限制矩阵每行的元素个数

Question

我有一个像这样的矩阵：

A
     [,1] [,2] [,3] [,4]
[1,]    1    3    5    7
[2,]    2    4    6    8

以及每行限制元素个数的向量（其他元素将被转换为0）。

 c
[1] 2 3

我想获得（至少有循环）：

B
     [,1] [,2] [,3] [,4]
[1,]    1    3    0    0
[2,]    2    4    6    0

Answer 1

m <- matrix(1:8, nrow = 2)
sel <- 2:3

#create a integer matrix of col numbers
#use this to create a logical matrix indicating 
#if the col numbers are greater than the threshold
#this relies on vector recycling
subs <- col(m) > sel

#assign to subset
m[subs] <- 0
#     [,1] [,2] [,3] [,4]
#[1,]    1    3    0    0
#[2,]    2    4    6    0