# [R] to modify a matrix : Summary

Mitsuo Igarashi mitsu5 at ruby.famille.ne.jp
Sun Feb 9 09:08:02 CET 2003

```I truly thank for evrybody to give me many very

I like to summarize the replies with their results
when excuted on R and with my comments.

My question is
> x <- matrix(1:10.,5)
> x
[,1] [,2]
[1,]    1    6
[2,]    2    7
[3,]    3    8
[4,]    4    9
[5,]    5   10
there is a matrix. On condition x[? ,1]=3, how to modify the
appropriate location to be x[? ,2] * 5.

SUMMARY
(1) x[,2] <- x[,2]*(1+4*(x[,1]==3))
good result, a little bit tricky.

(2) x[,2] <- ifelse(x[,1]==3, x[,2]*5, x[,2])
good result, easy to be understood.

(3) for(i in 1:5) {if (x[,1]==3) {x[,2] <- 5*x[,2]}}
unable to get any modification, theoretically easily undrstandable
from a programmer.

(4) rowstochange <- x[,1]==3
x[rowstochange,2] <- x[rowstochange,2]*5
good result, to give many suggetions.

When I excute "rowstochange <- x[,1]==3", I have
> rowstochange
[1] FALSE FALSE  TRUE FALSE FALSE  .

This is my first time to realize the meaning of the
expression of "x[,1]==3". This short sentence has inclusively
the way of "for", "repeat", and/or  "if" clauses.
This is the wonderful part of R and the difficult part.

(5) x[x[,1]==3,2] <- x[x[,1]==3,2]*5
good result, simple, direct and straight.

My Best Gegards to everyone concerned.

-------========--------
mitsu5
mitsu5 at ruby.famille.ne.jp

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