Question

I am having trouble with the lambda function and not really understanding it completely. I am trying to calculate the manhattan distance between two points, 1 point is the current location and the second point is from a list that contains multiple positions. Any help is appreciated, thank you

my function is:

toVisit.sort(toVisit,key = lambda x: util.manhattanDistance(curr,x))

I am trying to get a value out of the toVisit list but don’t know how to exactly. I was thinking of passing in x with the curr location but that does not work.

Answer 1

Look at the documentation (or built-in help, or how-to. The signature for list.sort is:

s.sort([cmp[, key[, reverse]]]) sort the items of s in place    (7)(8)(9)(10)

The notes explain, among other things, what each of those parameters mean. In particular, the first parameter, cmp:

specifies a custom comparison function of two arguments (list items)

You’re passing toVisit as the first argument. So, toVisit will be used as a comparison function. You can’t possibly have thought that was what you wanted. So, I’m guessing you probably don’t get the basics of objects and dot notation, and you need to read an appropriate tutorial. (Sorry, I don’t have one to recommend. I checked the official Python tutorial, but it seems to assume that dot notation, and what a method is, and so on are all just obvious and need no explanation…)

Here’s the right version:

toVisit.sort(key = lambda x: util.manhattanDistance(curr,x))

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Meanwhile, there doesn’t seem to be any problem with the lambda function itself, but you seem to be convinced, in two questions in a row, that the lambda is the part you’re having trouble with. If you ever have trouble with a lambda, the easiest thing to do is transform it into a normal named function, which is trivial to do.

Where you have this:

lambda ARGS: EXPR

Do this a line above it:

def good_name_for_what_expr_does(ARGS):
    return EXPR

Then, replace the lambda ARGS: EXPR with good_name_for_what_expr_does.

So, this is equivalent to your original code:

def manhattanDistanceFromCurr(x):
    return util.manhattanDistance(curr, x)
toVisit.sort(toVisit, key = manhattanDistanceFromCurr)

Can you see the problem there any more easily than in the lambda version?