CODEWITHSUNDEEP
r
yzhang
yzhang

Reputation: 541

how to set value of a matrix in the loop?

I have two matrix, same size, 2 million rows and 2 columns. matrix_a has values, while matrix_b is blank, the value of matrix_b will be decided by the vaule in matrix_a.

matrix_b <- matrix(, nrow(matrix_a),2)
for(rows in 1:nrow(matrix_a)){
    if (matrix_a[rows,1]==0){
         .....do something get values of x, 
         matrix_b[rows,2] = x
         }
    else {
          do sth else get value of y,
          then set matrix_b 
          }
    }

First, how can I speed up this loop, as there are 2 millions rows, it is very slow to process. Second, I noticed when I tried to simply copy one value from matrix_a to matrix_b, it seems does not work. For example, while in my data 

matrix_a[1,1] = 'user1'

after I copy the values as

matrix_b[1,1] = matrix_a[1,1]

however, the result turns on as 

matrix_b[1,1]='1'

How come? Why the values 'user1' did not set to matrix_b?

I just found out this might be related to the sorting, as I sort my data first to generate matrix_a. The value '1' here means the first row after sorting, but I still do not know how it ends up in matrix_b.

Upvotes: 2

Views: 5372

Answers (1)

IRTFM
IRTFM

Reputation: 263332

If the two "somethings" and "somethings-else" can be encapsulated into functions that deliver a vector of the same length as matrix_a[ ,1], you could construct the vectors, sth_a and sth_b and then run

matrix_b[ , 2] <- ifelse(matrix_a[ ,1] == 0,  sth_a , sth_b)

Upvotes: 2

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