[R] Find the max entry in column 2 - that satisfies a condition given a fixed entry in column 1

[Vangelis Litos]

I am using R and I created based on a vector x, a grid (named: plain) - where zero is included. This gives me a 9x2 ``matrix''.

> x_t = cbind(c(1),c(2));
> x = t(x_t);

> plain = expand.grid (sort (unique (c (0, x))), sort (unique (c (0, x))));

My aim is to focus on column 1 and take i.e the first entry (then the second, the third entry etc through a loop) of the unique (c (0, x)) vector (==0) [rows 1, 4 and 7] and find the maximum value (entry) in the second column of the matrix that satisfies a condition.

Let say that the condition is satisfied when at column 2 the value is 2. That is I want row 7 to be selected

Then I want to create a column vector b (9x1) that has zero everywhere apart from row 7.

Can anybody help me with this?

Thank you in advance.

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