r - How to recognize a category from a matrix -
i have matrix 6*4 matrix , stacked them 2 columns matrix. first column numerical values, , second column category names. want calculate mean of values each category don't know how characters column 2.
to aggregate values in data frame category, use aggregate function.  consider artificial data:
x <- data.frame(values=sample(1:6), categories=sample(c('a','b'), 6, replace=true)) x ##   values categories ## 1      4          b ## 2      1          b ## 3      5          ## 4      3          b ## 5      6          ## 6      2           aggregate(values~categories, data=x, fun=mean) ##   categories   values ## 1          4.333333 ## 2          b 2.666667 
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