# [R] how to see what's wrong with a self written function?

casperyc casperyc at hotmail.co.uk
Tue Dec 21 20:39:31 CET 2010

```Hi all,

I am writing a simple function to implement regularfalsi (secant) method.

###################################################
regulafalsi=function(f,x0,x1){
x=c()
x=x1
i=1
while ( f(x[i])!=0 ) {
i=i+1
if (i==2) {
x=x-f(x)*(x-x0)/(f(x)-f(x0))
} else {
x[i]=x[i-1]-f(x[i-1])*(x[i-1]-x[i-2])/(f(x[i-1])-f(x[i-2]))
}
}
x[i]
}
###################################################

These work fine,
regulafalsi(function(x) x^(1/2)+3*log(x)-5,1,10)
regulafalsi(function(x) x^(1/2)+3*log(x)-5,10,1)

For all x>0, the function is strictly increasing.

Then

regulafalsi(function(x) x^(1/2)+3*log(x)-5,1,100)

Error in while (f(x[i]) != 0) { : missing value where TRUE/FALSE needed
In log(x) : NaNs produced

I dont know what happened there, is there a way to find the value for
f(x[i])
that R can't determine TRUE/FALSE?

Thanks!

casper
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```