# C'est la Z

Day 2 of Advent of Code 2019 was also pretty straightforward and once again I wrote my solution in Clojure but in order to talk about this from a teacher's point of view, we'll look at a Python solution.

At its core, this is a simulation problem - read the data into an array or list and write a program to run through the steps. At first I was hoping that the solution would consume the data - that is, once you read past an instruction you don't go back to it. If it was, I was going to try to write some clever reduce. It wasn't. In this problem, any instruction could affect any part of the data set. This means you have to keep the entire data set in memory and preferably in a random access structure like an array.

Step 1 is to read in the data and convert it into a list of integers.

``data  = [ int(x) for x in open("day02.dat").read().split(",")]``

If you don't know comprehensions you can just do something like this:

``````  data = []
data.append(int(item))``````

Now we just have to run the simulation:

``````def run_program(program):
ip = 0
while True:
op = program[ip]
if op == 99:
break
a = program[ program[ip+1] ]
b = program[ program[ip+2] ]
dest = program[ip+3]
if op == 1:
program[dest] =  a + b
else:
program[dest] = a * b
ip = ip + 4
return program

def part1(program):
program=12
program=2
return run_program((program))``````

The most likely source of error here is forgetting to change the values in `program` and `program` in lines 18 and 19 as specified in the question. The other likely error is forgetting to do the two level indirection in lines 7 and 8. That is writing `program[ip+1]` and `program[ip+2]` by mistake. Truth be told, I made both of these mistakes but I didn't find them to be "interesting" mistakes.

The interesting part of this program is part 2 where instead of setting `program` to `12` and `program` to `2` you have to try values between 0 and 99 inclusive for each entry until you find a specific answer.

This is a great platform to talk about memory. The problem refers to the values you put into `program` as nouns and `program` as verbs. The issue is that every time you run your program for a specific noun verb combination you change the array (list) representing your program. If you don't reset the array to its original contents before the next run, the next run will be working off of new values and therefore give you incorrect results.

This is a great platform for discussion. Students usually start with pass by value semantics so passing a list to a function and then having the list change can seem weird even though it can still be pass by value:

``````  # bad example to follow
def change_list(l):
l = 9999

l=[1,2,3,4,5,6,7]
change_list(l)
# changes l to 9999``````

Here's a finished solution:

`````` 1data  = [ int(x) for x in open("day02.dat").read().split(",")]
2
3def run_program(program):
4    ip = 0
5    while True:
6        op = program[ip]
7        if op == 99:
8            break
9        a = program[ program[ip+1] ]
10        b = program[ program[ip+2] ]
11        dest = program[ip+3]
12        if op == 1:
13            program[dest] =  a + b
14        else:
15            program[dest] = a * b
16        ip = ip + 4
17    return program
18
19def part1(data):
20    program = data[:]
21    program=12
22    program=2
23    return run_program((program))
24
25
26def part2(data):
27    results = []
28    for noun in range(100):
29        for verb in range(100):
30            program = data[:]
31            program = noun
32            program = verb
33            r = run_program(program)
34            results.append( (100*noun+verb, r))
35    ans = [ x for x in results if x==19690720]
36    return ans
37
38print(part1(data))
39print(part2(data))``````

Note that in line 30 we copy over the original data for each noun,verb combo. also note the list comprehension in line 35 to find the final answer.

One last potential error point is notice that we had to add the array copy in line 20. Otherwise part1 will change the data and if we run it before part2 we won't get the correct result.

So there it is. Another nice problem and even though it's a straightforward problem, there's still some interesting meat to discuss with your students.

Tomorrow, snowstorm notwithstanding, I'll be heading up to Albany for the remainder of the week so might not be able to get to day 3 and beyond until Friday so who knows if I'll post any more of these write ups.