sum() down columns by subject dplyr

Question

I'm trying to use dplyr to summarize some data and can't work out how to sum values from part of a column. Normally I'd use tally(), but in this case I want to add up all of the 1's and 0's so tally() isn't appropriate.

My data looks something like this:

  subj | child | child_age | older | younger
    1      1        374        0        1
    1      2        465        1        0
    2      1        573        1        0
    2      2        583        1        0
    2      3        172        0        1

So, I want to create a dataset that shows, for each subj, how many 'older' children and how many 'younger' children they have. This should look something like this:

  subj | n_child | older | younger
    1      2        1         1
    2      3        2         1

This is the code I've used so far:

  child_ages <- data %>%
    group_by(subj) %>%
    mutate(nOlder = sum(older),
           nYounger = sum(younger)) %>%
    ungroup()

I've also tried summarize() in place of mutate(); both appear to be ignoring my group_by command and just give me totals across the data.

Many thanks!

Possible duplicate of [How to sum a variable by group?](https://stackoverflow.com/questions/1660124/how-to-sum-a-variable-by-group) — Kristofersen, May 25 '17 at 14:47
`data %>% group_by(subj) %>% summarise(n_child = n(),nOlder = sum(older),nYounger = sum(younger))` works for me — David Arenburg, May 25 '17 at 14:48
@DavidArenburg is right. It should work. You shouldn't `ungroup()` at the end, as you want the information after group. — A Gore, May 25 '17 at 14:51
@AGore `ungroup` isn't relevant here. It won't make the data long again. — David Arenburg, May 25 '17 at 14:53
Thanks! I got rid of ungroup() and it worked! Slowly getting to grips with dplyr :) — Catherine Laing, May 25 '17 at 15:08
@DavidArenburg So it does! Which is strange because in that case my original code should have worked. I re-booted R before trying again so perhaps something more to do with my R environment than the code itself. — Catherine Laing, May 25 '17 at 15:22

sum() down columns by subject dplyr

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