[R] how to do this trimming/selecting in canonical R?

[gildororonar at mail-on.us]

This is better explained by example:

> A <- data.frame(force = sort(runif(10, 0, 1)), condition =  
> sort(sample(0:100, 10)))
> B <- data.frame(counterforce = sort(runif(15, 0, 1), decreasing=T),  
> condition = sort(sample(0:100, 15)))

So we have:

> A
         force condition
1  0.03515542         1
2  0.13267882        13
3  0.26155689        24
4  0.37453142        38
5  0.39360520        45 <--- trim everything after this
6  0.43924737        48
7  0.47669800        50
8  0.57044795        51
9  0.81177499        61
10 0.98860450        94

> B
    counterforce condition
1   0.965769548         2
2   0.965266255         5
3   0.846941244         7
4   0.818013029        11
5   0.813139978        22
6   0.730599939        34
7   0.715985436        39
8   0.658073895        40
9   0.421264948        42 <--- trim everything after this
10  0.373774505        52
11  0.242191461        62
12  0.090584590        63
13  0.070020635        68
14  0.067366062        83
15  0.001585313        84

I need to trim away rows after No. 5, from A, trim away rows after No.  
9, from B.

Because

A[5, condition] > max(B[1:9, condition] && A[5, force] > B[9+1, counterforce]

In a general way, I am looking for x and y, where:

A[x, condition] > max(B[1:y, condition] && A[x, force] > B[y+1, counterforce]

and I will select A[1:x,] and B[1:y,], or trim away the rest, because  
they are irrelevent for the calculation onwards.

This is easy to do it in C, and I actually have done it in C-like R  
script, by looping through all rows of A, and breaking from the loop  
when finding the matching trim-point in B. But I am learning R, so  
what is the native way to do it in R?


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