# All Questions

**839**

votes

**9**answers

328381 views

### Grouping functions (tapply, by, aggregate) and the *apply family

Whenever I want to do something "map"py in R, I usually try to use a function in the apply family.
However, I've never quite understood the differences between them -- how {sapply, lapply, etc.} app...

**85**

votes

**6**answers

146795 views

### Apply a function to every row of a matrix or a data frame

Suppose I have a n by 2 matrix and a function that takes a 2-vector as one of its arguments. I would like to apply the function to each row of the matrix and get a n-vector. How to do this in R?
For ...

**15**

votes

**3**answers

23095 views

### Concatenate row-wise across specific columns of dataframe

I have a data frame with columns that, when concatenated (row-wise) as a string, would allow me to partition the data frame into a desired form.
> str(data)
'data.frame': 680420 obs. of 10 vari...

**5**

votes

**3**answers

2732 views

### Create frequency tables for multiple factor columns in R

I am a novice in R. I am compiling a separate manual on the syntax for the common functions/features for my work. My sample dataframe as follows:
x.sample <-
structure(list(Q9_A = structure(c(5L, ...

**4**

votes

**2**answers

6238 views

### weighted means by group and column

I wish to obtain weighted means by group for each of several (actually about 60) columns. This question is very similar to: repeatedly applying ave for computing group means in a data frame just aske...

**7**

votes

**2**answers

9984 views

### How do I count the number of words in a text (string)?

I have this string vector (for example):
str <- c("this is a string current trey",
"feather rtttt",
"tusla",
"laq")
To count the number of words in this vector I used this (as given ...

**18**

votes

**3**answers

5918 views

### Using "..." and "replicate"

In the documentation of sapply and replicate there is a warning regarding using ...
Now, I can accept it as such, but would like to understand what is behind it. So I've created this little contrived...

**26**

votes

**5**answers

52167 views

### How to subset from a list in R

I have a rather simple task but haven't find a good solution.
> mylist
[[1]]
[1] 1 2 3 4 5 6 7 8 9 10
[[2]]
[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r...

**5**

votes

**4**answers

4380 views

### Extracting nth element from a nested list following strsplit - R

I've been trying to understand how to deal with the output of strsplit a bit better. I often have data such as this that I wish to split:
mydata <- c("144/4/5", "154/2", "146/3/5", "142", "143/4...

**7**

votes

**1**answers

204 views

### Speeding up function that uses which within a sapply call in R

I have two vector e and g. I want to know for each element in e the percentage of elements in g that are smaller. One way to implement this in R is:
set.seed(21)
e <- rnorm(1e4)
g <- rnorm(1e4)...