[R] Average of a variable against another.

[Keeeeeeee]

Dear helpers,

FYI, I am a beginner of R, just have dealt with MATLAB or JAVA.

I want to know how to solve one problem given 4 variables: year_1, year_2,
tall_1, tall_2.
The tall_1 is measured at year_1 and tall_2 at year_2.
The tall has grown up such as uniformly 1 cm/yr. 

The data is like

year_1 year_2 tall_1 tall_2
2007    2010    12    15
1999    2009    6    16
2003    2005    11    13
2002    2009    3    10
    .    .    .    .    .
    .    .    .    .    .
    .    .    .    .    .
    .    .    .    .    .

So I need to get the average tall of the plant against year, for all the
years of available data. 
The year_1 and year_2 are recorded so that a plant is alive if the year of a
question is equal to or grater than the year_1 and equal to and less than
the year_2.

For example, 
1999    2000    2001    2002    2003    2004    2005    2006    2007    2008  
2009    2010
   6          7          8          9        10        11        12       
13        14       15      16             
                                       3         4          5          6         
7         8        9       10
                                                 11        12        13
                                                                                              
12       13      14        15

avg
  6        7            8         6         25/3 ...etc.

Since the amount of the data is too huge, I need to use appropriate
functions and algorithm but I am
not good at programming R. 

I wish you help me out from this hell problem please.

Thanks,
Keeeeeeee

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