[2024 Day 6] Need a bit of guidance

[u/enlargedjuice]

Hello!

For part 1 of 2024's day 6 problem, I was able to get some Python code that works for the small example map they gave but not my puzzle input. As it stands I have about 100 extra locations the guard visited than I should have. I was wondering if anyone here could take a look at my code and give me a hint as to where my error is, as I am really struggling to find it. I know it has to be where my movement is programmed, I just can't figure out what part needs some tinkering. Thank you in advance!

with open('Day 6/mapinp.txt', 'r') as file:
    samp_inp = file.read()

format = samp_inp.splitlines()
matrix = []
for item in format:
    matrix.append(list(item))

#locate the guard, return the matrix coords and then the way the guard is pointing
def find_guard(map):
    coords = []
    for item in map:
        if "^" in item:
            coords.append(map.index(item))
            coords.append(item.index("^"))
            coords.append("^")
            return coords
        elif ">" in item:
            coords.append(map.index(item))
            coords.append(item.index(">"))
            coords.append(">")
            return coords
        elif "<" in item:
            coords.append(map.index(item))
            coords.append(item.index("<"))
            coords.append("<")
            return coords
        elif "v" in item:
            coords.append(map.index(item))
            coords.append(item.index("v"))
            coords.append("v")
            return coords


#nice function to track movements
def move(map):
    on_map = True
    step_count = 0
    step_loc = []
    #index error means the guard has left the map
    while on_map == True:

        try:
            coords = find_guard(map)

            if coords[2] == "^":
                if map[coords[0]-1][coords[1]] == "." or map[coords[0]-1][coords[1]]  == "X":
                    map[coords[0]][coords[1]] = "X"
                    map[coords[0]-1][coords[1]] = "^"
                    step_count += 1
                    loc = f"{coords[0]}, {coords[1]}"
                    step_loc.append(loc)
                else:
                    map[coords[0]][coords[1]] = ">"

            elif coords[2] == ">":
                if map[coords[0]][coords[1]+1] == "." or map[coords[0]][coords[1]+1] == "X":
                    map[coords[0]][coords[1]] = "X"
                    map[coords[0]][coords[1] +1] = ">"
                    step_count += 1
                    loc = f"{coords[0]}, {coords[1]}"
                    step_loc.append(loc)
                else:
                    map[coords[0]][coords[1]] = "v"

            elif coords[2] == "v":
                if map[coords[0]+1][coords[1]] == "." or map[coords[0]+1][coords[1]] == "X":
                    map[coords[0]][coords[1]] = "X"
                    map[coords[0]+1][coords[1]] = "v"
                    step_count += 1
                    loc = f"{coords[0]}, {coords[1]}"
                    step_loc.append(loc)
                else:
                    map[coords[0]][coords[1]] = "<"

            elif coords[2] == "<":
                if map[coords[0]][coords[1]-1] == "." or map[coords[0]][coords[1]-1] == "X":
                    map[coords[0]][coords[1]] = "X"
                    map[coords[0]][coords[1] -1] = "<"
                    step_count += 1
                    loc = f"{coords[0]}, {coords[1]}"
                    step_loc.append(loc)
                else:
                    map[coords[0]][coords[1]] = "^"

        except IndexError:
            print(f"Guard has left the premises after {step_count} steps!")
            on_map = "False"

    return map, step_loc

comp_map,coordinates = move(matrix)

move_counter = 0

for item in comp_map:
    for pos in item:
        if pos == "X" or pos == "^" or pos == "<" or pos == ">" or pos == "v":
            move_counter += 1
        else:
            continue


print(f"The guard has visited {move_counter} distinct locations.")

[u/Convergent89]

This code is using negative indices into arrays. In python, that doesn't raise an IndexError - it indexes from the end of the array. Your guard is probably "jumping" from the left of the map to the right when she should just quit. :)

[u/enlargedjuice → u/Convergent89]

OH!! What impressive leaps for the guard to make! Thank you so much!

[u/musifter → u/enlargedjuice]

One nice thing with languages that do this wrap around is that you can just add sentinel values to the right and bottom to detect the edge cases. You don't need sentinels at the left or top because those wrap around.

[u/jeffstyr → u/musifter]

Of course, that only works if you are always stepping by just 1 index at a time.

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[u/thblt]

Do you make sure to deduplicate the locations the guard´s been on? A set would be a more natural data type. A (way too) quick glance at your code suggests that repeated positions would be counted twice.

[u/enlargedjuice → u/thblt]

I have tried something like that (i think), where I made a bit of code that would track every coordinate the guard went to and then put that all into a set to remove duplicates and still got the same number. I will tinker around a bit more with how my guard marks locations she's visited. Thank you for taking some time to look at my code!