Scratch of the rcg function (random count Gamma).

R/gammacount.R

+## Generate Gamma-Count random variables.
+
+n=1L; shape=1; rate=1
+n=1L; shape=1:4; rate=11:14
+n=4L; shape=1:4; rate=11:12
+size <- c(n=n, sh=length(shape), rt=length(rate))
+
+rgc <- function(n=1L, shape=1, rate=1){
+    rGammaCount <- function(shape=shape, rate=rate){
+        ## To save the sum of Gamma ramdom variables.
+        s <- 0
+        ## To keep each Gamma random number.
+        x <- numeric(0)
+        repeat {
+            ## Generate a Gamma.
+            u <- rgamma(n=1L, shape=shape, rate=rate)
+            ## Update the sum.
+            s <- s+u
+            if (s>=1) break
+            ## Append the new value.
+            x <- append(x, values=u)
+        }
+        return(length(x))
+    }
+    sizes <- c(n, length(shape), length(rate))
+    if (max(sizes[-1])>1L){
+        message(paste(
+            "`n` was ignored because some",
+            "parameter has length > 1."))
+        shapeRate <- cbind(shape=shape, rate=rate)
+        y <- mapply(FUN=rGammaCount,
+                    shape=shapeRate[, "shape"],
+                    rate=shapeRate[, "rate"])
+    } else {
+        y <- replicate(n, rGammaCount(shape=shape, rate=rate))
+    }
+    return(y)
+}
+
+L <- list(
+    y1=rgc(1000, shape=1, rate=1),
+    y2=rgc(1000, shape=5, rate=5),
+    y3=rgc(1000, shape=0.2, rate=0.2))
+lapply(L, FUN=function(x) c(mean(x), var(x)))
+
+##----------------------------------------------------------------------
+
+rgc <- function(n=1L, shape=1, rate=1){
+    ## To save the sum of Gamma ramdom variables.
+    s <- 0
+    ## To keep each Gamma random number.
+    x <- numeric(0)
+    repeat {
+        ## Generate a Gamma.
+        u <- rgamma(n=1L, shape=shape, rate=rate)
+        ## Update the sum.
+        s <- s+u
+        if (s>=n) break
+        ## Append the new value.
+        x <- append(x, values=u)
+    }
+    ## Tabulate (count) occurrences in each interval.
+    y <- tabulate(ceiling(cumsum(x)))
+    ## This is a Gamma Count random variable.
+    return(y)
+}
+
+L <- list(
+    y1=rgc(500, shape=1, rate=1),
+    y2=rgc(500, shape=5, rate=5),
+    y3=rgc(500, shape=0.2, rate=0.2))
+
+lapply(L, FUN=function(x) c(mean(x), var(x)))