Question

I am working on a loop coding on R language, and this code involves two functions. If value is radius, period function should be run instead of radius function, and so on.

I coded using my notes from class, but I think it is wrong. I do not get any warnings, but if I do on Console:

R<-98
orbit(R)

I will get this message:

Error in orbit(R) : attempt to apply non-function

This is my function code:

# Two functions: period and radius
# If a value you input is a period (in minutes), radius function should be used (radius(R))
# If a value you input is a radius (in km), period function should be used (period(R))
#   R is radius in km or period in minutes


orbit <- function(R){
    G <-6.673*10^-11
    M <- 5.972*10^24 # in kg

    if(R == 98){
            omega <- 2*pi/R # pr is period for one rotation

            Radi <- (G*M/omega^3)(1/3)
            print(Radi)
        }
    else {

        Peri <- 2*pi*sqrt(R^3/G*M)
        print(Peri)

    }

}

I don’t think I fully understand if and else statement. Does anyone explain it for me? Also what is difference between for statement and if statement?

Thank you for all help.

Answer 1

The problem in this line

Radi <- (G*M/omega^3)*(1/3)

where you missing * operation

if/else statement allows your program to decide which code to execute based on some condition. Like in your code, you have two blocks of code the first one is:

omega <- 2*pi/R # pr is period for one rotation
Radi <- (G*M/omega^3)(1/3)
print(Radi)

which you want to execute only if some condition is true i.e. R == 98, otherwise you execute the other block of code.

for statement is used when you want to repeatedly execute a block of code many times. Let’s say you want to print the numbers from 1-100, it is not feasible to write print(1) print(2) … 100 times!

You do this with a simple for loop, e.g.

for(i in 1:100){
   print(i)
}