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I have a 2D array A with 10,000 rows and 2 columns.

At a first instance I want to use only the first 200 rows of the array A. I did the following: New_array=A[A(1:200) ,]

Each time I want to increase the number of the rows by 50. i.e. in the second iteration I want to have access on the 250 rows of matrix A, third iteration 300 and so on until I reach the original size of the matrix.

I know that I have to create a for loop but I struggle. Any help will be highly appreciated

Gina
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    I think this question would benefit from sample data. Can you make it a [reproducible question](https://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example)? (For instance, are you really just asking for `rbind(A, emptymatrixhere)`? In general, appending to vectors/arrays/data.frames is discouraged since it it does a full copy; this is fine with small objects, but with larger objects you will feel the effect if repeated. In that case, I suggest over-allocating up front and cropping when complete.) – r2evans Jul 18 '17 at 21:48
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    `lapply(seq(200, NROW(m), 50), function(i) m[1:i,])` – d.b Jul 18 '17 at 21:55
  • simply using @d.b answer will solve you problem . – BENY Jul 18 '17 at 22:04

1 Answers1

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The seq function allows you to specify intervals in a sequence, as shown in @d.b's comment.

seq(0, 20, by = 5)
[1]  0  5 10 15 20

The output of seq can then be used to drive the loop. Here i is used as the endpoint for a sequence in each iteration.

for ( i in seq(5, 20, by = 5) ) {
    print(1:i) 
}

[1] 1 2 3 4 5
[1]  1  2  3  4  5  6  7  8  9 10
[1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
[1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20

Applied to your example, the sequences can be used to subset the matrix

# Example matrix

m <- 10000
n <- 2     
A <- matrix(1:(m*n), ncol = n)

head(A)
     [,1]  [,2]
[1,]    1 10001
[2,]    2 10002
[3,]    3 10003
[4,]    4 10004
[5,]    5 10005
[6,]    6 10006

# Iterate with a loop

jump <- 5  # I'm using 5 instead of 50

for ( i in seq(jump, m, by = jump) ) {
    print(paste("i =", i))
    print( A[1:i, ] )        # subset the matrix
    if ( i > 15 ) break      # limiting the output for readability
}

[1] "i = 5"
     [,1]  [,2]
[1,]    1 10001
[2,]    2 10002
[3,]    3 10003
[4,]    4 10004
[5,]    5 10005

[1] "i = 10"
      [,1]  [,2]
 [1,]    1 10001
 [2,]    2 10002
 [3,]    3 10003
 [4,]    4 10004
 [5,]    5 10005
 [6,]    6 10006
 [7,]    7 10007
 [8,]    8 10008
 [9,]    9 10009
[10,]   10 10010

[1] "i = 15"
      [,1]  [,2]
 [1,]    1 10001
 [2,]    2 10002
 [3,]    3 10003
 [4,]    4 10004
 [5,]    5 10005
 [6,]    6 10006
 [7,]    7 10007
 [8,]    8 10008
 [9,]    9 10009
[10,]   10 10010
[11,]   11 10011
[12,]   12 10012
[13,]   13 10013
[14,]   14 10014
[15,]   15 10015

[1] "i = 20"
      [,1]  [,2]
 [1,]    1 10001
 [2,]    2 10002
 [3,]    3 10003
 [4,]    4 10004
 [5,]    5 10005
 [6,]    6 10006
 [7,]    7 10007
 [8,]    8 10008
 [9,]    9 10009
[10,]   10 10010
[11,]   11 10011
[12,]   12 10012
[13,]   13 10013
[14,]   14 10014
[15,]   15 10015
[16,]   16 10016
[17,]   17 10017
[18,]   18 10018
[19,]   19 10019
[20,]   20 10020
Damian
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