How can I create this exact 5x5 matrix using only for loops?-CodePudding

So I was given this matrix and was told to create it using only for loops. What i have done so far is using cbind(0:4,1 (0:4),2 (0:4),3 (0:4),4 (0:4)) but i cant figure out a way to do so with the for function.

CodePudding user response：

You were on the right track. If you rewrite your current

cbind(0:4,1 (0:4),2 (0:4),3 (0:4),4 (0:4))

cbind(0 (0:4),1 (0:4),2 (0:4),3 (0:4),4 (0:4))

you might notice that the thing that you are adding to 0:4 is implicitly a loop index.

Make it explicit:

m = c()
for(i in 0:4){
  m = cbind(m,i (0:4))
} 
print(m)

Output:

     [,1] [,2] [,3] [,4] [,5]
[1,]    0    1    2    3    4
[2,]    1    2    3    4    5
[3,]    2    3    4    5    6
[4,]    3    4    5    6    7
[5,]    4    5    6    7    8

CodePudding user response：

One way:

mat <- matrix(0L, nrow=5, ncol=5) 
for (i in 0:4) {
  for (j in 0:4) {
    mat[i   1, j   1] <- i   j
  }
}
#      [,1] [,2] [,3] [,4] [,5]
# [1,]    0    1    2    3    4
# [2,]    1    2    3    4    5
# [3,]    2    3    4    5    6
# [4,]    3    4    5    6    7
# [5,]    4    5    6    7    8

And technically *apply functions are loops as well:

sapply(0:4, \(x) 0:4   x)

CodePudding user response：

You can just create an empty matrix first and then fill it with two for-loops iterating over rows and columns. Playing a little bit around with the variable to write into the matrix (count) I figured out that this is a suitable solution.

matrix2fill <- matrix(NA, 5,5)
count = 0
for (i in 1:5){
  for (j in 1:5){
    matrix2fill[j,i] = count
    count = count   1
  }
  count = i
}

matrix2fill
     [,1] [,2] [,3] [,4] [,5]
[1,]    0    1    2    3    4
[2,]    1    2    3    4    5
[3,]    2    3    4    5    6
[4,]    3    4    5    6    7
[5,]    4    5    6    7    8

CodePudding user response：

Try this:

a = matrix(1:25, nrow=5, ncol=5) 
for (i in 1:5) {
   for (j in 1:5) {
      a[i][j] = (i-1)   (j-1)
   }
}
print(a)

CodePudding user response：

Yet another way:

mymat <- matrix(NA, nrow = 5, ncol = 5)
i_mat <- 1
for (i in 0:4) {
  mymat[seq(i_mat, i_mat   4)] <- seq(i, i   4)
  i_mat <- i_mat   5
}
mymat

     [,1] [,2] [,3] [,4] [,5]
[1,]    0    1    2    3    4
[2,]    1    2    3    4    5
[3,]    2    3    4    5    6
[4,]    3    4    5    6    7
[5,]    4    5    6    7    8