[R] For Loop

[Richard M. Heiberger]

c1 <- 1:1000000
len <- 1000000
system.time(
s1 <- log(c1[-1]/c1[-len])
)
s <- c1[-len]
system.time(
for (i in 1:(len-1)) s[i] <- log(c1[i+1]/c1[i])
)
all.equal(s,s1)


>
> c1 <- 1:1000000
> len <- 1000000
> system.time(
+ s1 <- log(c1[-1]/c1[-len])
+ )
   user  system elapsed
  0.032   0.005   0.037
> s <- c1[-len]
> system.time(
+ for (i in 1:(len-1)) s[i] <- log(c1[i+1]/c1[i])
+ )
   user  system elapsed
  0.226   0.002   0.232
> all.equal(s,s1)
[1] TRUE
>

much faster, and much easier to understand when vectorized

On Sat, Sep 22, 2018 at 5:16 PM, rsherry8 <rsherry8 using comcast.net> wrote:
>
> It is my impression that good R programmers make very little use of the for
> statement. Please consider  the following
> R statement:
>         for( i in 1:(len-1) )  s[i] = log(c1[i+1]/c1[i], base = exp(1) )
> One problem I have found with this statement is that s must exist before the
> statement is run. Can it be written without using a for
> loop? Would that be better?
>
> Thanks,
> Bob
>
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