Nested for-loop with lists

Question

So I was trying to
(1) Use the vector function to create a list of size 3, name these as x,y,z
(2): Write a nested for loop, the outer loop iterating through the list (n=1:N), the inner from t=1:4
(3): Assign to the n−th list in each of the t−th positions in the vector, the value 10n+t

What I currently get is

N = 3
N_list <- vector(mode = "list", length = N) 
list_names <- c('x', 'y', 'z')
names(N_list) <- list_names
inner <- NULL
for (n in 1:N) {
    for (t in 1:4) {
        inner[[t]] <- t  
    }
    N_list[[n]] <- (10*n+inner[[t]]) 
}

Though I expect the list to be like:

$x  
[1] 11, 12, 13, 14  
$y    
[1] 21, 22, 23, 24  
$z  
[1] 31, 32, 33, 34

I actually only get 14, 24, 34 for each list.

Though I searched a lot of articles to learn about the logic of nested for-loop, I'm still not sure how to do it. Could somebody help me with this? Thank you in advance.

Onyambu · Accepted Answer · 2020-07-27 00:01:44Z

1

you are almost correct. check this out:

N = 3
N_list <- vector(mode = "list", length = N) 
list_names <- c('x', 'y', 'z')
names(N_list) <- list_names

for (n in 1:N) {
    inner <- NULL # You should define inner here.
    for (t in 1:4) {
        inner[t] <- t  
    }
    N_list[[n]] <- (10 * n + inner) 
}

N_list
$x
[1] 11 12 13 14

$y
[1] 21 22 23 24

$z
[1] 31 32 33 34

edited Jul 27, 2020 at 0:01

answered Jul 26, 2020 at 23:56

Onyambu

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4 Comments

Anaerobic Over a year ago

Thank you! Just for sure, suppose the "inner" is used in the outer for-loop, should I define this "inner" before the whole for-loop code?

Onyambu Over a year ago

then it wont be changing as expected. You should not define it out of the outer loop.

Anaerobic Over a year ago

Oh I got it thank you so much! Sorry I'm new to Stack Overflow, will the question automatically be closed if I accept the solution? I cannot find the close button on the page.

Onyambu Over a year ago

Once the an answer is accepted, it shows that the question has been solved.

Ronak Shah · Accepted Answer · 2020-07-26 23:51:23Z

1

You can do this with one for loop :

N = 3
N_list <- vector(mode = "list", length = N) 
list_names <- c('x', 'y', 'z')
names(N_list) <- list_names
inner <- 1:4

for (n in 1:N) {
   N_list[[n]] <- (10*n+inner) 
}
N_list
#$x
#[1] 11 12 13 14

#$y
#[1] 21 22 23 24

#$z
#[1] 31 32 33 34

You can also use lapply :

lapply(seq_len(N), function(x) 10 * x + inner)

answered Jul 26, 2020 at 23:51

Ronak Shah

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1 Comment

Anaerobic Over a year ago

Thank you for the comment! lapply seems a better and simpler method in this case.

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