in r, how to assess the two vectors based on other vectors

i am relatively new to r, so any help is appreciated

sample dataset, also attached an image of the dataset. image is of sample dataset

a           B           C           D
12.97221,   64.78909    1           2
69.64817,   321.90037   2           28
318.87946,  259.29946   3           5
326.17622,  94.7089     9           8
137.54006,  325.34917   5           88
258.06002,  94.77531    6           63
258.92824,  322.20164   7           64
98.57514,   12.96828    8           34
98.46303,   139.27264   9           21
317.22764,  261.25563   10          97

my Goal: i need to

1) look at value in column A
2) find the nearest/closest number in column B
3) test to see if the value in column B has already been selected
4) if the value in column B has already been selected, then ignore and choose the next closest value.
5) once a new, non-duplicated, value is chosen from column B, then
6) Test to see if the value in column C that is on the same row as the value of interest in column A is not the same as the value in column D on the same row as the nearest chosen value in column B
7) if the values in column C and D are NOT the same, then 
8) return the value from column B into a new column
9) if the value in column C and D are the same, then repeat steps 4-7 until a) a new, non-duplicated value is chosen, and b) the value in C and D are not equal. 

here is the code i have so far, this code solves the problem of finding the nearest number "without replacement" but does not tackle the issue of similar values in columns C and D being considered before the value in column B is chosen; developed by "Chase" from here: How to get the closest element in a vector for every element in another vector without duplicates?

foo <- function(a,b) {

  out <- cbind(a, bval = NA)

  for (i in seq_along(a)) {
    #which value of B is closest?
    whichB <- which.min(abs(b - a[i]))
    #Assign that value to the bval column
    out[i, "bval"] <- b[whichB]
    #Remove that value of B from being chosen again
    b <- b[-whichB]
  }

  return(out)

hopefully this (below) is a better description and example of my problem.

see adjusted table to better show my problem. look at value in column A of 12.97221, then evaluate column B and choose the value of 12.96828. then evaluate the values in column C that correspond to 12.97221, which is 1; then look at the value in column D (value in d=34) that corresponds to 12.96828. Since the value in column B of 12.96828 has not been selected and the values in column C and D do not match, then I would expect it to return 12.96828 in column E. Next it would look at 2nd value in column A of 69.64817 and compare it to the values in column B, it should select 64.78909, then evaluate if it has been chosen. then evaluate value in column C (2) that corresponds to value in column B and evaluate value in column D (2) that corresponds to chosen value in column C. although this is the first time 64.78909 is selected, the values in column C and D are the same, and therefore i need to pick the next closest value from column B of 94.7089, then evaluate whether it has been chosen; it has not. then evaluate the value in column C that corresponds to the value in column A (value in C = 2) and evaluate the value in column D that corresponds to 94.7089 (value in D of 34) and compare them. Since the value of 94.7089 was not chosen yet and the values in columns C (value in C = 2) and D (value in D = 34) are not the same, return 94.7089 into column E.

again, thanks in advance and hopefully i described my problem sufficiently