Change some rows to columns, keep some rows in rows in R dataframe
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Every time I look at this problem, my brain shuts down.  I think it will be simple for some one else.  
I have a very large dataframe that can be simplified as so:
  
    day	subject	y variable
    1	a	2.1
    1	b	3.5
    1	c	2.2
    2	a	3.3
    2	b	4.5
    2	c	3.8
    3	a	5.1
    3	b	2.8
    3	c	2.3
    4	a	4.2
    4	b	1.9
    4	c	2.5
    5	a	4.9
    5	b	3.8
    5	c	3.4

I want columns by subject, eg:

    a	  b	    c
    2.1	  3.5	2.2
    3.3	  4.5	3.8
    5.1	  2.8	2.3
    4.2	  1.9	2.5
    4.9	  3.8	3.4

If some one gets me to this point, I can easily create a new column for days.  I am getting better at data manipulation, but sometimes I still get so stuck on something like this. 