Note that ?all.equal clearly says that it tests for **approximate equality
only** with tolerance "close to 1.5 e-8.
So..
> all.equal(z,pH, tol = 1e-15)
[1] "Mean relative difference: 6.732527e-11"
and
> print(pH, digits =15)
## output omitted
Shows you what's going on.
Cheers,
Bert
Bert Gunter
"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
On Mon, Dec 25, 2017 at 11:00 AM, Troels Ring <tr...@gvdnet.dk> wrote:
> Dear friends - copy paste missed
>
> SID <- c() before the first loop - sorry
>
> BW Troels
>
>
>
> Den 25-12-2017 kl. 19:12 skrev Troels Ring:
>
>>
>> Dear friends - merry Christmas and thanks a lot for much help during the
>> year!
>>
>> In the example below I fail to understand how the calculated value pH is
>> represented in a simple plot - also included. The calculations are useful
>> in practice and likely to be right in principle but I cannot see how this
>> occurs: why a calculated value of 7.4 known as numeric is not simply
>> plotted as such. It happened on Windows both 7 and 10 with R version 3.4.1.
>>
>> All best wishes
>>
>> Troels
>>
>> ff <- function(H,SID,ATOT,ka) H + SID - kw/H - ka*ATOT/(H+ka)
>> ka <- 1e-7
>> kw <- 1e-14
>> ATOT <- seq(0,0.3,length=100)*1e-3
>>
>> for (i in 1:length(ATOT)) {
>> SID[i] <- uniroot(ff,c(-1,1),tol=.Machine$double.eps,maxiter=100000,
>> ka=ka,
>> ATOT=ATOT[i], H = 10^-7.4)$root}
>> ATOT
>>
>> #confirm pH 0 7.4
>>
>> H <- c()
>> for (i in 1:length(ATOT)) {
>> H[i] <- uniroot(ff,c(1e-19,1),tol=.Machine$double.eps,maxiter=100000
>> ,ka=ka,
>> ATOT=ATOT[i], SID = SID[i])$root}
>>
>> (pH <- -log10(H))
>> plot(pH)
>> str(pH)
>> # num [1:100] 7.4 7.4 7.4 7.4 7.4 ...
>> z <- rep(7.4,length(ATOT))
>> all.equal(z,pH)
>> #TRUE
>> points(z,col="red")
>>
>>
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