group_by

``` r
library(tidyverse)


df <- tibble(x=1:100, y = rep(LETTERS[1:20],5) )

df
#> # A tibble: 100 × 2
#>        x y
#>    <int> <chr>
#>  1     1 A
#>  2     2 B
#>  3     3 C
#>  4     4 D
#>  5     5 E
#>  6     6 F
#>  7     7 G
#>  8     8 H
#>  9     9 I
#> 10    10 J
#> # ℹ 90 more rows

df_share <- df |>
    group_by(y) |>
    mutate(share=x/sum(x)) |>
    ungroup()

df_share
#> # A tibble: 100 × 3
#>        x y       share
#>    <int> <chr>   <dbl>
#>  1     1 A     0.00488
#>  2     2 B     0.00952
#>  3     3 C     0.0140
#>  4     4 D     0.0182
#>  5     5 E     0.0222
#>  6     6 F     0.0261
#>  7     7 G     0.0298
#>  8     8 H     0.0333
#>  9     9 I     0.0367
#> 10    10 J     0.04
#> # ℹ 90 more rows

sessionInfo()
#> R version 4.4.2 (2024-10-31)
#> Platform: x86_64-pc-linux-gnu
#> Running under: Debian GNU/Linux 12 (bookworm)
#>
#> Matrix products: default
#> BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.11.0
#> LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.11.0
#>
#> locale:
#>  [1] LC_CTYPE=en_GB.UTF-8       LC_NUMERIC=C
#>  [3] LC_TIME=en_GB.UTF-8        LC_COLLATE=en_GB.UTF-8
#>  [5] LC_MONETARY=en_GB.UTF-8    LC_MESSAGES=en_GB.UTF-8
#>  [7] LC_PAPER=en_GB.UTF-8       LC_NAME=C
#>  [9] LC_ADDRESS=C               LC_TELEPHONE=C
#> [11] LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C
#>
#> time zone: Europe/Brussels
#> tzcode source: system (glibc)
#>
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base
#>
#> other attached packages:
#>  [1] lubridate_1.9.3 forcats_1.0.0   stringr_1.5.1   dplyr_1.1.4
#>  [5] purrr_1.0.2     readr_2.1.5     tidyr_1.3.1     tibble_3.2.1
#>  [9] ggplot2_3.5.1   tidyverse_2.0.0
#>
#> loaded via a namespace (and not attached):
#>  [1] gtable_0.3.5       compiler_4.4.2     reprex_2.1.0       tidyselect_1.2.1
#>  [5] scales_1.3.0       yaml_2.3.8         fastmap_1.1.1      R6_2.5.1
#>  [9] generics_0.1.3     knitr_1.46         munsell_0.5.1      R.cache_0.16.0
#> [13] pillar_1.10.1.9000 tzdb_0.4.0         R.utils_2.12.3     rlang_1.1.4
#> [17] utf8_1.2.4         stringi_1.8.4      xfun_0.43          fs_1.6.4
#> [21] timechange_0.3.0   cli_3.6.3          withr_3.0.0        magrittr_2.0.3
#> [25] digest_0.6.35      grid_4.4.2         hms_1.1.3          lifecycle_1.0.4
#> [29] R.methodsS3_1.8.2  R.oo_1.26.0        vctrs_0.6.5        evaluate_0.23
#> [33] glue_1.7.0         styler_1.10.3      colorspace_2.1-0   rmarkdown_2.26
#> [37] tools_4.4.2        pkgconfig_2.0.3    htmltools_0.5.8.1
```

<sup>Created on 2025-01-01 with [reprex v2.1.0](https://reprex.tidyverse.org)</sup>