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return value of {....}

Sorkin, John 
Richard,
I sent my prior email too quickly:

A slight addition to your code shows an important aspect of R, local vs. global variables:

x <- 137
f <- function () {
       a <- x
       x <- 42
       b <- x
       list(a=a, b=b)
   }
 f()
print(x)

When run the program produces the following:
+        a <- x
+        x <- 42
+        b <- x
+        list(a=a, b=b)
+    }
$a
[1] 137

$b
[1] 42
[1] 137

The fist x, a <- x, invokes an x variable that is GLOBAL. It is known both inside and outside the function.
The second x, x <- 42, defines an x that is LOCAL to the function, it is not known to the program that called the function. The LOCAL value of x is used in the expression  b <- x. As can be seen by the print(x) statement, the LOCAL value of x is NOT known by the program that calls the function. The class of a variable, scoping (i.e. local vs. variable) can be a source of subtle programming errors. A general recommendation is to AVOID use of a global variable in a function, i.e. don't use a variable in function that is not passed as a parameter to the function (as was done in the function above in the statment a <- x). If you need to use a variable in a function that is known by the program that calls the function, pass the variable as a argument to the function e.g. 

Use this code:

# Set values needed by function
y <- 2
b <- 30

myfunction <- function(a,b){
cat("a=",a,"b=",b,"\n")
  y <- a
  y2 <- y+b
  cat("y=",y,"y2=",y2,"\n")
}
# Call the function and pass all needed values to the function
myfunction(y,b)
 
Don't use the following code that depends on a global value that is known to the function, but not passed as a parameter to the function:

y <- 2
myNGfunction <- function(a){
  cat("a=",a,"b=",b,"\n")
  y <- a
  y2 <- y+b
  cat("y=",y,"y2=",y2,"\n")
}
# b is a global variable and will be know to the function, 
# but should be passed as a parameter as in example above.
b <- 100
myNGfunction(y)

John
Message-ID: <MW4PR03MB63634102EDFD608D21BCE633E2C19@MW4PR03MB6363.namprd03.prod.outlook.com>
In-Reply-To: <MW4PR03MB636390FD5934FED2BFDD81D9E2C19@MW4PR03MB6363.namprd03.prod.outlook.com>
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