Here's the script of the basic Bollinger Bands Width indicator (plus my commented explanation):
//@version=5
indicator(title="Bollinger Bands Width", shorttitle="BBW", format=format.price, precision=2, timeframe="", timeframe_gaps=true)
length = input.int(20, minval=1)
src = input(close, title="Source")
mult = input.float(2.0, minval=0.001, maxval=50, title="StdDev")
basis = ta.sma(src, length)
dev = mult * ta.stdev(src, length)
upper = basis + dev
lower = basis - dev
bbw = (upper-lower)/basis
// Explanation of what I seek:
// bbwd = bbw minus previousbbw
plot(bbwd, "Bollinger Bands Width Differenctial", color=#138484)
There's a commented line starting "Example of what I need". I would like to make a variable by subtracting the previous value of Width from the current value of Width. To measure speed.
This shows two different ways to achieve your goal. The precision was increased to 6 because the values are smallish:
//@version=5
indicator(title="Bollinger Bands Width", shorttitle="BBW", format=format.price, precision=6, timeframe="", timeframe_gaps=true)
length = input.int(20, minval=1)
src = input(close, title="Source")
mult = input.float(2.0, minval=0.001, maxval=50, title="StdDev")
basis = ta.sma(src, length)
dev = mult * ta.stdev(src, length)
upper = basis + dev
lower = basis - dev
bbw = (upper-lower)/basis
// Explanation of what I seek:
// bbwd = bbw minus previousbbw
bbwd1 = ta.change(bbw)
bbwd2 = bbw - bbw[1]
plot(bbwd1, "Bollinger Bands Width Differenctial", color=#13848450, linewidth=6)
plot(bbwd2, "Bollinger Bands Width Differenctial", color=#138484)
This is the refman entry for ta.change(). This is the usrman section that explains the [] history-referencing operator.