Nested "for" loops in R - r

Question

I'm an R novice with what I hope is a simple question. I've got a couple of nested loops I'm running and can't seem to get the output that I'm expecting.
I want to keep track of the balance in the internal loop. Each iteration represents 1 year. At the end of 15 years, I want to write the final balance to InvTotal for each of 4 simulations.
My final output should be 3 vectors (1 for each investment) of length 4, showing the final cumulative value for each investment at the end of 4 simulations.
My code is below. Any assistance you could provide would be a huge help.
Thank you!
Investment1_Balance <- 10000
Investment2_Balance <- 10000
Investment3_Balance <- 10000
Inv1Returns <- c(0, 1000, -500, 500)
Inv2Returns <- c(0, -9000, 30000, 10000)
Inv3Returns <- c(0, 4000, -1000, -2000)
Inv1Outcome = NULL
Inv2Outcome = NULL
Inv3Outcome = NULL
Inv1Total = NULL
Inv2Total = NULL
Inv3Total = NULL
random = NULL
for (j in 1:4)
{
for (i in 1:15 )
{
random[i] = runif(1, 0, 1)
Inv1Outcome[i] = if (random[i] <= .25){Investment1_Balance + Inv1Returns[1]}
else if (random[i] > .25 & random[i] <= .50){Investment1_Balance + Inv1Returns[2]}
else if (random[i] > .50 & random[i] <= .75){Investment1_Balance + Inv1Returns[3]}
else {Investment1_Balance + Inv1Returns[4]}
Inv2Outcome[i] = if (random[i] <= .20){Investment2_Balance + Inv2Returns[1]}
else if (random[i] > .20 & random[i] <= .30){Investment2_Balance + Inv2Returns[2]}
else if (random[i] > .30 & random[i] <= .70){Investment2_Balance + Inv2Returns[3]}
else ({Investment2_Balance + Inv2Returns[4]})
Inv3Outcome[i] = if (random[i] <= .50){Investment3_Balance + Inv3Returns[1]}
else if (random[i] > .50 & random[i] <= .70){Investment3_Balance + Inv3Returns[2]}
else if (random[i] > .70 & random[i] <= .90){Investment3_Balance + Inv3Returns[3]}
else ({Investment3_Balance + Inv3Returns[4]})
Investment1_Balance[i] =+ Inv1Outcome[i]
Investment2_Balance[i] =+ Inv2Outcome[i]
Investment3_Balance[i] =+ Inv3Outcome[i]
}
Inv1Total[j] = Investment1_Balance[15]
Inv2Total[j] = Investment2_Balance[15]
Inv3Total[j] = Investment3_Balance[15]
}
Inv1Total
Inv2Total
Inv3Total

Answer 1

Welcome to stackoverflow! There are a couple of points that you should consider while trying to figure out why your code isn't working as expected:
"Start small" : In your case here, I would start with one investment, maybe even with one simulation. This would verify if the logic of the calculation is fine, without distracting myself with other numbers.
"Seeing is believing" : Try to print intermediate results, so you can trace the calculation bugs.
Applying the previous, a first debugging would would look something like:
Investment1_Balance <- 10000
Inv1Returns <- c(0, 1000, -500, 500)
Inv1Outcome = NULL
Inv1Total = NULL
random = NULL
for (j in 1:4) { # Loop over simulations
cat("\n\n\nSimulation ", j, ": Investment balance:" , Investment1_Balance)
for (i in 1:15) { # Loop over years
random[i] = runif(1, 0, 1)
Inv1Outcome[i] = if (random[i] <= .25){Investment1_Balance + Inv1Returns[1]}
else if (random[i] > .25 & random[i] <= .50){Investment1_Balance + Inv1Returns[2]}
else if (random[i] > .50 & random[i] <= .75){Investment1_Balance + Inv1Returns[3]}
else {Investment1_Balance + Inv1Returns[4]}
Investment1_Balance[i] =+ Inv1Outcome[i]
cat("\n Year: ", i, "- Outcome:", Inv1Outcome[i], "- Final balance: " , Investment1_Balance[i])
}
Inv1Total[j] = Investment1_Balance[15]
}
Inv1Total
.
You can see a couple of problems with the numbers printed :
The outcome is always the same as the balance. This suggests that there might be a problem with the addition.
Let's try to debug that:
> x = 10
> x =+ 2
> x
[1] 2
Apparently, the =+ isn't working as expected. So we have to correct that : Investment1_Balance[i+1] = Investment1_Balance[i] + Inv1Outcome[i]
.
When printing the initial investment balance for each simulation, we can see that the first simulation is fine (Simulation 1 : Investment balance: 10000). But for the other simulations we get (Simulation 2 : Investment balance: 10000 9500 10000 10000 10500 11000 9500 11000 10000 10500 10500 10000 10000 11000 11000).
This suggests a problem with the initialization. Each simulation takes the output of the previous and starts with it. The logic of that is not quite correct. The simple solution would be resetting Investment1_Balance <- 10000 at the beginning of each simulation. It would even make sense in this case to reset all the Balance and Outcome variables.
.
Here is the new code after debugging :
Inv1Returns <- c(0, 1000, -500, 500)
random = NULL
Inv1Total = NULL
for (j in 1:4) { # Loop over simulations
Investment1_Balance <- 10000
Inv1Outcome = NULL
cat("\n\n\nSimulation ", j, ": Investment balance:" , Investment1_Balance)
for (i in 1:15) { # Loop over years
random[i] = runif(1, 0, 1)
Inv1Outcome[i] = if (random[i] <= .25){Investment1_Balance + Inv1Returns[1]}
else if (random[i] > .25 & random[i] <= .50){Investment1_Balance + Inv1Returns[2]}
else if (random[i] > .50 & random[i] <= .75){Investment1_Balance + Inv1Returns[3]}
else {Investment1_Balance + Inv1Returns[4]}
Investment1_Balance[i+1] = Investment1_Balance[i] + Inv1Outcome[i]
cat("\n Year: ", i, "- Outcome:", Inv1Outcome[i], "- Final balance: " , Investment1_Balance[i])
}
Inv1Total[j] = Investment1_Balance[15]
}
Inv1Total
Check if the output is now correct. If so, you can go ahead and add Inv2 and Inv3 as well.