[R] Transformation of a data frame

[Des Callaghan]

Dear R-helpers,

Apologies in advance for this (probably) simple question.  I've searched the
R Archive and can't seem to find a solution to my problem.  

I have a data frame of vegetation quadrat data with the following format:

Q S C
1 A 5
1 B 10
1 C 50
1 D 10
2 A 20
2 E 10
2 C 40
3 D 5
3 F 1
3 G 5
3 B 75

Where Q is the sample (vegetation quadrats), S is the species and C is the
percentage cover of each species within the sample.  I wish to transform
this into a community data matrix for analysis within the vegan package,
which needs the following data frame format:

Q A  B  C  D  E  F  G
1 5  10 50 10 0  0  0
2 20 0  40 0  10 0  0
3 0  75 0  5  0  1  75

I can't manage to do this transformation so your help would be much
appreciated.  Thanks very much in advance.

All the best,
Des