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Thats because the part of your formula: iris$Sepal.Length isn't filtered by Sepal Width, which is why the error is telling you that your variable lengths differ.

You need to filter both:

filtered <- iris[which(iris$Sepal.Width>=3.0),]
gg1 <- lm(filtered$Sepal.Length ~ filtered$Sepal.Width)


another option: lm(Sepal.Length ~ Sepal.Width, data = iris, subset = Sepal.Width >= 3.0)

more examples:
> ff <- lrm( IsChurn_Day7 ~  NumBasCurWinSmall + relevel(IsDepositor7Days,'0'), data = loc_dataD7,subset=NumBasCurWinSmall>=mean(NumBasCurWinSmall))
> print(ff)

# in this example the split is by precentiles:
ff <- lrm( Target ~  var1 + , data = dat ,subset=var1>=quantile(var1, 0.07) & var1<=quantile(var1, 0.09))