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copied!<p>I suggest to recode myfun to take a matrix and use pasteCols from plotrix package.</p> <pre><code>library(plotrix) myfun = function(x){ out = pasteCols(t(x), sep = ":") # some code return(out) } </code></pre> <p>then, its very easy: for each group, compute the index of the first and of the last column you want to use when you call myfun, using modulus and integer division:</p> <pre><code>rubiques_solution = function(group, myd, num_to_group){ # loop over groups for(g in unique(group)){ var_index = which(group == g) num_var = length(var_index) # test to make sure num_to_group is smaller than the number of variable if(num_var < num_to_group){ stop("num_to_group > number of variable in at least one group") } # number of calls to myfun num_calls = num_var %/% num_to_group # the idea here is that we create the first and last column # in which we are interested for each call first = seq(from = var_index[1], by = num_to_group, length = num_calls) last = first + num_to_group -1 # the last call will contain possibly more varialbe, we adjust here: last[length(last)] = last[length(last)] + (num_var %% num_to_group) for(i in num_calls){ # maybe do something with the return value of myfun ? myfun(myd[,first[i]:last[i]]) } } } group = rep(1:10, each = 10) # same than yours myd = data.frame (matrix (sample (c("AB", "BB", "AA"), 100*100, replace = T), ncol = 100)) # same than yours num_to_group = 2 # this is your first example rubiques_solution(group, myd, num_to_group) </code></pre> <p>hope i understood the problem right.</p>

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