index of the array is always equal to zero. The 0 index in the timeseries array denotes data of the current bar, i.e. the bar that corresponds to the unfinished time interval in this timeframe. A timeframe is the time period, during which a single price bar is formed. There are several predefined standard timeframes.
Despite the fact that by using the ArraySetAsSeries() function it is possible to set up in arrays access to elements like that in timeseries, it should be remembered that the array elements are physically stored in one and the same order - only indexing direction changes. To demonstrate this fact let's perform an example:
As a result we will get the output like this:
As we see from the output, as the index of TimeAsSeries array increases, the time value of the index decreases, i.e. we move from the present to the past. For the common array ArrayNotSeries the result is different - as index grows, we move from past to present. See Also ArrayIsDynamic(), ArrayGetAsSeries(), ArraySetAsSeries(), ArrayIsSeries() |
