python 2d dict

Author: mstathers

_posts/2025-12-04-2d-array-python-dict.md

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+---
+layout: posts
+title: Using a Python Dictionary to Represent a 2D Grid
+tags: python TIL
+---
+
+It's <a href="https://adventofcode.com/">Advent of Code</a> time! Problems featuring a two dimensional grid are not uncommon, so developing a strategy to deal with this is useful.
+
+In the past I would use a pair of nested arrays to represent this grid. This was a bit messy, you would have to reference grid coordinates "backwards" as `Y, X`, and it would require all sorts of additional out-of-bounds checking.
+
+An improvement is to use a dictionary where the key is a tuple of "X,Y" coordinates.
+
+### Sample Input
+Just a simple 3x3 grid of sequential whole numbers. I'm not going to actually bother to convert these to integers for our example, because the value actually doesn't matter for this strategy.
+{% highlight python %}
+>>> print(input)
+012
+345
+678
+{% endhighlight %}
+
+### Populating the Dictionary
+{% highlight python %}
+grid = {}
+for y, line in enumerate(input.split('\n')):
+    for x, val in enumerate(line):
+        grid[(x, y)] = val
+{% endhighlight %}
+
+### Accessing the Data
+Use a tuple for the dictionary key. `x=1, y=2`
+{% highlight python %}
+>>> grid[(1, 2)]
+'7'
+{% endhighlight %}
+
+### Searching the Grid
+Dictionary comprehension can be used to search the grid for keys that hold the desired value.
+{% highlight python %}
+>>> [key for key, val in grid.items() if val == '7']
+[(1, 2)]
+{% endhighlight %}
+
+### Finding Neighbors
+In the Advent of Code problems, finding the contents of neighboring locations is also very common. This function will return a list of all neighboring locations, for a given position.
+{% highlight python %}
+def get_neighbors(grid: dict, pos: tuple) -> list[tuple]:
+    x0, y0 = pos
+    candidates = [
+        (x0 - 1, y0 + 1),(x0, y0 - 1),(x0 + 1, y0 - 1),
+        (x0 - 1, y0),                 (x0 + 1, y0),
+        (x0 - 1, y0 - 1),(x0, y0 + 1),(x0 + 1, y0 + 1),
+    ]
+    return [p for p in candidates if p in grid]
+{% endhighlight %}
+
+We can see that the `1,2` coordinate location (value: `7`) has five valid neighboring locations:
+{% highlight python %}
+>>> get_neighbors(grid, (1,2))
+[(1, 1), (2, 1), (0, 2), (2, 2), (0, 1)]
+{% endhighlight %}
+
+
+
+
+#### Future Learning - Complex Numbers
+I've seen people discuss using <a href="https://en.wikipedia.org/wiki/Complex_number">complex numbers</a> to represent the grids. I honestly haven't wrapped my head around the math for that, but I would like to learn.

tag/TIL.md

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+---
+layout: tagpage
+title: "Tag: #TIL"
+tag: TIL
+robots: noindex
+---

tag/python.md

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+---
+layout: tagpage
+title: "Tag: #python"
+tag: python
+robots: noindex
+---