R: Simple Function with For Loop

Question

I have an elementary question that I sadly cannot figure out. I have a set of numeric vector of 1s and 0s that are stored in the return variable below and whose sums are stored in the totals variable. I would like to check each of these individual vectors to see if there were consecutive zeroes in the result, and then return the total number of times this occurred. However, I'm quite rusty and/or bad at for loops/functions and cannot get this result. My latest attempt is below. Any suggestions are welcome - appreciate the help.

set.seed(1)
return = ifelse(runif(10) <= 0.6, 1, 0)
totals = sapply(1:10, function (x) sum(ifelse(runif(10)<=0.6,1,0)))

sums = function (x) { 
  g = 0
  for (i in 1:length(x)-1) {
    sum(ifelse (x[i]+x[i+1]=0,1,0)) 
  }
  return (g)
}

Florian · Accepted Answer · 2017-07-23 15:43:26Z

1

Although this is not the most efficient way to do so (see akrun's answer), we can get your for loop to work:

sums=function (x)
{ 
  g=0
  # watch your brackets! 1:3-1 returns c(0,1,2), not c(1,2)!
  for (i in 1:length(x)-1)  
  {
    # To test for equality, use a double ==, rather than a single.
    # also, your 'g' variable is not updated, which is what you want to do.
    sum(ifelse (x[i]+x[i+1]=0,1,0)) 
  }
  return (g) 
}

Corrected:

sums <-function(x)
{ 
  g=0
  for (i in 1:(length(x)-1)) 
    {
    g= g+ifelse(x[i]+x[i+1]==0,1,0)
    }
  return (g) 
}

You can call your function by:

return=ifelse(runif(10)<=0.6,1,0)
sums(return)

Or to generate ten vectors with random 1's and 0's, and apply your function to them, you could do:

totals = lapply(1:10, function (x) ifelse(runif(10) <= 0.6, 1, 0))
sapply(totals,sums)

Hope this helps!

edited Jul 23, 2017 at 15:43

answered Jul 23, 2017 at 15:28

Florian

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3 Comments

Z_D Over a year ago

Thank you. Those are sloppy mistakes - appreciate your help. And while this seems even less efficient, would you recommend making another for loop to then check the number of times this happens in each of the ten runs stored in the 'totals' variable?

Florian Over a year ago

No, in general there is often a better way than a for loop in R. I modified my answer to show how you can call your function to ten random vectors. With the lapply statement we create ten vectors, en with sapply we apply your function to these vectors.

Z_D Over a year ago

Thanks, understood

akrun · Accepted Answer · 2017-07-23 16:10:54Z

1

If we are looking for the number of times consecutive 0's occur (i.e. greater than 1) and its length, then use rle

with(rle(return), lengths[values==0 & lengths > 1])
#[1] 4

The return vector is

return
#[1] 1 1 1 0 1 0 0 0 0 1

Now, we can see the 4 consecutive number of 0's. Just to show that the answer matches the initial vector

A for loop (incorrect answer just for the sake of answering)

sums <- function (x) { 
  g  <- 0
   for (i in tail(seq_along(x), -1)) {
    if(x[i-1]==0 & x[i]==0) {
    g <- g+1
  }
 }
  g
}

sums(return)

edited Jul 23, 2017 at 16:10

answered Jul 23, 2017 at 15:20

akrun

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