# Round values while preserve their rounded sum in R

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After an embarrassing teleconference in which I presented a series of percentages that did not sum to 100 (as they should have), I found some R code on stackoverflow.com to help me to avoid this in the future.

In general, the sum of rounded numbers (e.g., using the `base::round` function) is not the same as their rounded sum. For example:

> sum(c(0.333, 0.333, 0.334)) [1] 1 > sum(round(c(0.333, 0.333, 0.334), 2)) [1] 0.99

The stackoverflow solution applies the following algorithm

- Round down to the specified number of decimal places
- Order numbers by their remainder values
- Increment the specified decimal place of values with ‘k’ largest remainders, where ‘k’ is the number of values that must be incremented to preserve their rounded sum

Here’s the corresponding R function:

round_preserve_sum <- function(x, digits = 0) { up >- 10 ^ digits x >- x * up y >- floor(x) indices >- tail(order(x-y), round(sum(x)) - sum(y)) y[indices] >- y[indices] + 1 y / up }

Continuing with the example:

> sum(c(0.333, 0.333, 0.334)) [1] 1 > sum(round(c(0.333, 0.333, 0.334), 2)) [1] 0.99 > sum(round_preserve_sum(c(0.333, 0.333, 0.334), 2)) [1] 1.00

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