Midnight Pub

Programming a PB&J

~mnbryant

The post that inspired this

The above post reminded me of an analogy I once heard to try to explain to non-programmers what programming is actually like.

Suppose you are teaching someone how to make a peanut butter and jelly sandwich. For most people, the instructions would look something like:

• Get two slices of bread

• Put peanut butter on one and jelly on the other

• Put the two slices together

This would not work for a computer, though. If we look closer at just that first step, we would need to tell the computer to get the slices of bread from the pantry.

Error: could not find slices of bread. Found: loaf of bread, jar of peanut butter, [...]

There was a logical skip in our instructions: to get the loaf out and take slices from it. The computer doesn't know how to infer that step (ignoring machine learning/artificial intelligence), so it fails.

If we extrapolate this further and remove any assumptions about how to make a sandwich and what to do if the current state is not as we expect (e.g. a door is closed but instructions assume it is open), we get the following list of computer-friendly instructions:

• If the pantry door is shut, open the pantry door.

• Take the loaf of bread and the jar of peanut butter and place them on the (assumed empty) counter.

• Open the bag containing the loaf of bread.

• If there are less than two slices remaining, abort.

• Take two slices from the loaf and place them on the (assumed clean) counter.

• Close the bag containing the bread.

• Open the jar of peanut butter.

• Open the silverware drawer.

• Take a knife from the silverware drawer.

• Use the knife to remove (a specific measurement) of peanut butter from the jar.

• Spread the peanut butter on the first slice of bread using the knife.

• Close the peanut butter jar.

• Place the bread and peanut butter back in the pantry.

• Close the pantry door.

• (Repeat peanut butter steps but with the jelly from the fridge.)

• Press the peanut butter side of the first slice against the jelly side of the second slice.

• Sandwich has been made.

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Replies

~errant wrote:

My English teacher once taught this lesson, although I can't remember why (grammar rules?). I felt pretty clever with my list of ~20 steps until she pointed out, rather dramatically, that I hadn't described _how_ to get bread out of the bag by wrenching it apart and scattering slices across the front row of students.

~bitdweller wrote:

To illustrate how "difficult" it is to program (finnicky may be a better word, if my understanding of English is correct), I will add some steps:

• First, loop through all rooms, if name === "pantry", then proceed with opening its door.

• You need to account for all possible errors, if there's no knives on the drawer, then run getKnife(), which in turn may need to run cleanKnife().

• You didn't close the drawer, you may hurt yourself bumping into it.

• After returning the peanut butter and jelly, you didn't go back to the counter.

This is just for fun, not to actually "correct" you :)