[R] lm question

[Bill Simpson]

I have not really used lm before and I was hoping for some help on a
simple problem.

Here is a toy version of the problem I want to solve.

y       x       grp
-.9     1       a
-.8     2       a
-.7     3       a
-.7     1.5     b
-.5     2.5     b
-.3     3.5     b
-.19    2.7     c
-.11    3.7     c
-.41    4.7     c

I want to fit a model that has one y-intercept and three slopes, one for
each group.

Could someone please show me how to do this?  Thanks very much for any
help.

Bill Simpson

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