Jeff - 1 year ago 156

R Question

Using R I have data in a matrix with three columns, and I would like to replace the value in the first two columns conditional on the value in the third. If the third column is '1', I want to replace the values in the first and second columns both with 0.5 (the third column is irrelevant, I will be dropping it later), otherwise, I want to leave them as they are.

`> MAT`

a b c

t4 0 1 0

t5 1 0 0

t6 0 1 0

t7 0 1 0

t8 0 1 0

t3 0 1 0

t1 1 0 0

t2 0 1 0

t9 0 0 1

t10 0 0 1

I can do this manually:

`> MAT["t9",1:3]=c(.5,.5,0)`

> MAT["t10",1:3]=c(.5,.5,0)

> MAT

a b c

t4 0.0 1.0 0

t5 1.0 0.0 0

t6 0.0 1.0 0

t7 0.0 1.0 0

t8 0.0 1.0 0

t3 0.0 1.0 0

t1 1.0 0.0 0

t2 0.0 1.0 0

t9 0.5 0.5 0

t10 0.5 0.5 0

but I have about 20 sets of data and for some of them I will have to replace about 100 rows. I was wondering whether there is code I can use to automate it. I've been trying ifelse() but I don't understand the results:

`>ifelse(MAT[,3]==1,MAT[,1:3]==c(0.5,0.5,0),MAT[,3]==0)`

t4 t5 t6 t7 t8 t3 t1 t2 t9 t10

TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE

> ifelse(MAT[,3]==1,MAT[,1:3]==c(0.5,0.5,0),ifelse(MAT[,2]==1,MAT[,2]==1,MAT[,1]==1))

t4 t5 t6 t7 t8 t3 t1 t2 t9 t10

TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE FALSE FALSE

> ifelse(MAT[,3]==1,c(0.5,0.5,0),MAT)

t4 t5 t6 t7 t8 t3 t1 t2 t9 t10

0.0 1.0 0.0 0.0 0.0 0.0 1.0 0.0 0.5 0.5

> ifelse(MAT[,1:3]==c(0,0,1),MAT[,1:3]==c(0.5,0.5,0),MAT)

a b c

t4 0.0 1.0 0

t5 1.0 0.0 0

t6 0.0 1.0 0

t7 0.0 1.0 0

t8 0.0 0.0 0

t3 0.0 1.0 0

t1 1.0 0.0 0

t2 0.0 0.0 0

t9 0.5 0.5 0

t10 0.5 0.5 0

I thought the first would would work by checking for whether the 3rd column = 1, and if so replacing the row with 0.5, 0.5, 0.0, and if not leaving it at 0. I thought the second was clunkier but might accomplish the same thing. The last successfully made the changes, but also eliminated other data that I wanted to keep. I'm guessing at this point that ifelse is not the way to go about this, but I'm not sure how else to do it.

Thanks in advance.

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Answer Source

Are you happy with the following?

```
a<-c(0,1,0,0,0,0,1,0,0,0)
b<-c(1,0,1,1,1,1,0,1,0,0)
c<-c(0,0,0,0,0,0,0,0,1,1)
MAT<-data.frame(a,b,c)
colnames(data)<-c("a","b","c")
MAT$a[data$c == 1] <- 0.5
MAT$b[data$c == 1] <- 0.5
MAT<-data[,1:2]
```

Or you can even do:

```
a<-c(0,1,0,0,0,0,1,0,0,0)
b<-c(1,0,1,1,1,1,0,1,0,0)
c<-c(0,0,0,0,0,0,0,0,1,1)
MAT<-cbind(a,b,c)
for(i in 1:2){
MAT[,i][MAT[,3] == 1] <- 0.5
}
MAT<-MAT[,1:2]
```

Notice: 3 column is only used for the logical test, its values are unchanged.

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