Distance of Robot from Starting Point after a Series of Movements

Question

I am trying to write a program which takes a list of directions and magnitudes and outputs the distance of the robot from its starting position.

I get an error when executing the following code but I cannot identify why I get the error.

import math

position = [0,0]
direction = ['+Y','-X','-Y','+X','-X','-Y','+X']
magnitude = [9,7,4,8,3,6,2]
i = 0

while i < len(direction):
    if direction[i] == '+Y': position[0] += magnitude[i]
    elif direction[i] == '-Y': position[0] -= magnitude[i]
    elif direction[i] == '+X': position[1] += magnitude[i]
    elif direction[i] == '-X': position[1] -= magnitude[i]
    else: pass
    i += 1

print float(math.sqrt(position[1]**2+position[0]**2))

Edit:

I get this error:

IndentationError: unindent does not match any outer indentation level

@user2357 You will probably get correct distance. But the position will be a bit wrong, as on lines with `+X` and `-X` you swapped `-=` and `+=`. — Jan Vlcinsky, Jul 14 '14 at 05:46
Yes, I just noticed that mistake, I should have used up, down, etc. +x, -x etc. is a bit confusing. I don't get any distance at all though. — , Jul 14 '14 at 05:48
@user2357 you probably mixed tabs and spaces. Check out http://stackoverflow.com/questions/492387/indentationerror-unindent-does-not-match-any-outer-indentation-level — Ashoka Lella, Jul 14 '14 at 05:48
You're mixing tabs and spaces. You use spaces to begin with, but switch to tabs for the `if` part. Use spaces throughout. — Jayanth Koushik, Jul 14 '14 at 05:49
Thank you, I didn't know that I could get an error from mixing tabs and spaces. Good to know. — , Jul 14 '14 at 05:51

score 0 · Accepted Answer · answered Jul 14 '14 at 06:24

Most probably you have mixed up your spaces and tabs. In this instance, it might be easier to put the sign within the magnitude and filter with x and y like so:

In [15]: mDr = [ (int(d[0]+m), d[1]) for (d, m) in zip(direction, map(str, magnitude))]

In [16]: mDr
Out[16]: [(9, 'Y'), (-7, 'X'), (-4, 'Y'), (8, 'X'), (-3, 'X'), (-6, 'Y'), (2, 'X')]

In this case, you can get to total x and y distances pretty easily. For example, the y distances:

In [17]: [md[0]  for md in mDr if md[1] =='Y']
Out[17]: [9, -4, -6]

And the total y distance in the particular direction:

In [18]: sum( [md[0]  for md in mDr if md[1] =='Y'] )
Out[18]: -1

You can do the same for x and then calculate the distance that way.

Jan Vlcinsky · Answer 2 · 2014-07-14T07:25:27.287

Here comes my offtopic reaction (your problem was mixing tabs and spaces, my answer is simple rewrite).

import math

xymoves = {"+X": (1, 0), "-X": (-1, 0), "+Y": (0, 1), "-Y": (0, -1)}

position = [0, 0]
directions = ['+Y', '-X', '-Y', '+X', '-X', '-Y', '+X']
assert all(xymove in xymoves for xymove in directions)
magnitudes = [9, 7, 4, 8, 3, 6, 2]

for direction, magnitude in zip(directions, magnitudes):
    xmove, ymove = xymoves[direction]
    position[0] += magnitude * xmove
    position[1] += magnitude * ymove

print math.sqrt(position[1]**2+position[0]**2)

Changes:

looping using for and not while with incrementing index.
the logic "where to move" moved from if elif elif into dictionary xymoves
rejecting to process direction, which is not expected
math.sqrt always returns float, so conversion to float removed

Note, that the dictionary with xymoves could be extended with other directions, e.g. using "N" for North, "NE" for North-East etc.

I don't get it. Why shouldn't I upvote this? Is it not an answer? — Emil Vikström, Jul 14 '14 at 07:22
@EmilVikström Of course you can upvote it. I just wanted to highlight, that my answer is not reacting to real problem from the question. — Jan Vlcinsky, Jul 14 '14 at 07:24

Distance of Robot from Starting Point after a Series of Movements

2 Answers2