Creating a vector in a for loop

Question

I am ashamed I need assistance on such a simple task. I want to create 20 normal distributed numbers, add them, and then do this again x times. Then plot a histogram of these sums. This is an exercise in Gilman and Hills text "Data Analysis Using Regression and Multilevel/Hierarchical Models".

I thought this would be simple, but I am into it about 10 hours now. Web searches and looking in "The Art of R Programming" by Norman Matloff and "R for Everyone" by Jared Lander have not helped. I suspect the answer is so simple that no one would suspect a problem. The syntax in R is something that I am having difficulty with.

    > #  chapter 2 exercise 3
    > n.sim <- 10  #  number of simultions
    > 
    > sumNumbers <- rep(NA, n.sim) #  generate vector of NA's
    > for (i in 1:n.sim)  #  begin for loop
     +{
     +     numbers <- rnorm(20,0,1)  
     +     sumNumbers(i) <- sum(numbers) #  defined as a vector bur R 
     +                                   #  thinks it's a function
     + }
 Error in sumNumbers(i) <- sum(numbers) :     
 could not find function "sumNumbers<-"
 > 
 > hist(sumNumbers)    
 Hide Traceback

 Rerun with Debug    
 Error in hist.default(sumNumbers) : 'x' must be numeric     
 3 stop("'x' must be numeric")   
 2 hist.default(sumNumbers)  
 1 hist(sumNumbers) 
 >

You might want to tag with the language you are using in the future. It makes it a lot easier for your question to be seen by the right people. — jpw
– jpw, Commented May 17, 2015 at 0:43

Max Candocia · Accepted Answer · 2015-05-17 00:50:19Z

1

A few things:

When you put parentheses after a variable name, the R interpreter assumes that it's a function. In your case, you want to reference an index of a variable, so it should be sumNumbers[i] <- sum(numbers), which uses square brackets instead. This will solve your problem.
You can initiate sumNumbers as sumNumbers = numeric(n.sim). It's a bit easier to read in simple case like this.
By default, rnorm(n) is the same as rnorm(n,0,1). This can save you some time typing.

answered May 17, 2015 at 0:50

Max Candocia

4,44341 silver badges61 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

josliber · Accepted Answer · 2015-05-17 00:55:04Z

0

You can replicate an operation a specified number of times with the replicate function:

set.seed(144) # For consistent results
(simulations <- replicate(10, sum(rnorm(20))))
#  [1] -9.3535884  1.4321598 -1.7812790 -1.1851263 -1.9325988  2.9652475  2.9559994
#  [8]  0.7164233 -8.1364348 -7.3428464

After simulating the proper number of samples, you can plot with hist(simulations).

answered May 17, 2015 at 0:55

josliber

44.4k12 gold badges103 silver badges136 bronze badges

Collectives™ on Stack Overflow

Creating a vector in a for loop

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related