writing a function to calculate the mean of columns in a dataframe in R

Question

I have to calculate the mean of the columns in a dataframe by writing a function and then applying it. I understand that this is easy to do with mean and

Answer 1

Here's an example by slightly modifying your second attempt

mean_fun<-function(x){
    mean_c = numeric(0)
    for( i in 1:ncol(x)){
        s = sum(x[,i], na.rm=TRUE)
        l = length(x[,i][is.na(x[,i]) == FALSE])
        mean_c[i] = s/l
    }
    return (mean_c)
}

USAGE

mean_fun(mtcars)
# [1]  20.090625   6.187500 230.721875 146.687500   3.596563   3.217250  17.848750   0.437500   0.406250
#[10]   3.687500   2.812500

Answer 2

Why not to use dplyr?

You can get the mean for all columns in your data.frame using

summarise_each(funs(mean))

If we apply it to mtcars

library(dplyr)
mtcars %>% summarise_each(funs(mean))

#       mpg    cyl     disp       hp     drat      wt     qsec     vs      am   gear   carb
#1 20.09062 6.1875 230.7219 146.6875 3.596563 3.21725 17.84875 0.4375 0.40625 3.6875 2.8125

Answer 3

Assuming that all the columns in your data frame are numeric, here is a tweak of your first function, where x is a vector (a column in mydataframe).

mean_fun<-function(x){
    mean_c= sum(x,na.rm=TRUE)/length(!is.na(x))
    return(mean_c)
}

apply(mydataframe,2,mean_fun)