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Bit of a basic problem (I think), I have 2 columns with say, 1 with gender (1=girl,0=boy) and the other with age. How would I go about calculating the mean age of girls?

stats123
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    Something like: `tapply(age, gender, mean)` – DatamineR Feb 06 '16 at 20:08
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    if you're new, the "frequent" tab may be useful including [this question](http://stackoverflow.com/questions/3505701/r-grouping-functions-sapply-vs-lapply-vs-apply-vs-tapply-vs-by-vs-aggrega) – rawr Feb 06 '16 at 20:26

2 Answers2

2

aggregate is one option

weight <- c(12,34,56,78,33)
sex <- c('m','f','f','f','m')
df <- data.frame(weight,sex)
aggregate(df$weight, list(sex = df$sex), mean)
#  sex    x
#1   f 56.0
#2   m 22.5
Brandon Bertelsen
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tagoma
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0

base

I like by when I'm working in the console too: 

weight <- c(12,34,56,78,33)
sex <- c('m','f','f','f','m')
df <- data.frame(weight,sex)
by(df,INDICES = df$sex, function(x) mean(df$weight))

dplyr

library(dplyr)
df %>% group_by(sex) %>% summarize(mean = mean(weight))

data.table

library(data.table)
dt <- setDT(df)
dt[,mean(weight),by = sex]
Brandon Bertelsen
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