What is the purpose of using the mod operator in this line of code?

[user366312]

TL;DR

Kindly explain the highlighted lines.

[CODE title="R code" highlight="7,11"]#==================================================================
# Build the transition matrix for random walk on n-cycle
#------------------------------------------------------------------
n <- 25 # we are creating (nXn) transition matrix
r1 <- c(0, 1/2, rep(0,n-3), 1/2) # create same values 22 times...
tmat <- matrix(0, nrow=n, ncol=n) # create a 0-matrix
tmat[1,] <- r1 # inject values in 'r1' into 'tmat' along the 1st row

for (i in 1:(n-1))
{
tmat[26-i,] <- r1[1 + (i:(n+i-1)) %% n] # ? mod operator
}
#=================================================================[/CODE]

What are being done in line#11?

How are elements being selected using mod operator?

 

[FactChecker]

It really helps to print results and intermediate calculations.
I ran it with this code right after the calculation of r1:
[CODE highlight="R code to print r1 values"]
print("r1")
print( r1 )
[/CODE]
And this at the bottom of the loop:
[CODE title="R print code at bottom of loop"]
print("1 + (i:(n+i-1)) %% n")
print(1 + (i:(n+i-1)) %% n)
print("26-i")
print(26-i)
print("tmat")
print(tmat[26-i,])
[/CODE]

The beginning of the print results are:
[CODE title="Start of print results"]
[1] "r1"
[1] 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5
[1] "1 + (i:(n+i-1)) %% n"
[1] 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1
[1] "26-i"
[1] 25
[1] "tmat"
[1] 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0
[1] "1 + (i:(n+i-1)) %% n"
[1] 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1 2
[1] "26-i"
[1] 24
[1] "tmat"
[1] 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.5
[/CODE]