Advent Of Code Day 2

Second day of Advent of Code. This time our computer burst into flames and we have to reset it. I’m not sure how the code is meant to work if the computer just got burnt…

R has lots of different ways of doing the same thing: there are a lot of ways to read in a file and I’m not really sure what the differences are. There is read.csv and read.csv2. Why have both? I used scan and that gave me the data as a list, rather than a table which was better for this problem.

I also had to remember that R indices start at 1 rather than 0, which meant I had to remember to offset each of the indices.

My first solution was 466644 which was too low. I tried some of the examples on the problem and they all worked fine. Then I read the last paragraph of the problem - and found out I had to change the input file. And then it worked.

# I changed the actual file with the correct input
op_codes <- scan('day2.txt', sep=',')

i <- 1
while(i <= length(op_codes))
{
  # Adds
  if(op_codes[i]==1)
  {
    op_codes[op_codes[i+3]+1] <- op_codes[op_codes[i+1]+1] + op_codes[op_codes[i+2]+1]
  }
  else if (op_codes[i]==2)
  {
    op_codes[op_codes[i+3]+1] <- op_codes[op_codes[i+1]+1] * op_codes[op_codes[i+2]+1]
  }
  else if (op_codes[i]==99)
  {
    print('Got to 99, broke')
    break
  }
  else
  {
    print("Didn't meet conditions, investigate")
  }
  print(i)
  print(length(op_codes))
  i <- i + 4
}
op_codes[1]

For part 2, I couldn’t see of a good way other than brute forcing all options. I was tempted to try a solution in an alebraic system, but there is a good chance that the solution would be a not so nice polynomial.

This one was straight forward. The only thing I found strange was that sprintf doesn’t actually print to the screen, I still needed to wrap print around it. This old answer on the r-help mail list explained it.

run_ops_codes <- function(noun, verb)
{
  op_codes <- scan('day2.txt', sep=',', quiet=TRUE)
  op_codes[2] <- noun
  op_codes[3] <- verb
  
  i <- 1
  while(i <= length(op_codes))
  {
    # Adds
    if(op_codes[i]==1)
    {
      op_codes[op_codes[i+3]+1] <- op_codes[op_codes[i+1]+1] + op_codes[op_codes[i+2]+1]
    }
    else if (op_codes[i]==2)
    {
      op_codes[op_codes[i+3]+1] <- op_codes[op_codes[i+1]+1] * op_codes[op_codes[i+2]+1]
    }
    else if (op_codes[i]==99)
    {
      break
    }
    else
    {
      print("Didn't meet conditions, investigate")
    }
    i <- i + 4
  }
  op_codes
}

target_value <- 19690720
found=FALSE
for(noun in 0:99)
{
  for(verb in 0:99)
  {
    result <- run_ops_codes(noun, verb)
    if(target_value == result[1])
    {
      print(sprintf('Found target, %s, %s', noun, verb))
      found=TRUE
      break
    }
  }
  if(found)
  {
    break
  }
  
}
print(100*noun + verb)?