# tests/testthat/pli-R-v0.0.3-2007-07-25/powerexp-exponential-integral.R In spatialwarnings: Spatial Early Warning Signals of Ecosystem Degradation

```# Calculate the exponential integral function, for use with the continuous
# powerexp distribution
# Code stolen from numerical recipes and omitted from public distribution

# The procedure is based on that given in Numerical Recipes, sec. 6.3
# This includes the second (n) argument
# We only ever use the n=1 case here (hence the default argument), but may as
# well be completist
# However (BAD!) this omits ALL sanity-checks on the arguments
# In particular x must be STRICTLY positive

# Compute the exponential integral _once_, using a numerical recipe
# Input: Real-valued argument (x), integer-valued order (n)
# Output: Real value
exp_int_once.nr <- function(x, n=1) {
maximum_iterations <- 100
EULER <- 0.5772156649
FPMIN <- 1.0e-30
EPS <- 1.0e-7
n <- 1
n_minus_one <- n-1   # i.e., zero, but let's preserve the general case
if (x > 1.0) {
b <- x+n
c <- 1.0/FPMIN
d <- 1.0/b
h <- d
for (i in 1:maximum_iterations) {
a <- -i*(n_minus_one+i)
b <- b+2.0
d <- 1.0/(a*d+b)
c <- b+a/c
del <- c*d
h <- h*del
if(abs(del-1.0) < EPS) {
return(h*exp(-x))
}
}
} else {
if (n_minus_one != 0) {
ans <- 1.0/n_minus_one
} else {
ans <- -log(x) - EULER
}
fact <- 1.0
for (i in 1:maximum_iterations) {
fact <- fact * (-x/i)
if (i != n_minus_one) {
del <- -fact/(i-n_minus_one)
} else {
psi <- -EULER + sum(1/(1:n_minus_one))
# Original code follows in ####
#### psi <- -EULER
#### for (j in 1:n_minus_one) {
####  psi <- psi + 1.0/j
#### }
del <- fact*(-log(x) + psi)
}
ans <- ans + del
if (abs(del) < abs(ans)*EPS) {
return(ans)
}
}
}
return(NA) # i.e., something screwed up
}
```

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spatialwarnings documentation built on May 2, 2019, 5:16 p.m.