Round to first two digits

To improve the readability of a table, I wanted to round various numbers between 100 and 10,000,000 to their first two digits. 123,456 would become 120,000 and 987 would become 990.

Here is the solution I came up with.

\[ \text{scale} \leftarrow 10^{\lceil \log_{10}(x) \rceil} \]

\[ \text{round}\left(\frac{x}{\text{scale}}, 2\right) \times \text{scale} \]

library(purrr)

options(scipen = 999)
set.seed(0)

round_first_two <- function(x) {
  scale <- 10^ceiling(log10(x))
  round(x / scale, 2) * scale
}

x1 <- c(113, 2795, 42459, 442060, 6870466)

x1

[1]     113    2795   42459  442060 6870466

map_dbl(x1, round_first_two)

[1]     110    2800   42000  440000 6900000

Copilot

GitHub Copilot suggested a more general solution that works for numbers smaller and equal to zero.

x2 <- c(113, -2795, -42459, -442060, 6870466, 0)

x2

[1]     113   -2795  -42459 -442060 6870466       0

map_dbl(x2, round_first_two)

[1]     110     NaN     NaN     NaN 6900000     NaN

round_first_two_digits <- function(x) {
  if (x == 0) {
    return(0)
  }

  num_digits <- floor(log10(abs(x))) + 1
  scale_factor <- 10^(num_digits - 2)
  round(x / scale_factor) * scale_factor
}

x2

[1]     113   -2795  -42459 -442060 6870466       0

map_dbl(x2, round_first_two_digits)

[1]     110   -2800  -42000 -440000 6900000       0

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