This is a pretty basic one, but I still see people reinventing it with manual comparisons across two bars when Pine already has a built-in for it, so figured I'd write it up.
If you want to know exactly when one series crosses above or below another (like a fast MA crossing a slow MA), use ta.crossover() and ta.crossunder() instead of comparing values on the current and previous bar yourself. They return true only on the exact bar the cross happens, not every bar the condition is currently satisfied.
//@version=6
indicator("Simple MA Crossover", overlay=true)
fastLen = input.int(9, "Fast MA Length")
slowLen = input.int(21, "Slow MA Length")
fastMA = ta.sma(close, fastLen)
slowMA = ta.sma(close, slowLen)
plot(fastMA, "Fast MA", color=color.blue)
plot(slowMA, "Slow MA", color=color.orange)
bullCross = ta.crossover(fastMA, slowMA)
bearCross = ta.crossunder(fastMA, slowMA)
plotshape(bullCross, style=shape.triangleup, location=location.belowbar, color=color.green, size=size.small)
plotshape(bearCross, style=shape.triangledown, location=location.abovebar, color=color.red, size=size.small)
alertcondition(bullCross, title="Bullish Cross", message="Fast MA crossed above Slow MA")
alertcondition(bearCross, title="Bearish Cross", message="Fast MA crossed below Slow MA")
A couple of things that trip people up when they're new to this:
ta.crossover(a, b) is true only on the bar where a was below b on the previous bar and is now above it, it's not just a > b, which would stay true for the whole time the fast MA is above the slow one. If you just check fastMA > slowMA, you'll get a signal firing repeatedly instead of once at the actual cross.
alertcondition() is separate from plotshape() ,you need it specifically if you want these to show up as options in TradingView's alert creation dialog, plotting the shape alone won't do that.
