CODEWITHSUNDEEP
rfor-loopif-statementiteration
Jessm
Jessm

Reputation: 21

Use values from one vector to change values in another vector in R (for loop)

I need to change certain values in one vector depending on what the values are in the same places on in another vector. Below are my vectors:

r <- (1:20)
a <- c(54,54,54,54,55,55,50,0,0,0,0,0,0,1,1,1,1,1,56,57)

Basically if any of the values in 'a' are greater than or equal to and less than 20 (so any value in a that is 0-20) I want to change that value in 'r' to be itself -1. If the value in 'a' is greater than 20 or less than 0, then I want its value in 'r' to remain the same. So for the 8th spot in 'a' the value is 0 which is greater than/equal to 0 and less than 20 so I want the 8th spot in 'r' (has a value of 8) subtract by 1 (so now its value will be 7). But for the first spot in 'a', the value is 54 which is greater than 20 so the 1st value in r will remain the same. I assumed I needed to write a for loop for this and I started to but it's not doing what I need it to do. This is what I have so far.

for(i in a){
  if (i >= 0 && i < 20){
    r[i] = r[i]-1
  } else {
    r[i] = r[i]
  }
}

When I run this code it returns r as 

[1] -4  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 NA NA NA NA NA NA NA NA NA NA NA
[32] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

How can I get it to return the correct result which should look like this:

[1] 1 2 3 4 5 6 7 7 8 9 10 11 12 13 14 15 16 17 19 20

Thank you!

Upvotes: 1

Views: 1103

Answers (4)

lroha
lroha

Reputation: 34441

You can do:

r - (a >= 0 & a < 20) 

[1]  1  2  3  4  5  6  7  7  8  9 10 11 12 13 14 15 16 17 19 20

Upvotes: 1

ThomasIsCoding
ThomasIsCoding

Reputation: 101403

I guess a compact base R solution is to use ifelse

ifelse(a>=0 & a< 20,r-1,r)

which gives

> ifelse(a>=0 & a< 20,r-1,r)
 [1]  1  2  3  4  5  6  7  7  8  9 10 11 12 13 14 15 16 17 19 20

Upvotes: 0

MarcW
MarcW

Reputation: 118

Maybe this would help,

r <- (1:20)
a <- c(54,54,54,54,55,55,50,0,0,0,0,0,0,1,1,1,1,1,56,57)

r[a >= 0 & a < 20] <- r[a >= 0 & a < 20] - 1

# [1]  1  2  3  4  5  6  7  7  8  9 10 11 12 13 14 15 16 17 19 20

You don't need a loop here the above answer is the most efficient.

Upvotes: 3

akrun
akrun

Reputation: 887148

We can loop through the sequence 

for(i in seq_along(a)) if(a[i] >=0 && a[i] < 20) r[i] <- r[i] -1 
r
#[1]  1  2  3  4  5  6  7  7  8  9 10 11 12 13 14 15 16 17 19 20

instead of the values of 'a' because r[54] doesn't exist and assigning on that element results in NA

Upvotes: 1

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