Expert Advisor Programming by Andrew R. Young

EXPERT ADVISOR PROGRAMMING
We'll use the identifiers SlopePeriod, SlopeMethod and SlopePrice for the external variables in our
expert advisor.
// External variables
extern int SlopePeriod = 80;
extern int SlopeMethod = 3;
extern int SlopePrice = 0;
Here is how the iCustom() function will look for this particular indicator, along with the external
variables:
iCustom(NULL,0,"Slope Direction Line",SlopePeriod,SlopeMethod,SlopePrice,0,0);
NULL indicates that we are using the current chart symbol, and 0 is the current chart period. "Slope
Direction Line" is the name of the indicator file. SlopePeriod, SlopeMethod and SlopePrice
are the three indicator parameters. We are using the default Mode index of 0, and the Shift is the
current bar.
Although the Slope Direction Line indicator is drawn as a single line, it is actually composed of two
different buffers. Depending on whether the indicator price is moving up or down, the color (and the
buffer) change.
If you attach the indicator to a chart and view the Data
Window in MetaTrader, you'll see two values for the Slope
Direction Line. The first value displays a price when the
indicator value is increasing. The line is blue by default. The
second value displays a price when the indicator value is
decreasing. This line is red by default.
We need to determine the Mode index for both of these
lines. The easiest way to do this is to look at the source
code. In the init() function, you will see several lines of
code that are used to declare and set the properties for the
indicator buffers:
Fig 6.2 – Data Window
SetIndexBuffer(0, Uptrend);
SetIndexBuffer(1, Dntrend);
SetIndexBuffer(2, ExtMapBuffer);
...
SetIndexStyle(0,DRAW_LINE,STYLE_SOLID,2);
SetIndexStyle(1,DRAW_LINE,STYLE_SOLID,2);
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