[R] liner regression for multiple keys

[newbie_2010]

a1 is the first key in input. second column is x-axis and 3rd is y-axis and
4th is its corresponding key.
Now for every key in 1st column I would like to calculate LR that gives p
value. I tried to manage with a single key. But my problem is that How could
I manage multiple keys in input.
Thanx in advance

input

a1  1  2.5  x
a1  2  3.5  x
a1  3  4.5  x
a1  1  2.6  x
a1  2  3.8  x
b1  1  4.6  y
b1  2  5  y
b1  3  6  y
b1  6  7  y

desiredoutput

a1  pvalue(LR)  x
b1  pvalue(LR)  y

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