[R] Confused by code?

[Bazman76]

x<-matrix(c(1,0,0,0,1,0,0,0,1),nrow=3)
> y<-matrix(c(0,0,0,1,0,0,1,1,0),nrow=3)
> z<-matrix(c(0,1,0,0,1,0,1,0,0),nrow=3)
> x[z]<-y[z]

The resultant matrix x is all zeros except for the last two diagonal cells
which are 1's.
While y is lower triangualr 0's with the remaining cells all ones.

I really don't understand how this deceptively simple looking piece of code
is giving that result can someone explain please.
I'm obviously missing something pretty basic so please keep your answer
suitably basic.



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