How to add first number in row in a column in R-CodePudding

Lets say we have the following df

letter   number
a         10
b         11

and I wanted to add only the first number in a column. How would you do that

Result

= number column should have a sum of two (10 11) = 2

CodePudding user response：

A faster option is to get the substring, convert to numeric and get the sum

with(df, sum(as.numeric(substring(number, 1, 1))))
[1] 2

Or we may use

with(df, sum(number %/% 10))
[1] 2

Or as @rg255 mentioned in the comments, if the number of digits can be different (other than the 2 digit mentioned in the example)

with(df, sum(number %/% 10^(nchar(number)-1)))

CodePudding user response：

Substitute x here for you column (e.g. df$v2). This creates a list of the digits within each element, then extracts the first and sums them.

sum(sapply(strsplit(as.character(x), ""), function(i){as.numeric(i[1])}))

CodePudding user response：

Using tidyverse:

library(tidyverse)

set.seed(111)

df <- tibble(letter = letters[1:10], number = sample(1:100, 10))

summarise(df, sum_first = str_sub(number, 1, 1) %>%
                           as.numeric() %>% 
                            sum() )
#> # A tibble: 1 × 1
#>   sum_first
#>       <dbl>
#> 1        52

^{Created on 2021-11-24 by the reprex package (v2.0.1)}