[R] expand a matrix

[Jinsong Zhao]

On 2022/9/6 14:30, Deepayan Sarkar wrote:
> On Tue, Sep 6, 2022 at 8:16 AM Jinsong Zhao <jszhao using yeah.net> wrote:
>>
>> Hi there,
>>
>> I have a matrix likes:
>>   > m
>>        [,1] [,2] [,3] [,4]
>> [1,]   NA    1    2    3
>> [2,]    1   NA    6    5
>> [3,]    2    6   NA    4
>> [4,]    3    5    4   NA
>>
>> I hope to expand it to 10 by 10 matrix, M, likes:
>>   > M
>>         [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
>>    [1,]   NA   NA   NA   NA    1    1    2    3    3     3
>>    [2,]   NA   NA   NA   NA    1    1    2    3    3     3
>>    [3,]   NA   NA   NA   NA    1    1    2    3    3     3
>>    [4,]   NA   NA   NA   NA    1    1    2    3    3     3
>>    [5,]    1    1    1    1   NA   NA    6    5    5     5
>>    [6,]    1    1    1    1   NA   NA    6    5    5     5
>>    [7,]    2    2    2    2    6    6   NA    4    4     4
>>    [8,]    3    3    3    3    5    5    4   NA   NA    NA
>>    [9,]    3    3    3    3    5    5    4   NA   NA    NA
>> [10,]    3    3    3    3    5    5    4   NA   NA    NA
>>
>> I use the following code:
>>
>> M <- matrix(NA, 10, 10)
>>
>> for (i in 1:10) {
>>      for (j in 1:10) {
>>         if (i %in% 1:4 & j %in% 5:6) M[i,j] <- M[j,i] <- m[1,2]
>>         if (i %in% 1:4 & j == 7) M[i,j] <- M[j,i] <-m[1,3]
>>         if (i %in% 1:4 & j %in% 8:10) M[i,j] <- M[j,i] <-m[1,4]
>>         if (i %in% 5:6 & j == 7) M[i,j] <- M[j,i] <-m[2,3]
>>         if (i %in% 5:6 & j %in% 8:10) M[i,j] <- M[j,i] <-m[2,4]
>>         if (i == 7 & j %in% 8:10) M[i,j] <- M[j,i] <-m[3,4]
>>      }
>> }
>>
>> Is there any convenience way to do it? Thanks!
> 
> Assuming I understand what you are trying to do, perhaps
> 
> i <- rep(1:4, c(4, 2, 1, 3))
> m[i, i]
> 
> Like most operations in R, indexing is vectorized.

That is exactly what I expected. Thanks a lot!

Best,
Jinsong

> 
> Best,
> -Deepayan
> 
> 
>> Best,
>> Jinsong
>>
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