If statement R missing value - r

Question

I was trying to execute this functions, but I kept on getting an error with my if statment: Error in if (value[1][i] < 0) { : missing value where TRUE/FALSE needed:
Monte_Carlo <- function(trial)
{
S_T <- S_o*exp((r - q - (1/2)*sigma^2)*period + sigma*rnorm(trial, mean = 0, sd = 1))
K <- matrix(100, nrow = 1, ncol = 20)
value <- K - S_T
for(i in 1:trial)
{
if(value[1][i] < 0)
{
value[1][i] = 0;
}
}
return (mean(value)*exp(-r))
}

Answer 1

You are indexing your matrix incorrectly. value[1] will return a single value, which you are then trying to assign the ith element of, for i up to trial
If you assign to the ith element in the 1st row (which looks as if you are trying to do), then it will work
Monte_Carlo <- function(trial)
{
S_T <- S_o*exp((r - q - (1/2)*sigma^2)*period + sigma*rnorm(trial, mean = 0, sd = 1))
K <- matrix(100, nrow = 1, ncol = 20)
value <- K - S_T
for(i in 1:trial)
{
if(value[1, i] < 0)
{
value[1,i] = 0;
}
}
return (mean(value)*exp(-r))
}
you can remove the for loop and if statement with pmax which is vectorized and will return elementwise the maximum of value and 0.
Monte_Carlo <- function(trial)
{
S_T <- S_o*exp((r - q - (1/2)*sigma^2)*period + sigma*rnorm(trial, mean = 0, sd = 1))
K <- matrix(100, nrow = 1, ncol = trial)
value <- K - S_T
.value <- pmax(value,0)
return (mean(.value)*exp(-r)
}
As a matter of good programming, I would add r,S_o, q, sigma and period as arguments to your function, so that it doesn't depend on global variables