Line Equations in Python

The other day I was playing with some Python challenges found on a popular sites, this challenge worked with xy coordinates on a 2D plane.

I wanted to show some of the code I wrote and how they work.

The Plane

Being the data is a tuple of x, y coordinates we will use the Cartesian Coordinate System.

I started my code off as a simple Python class:

class Line(object):

    def __init__(self, data):
            self.first, self.second = data

Using this Class I can input two points in a tuple like this:

data = ((1,1), (2,3))

These coordinates would look like this on a 2D plane:

plane1

The Line

The coordinates ((1,1), (2,3)) holds quite a bit of data when it comes to the terms of Algebra.

Slope

The Slope of a line will tell us how steep it is, and can be calculated with the change in Y / change int X.

def slope(self):
        '''Get the slope of a line segment'''
        (x1, y1), (x2, y2) = self.first, self.second
        try:
                return (float(y2)-y1)/(float(x2)-x1)
        except ZeroDivisionError:
                # line is vertical
                return None

Using the slope method tells us this line has a slope of 2.

>>> data = ((1,1), (2,3))

>>> line = Line(data)

>>> m = line.slope()

>>> print m
2.0

Y Intercept

The Y Intercept tells us at what point a line will meet the Y axis. To get a Y Intercept we use the equation b = y – mx where m is our slope.

def yintercept(self, slope):
        '''Get the y intercept of a line segment'''
        if slope != None:
                x, y = self.first
                return y - slope * x
        else:
                return None

And if we plug all our data back in we get a intercept of -1:

>>> b = line.yintercept(slope)

>>> print b
-1.0

Solve for Y

Now that we know the slope of our line, and where our line meets the Y Axis we can plug in any X coordinate and solve for where Y will be:

def solve_for_y(self, x, slope, yintercept):
        '''Solve for Y cord using line equation'''
        if slope != None and yintercept != None:
                return float(slope) * x + float(yintercept)
        else:
                raise Exception('Can not solve on a vertical line')

And just like that we can when X is equal to 3 our Y will be 5, just look at the graph above and imagaine.

>>> line.solve_for_y(3, m, b)
5.0

Solve for X

And lastly using our slope and Y intercept we can solve for X when Y is some value:

def solve_for_x(self, y, slope, yintercept):
        '''Solve for X cord using line equatio'''
        if slope != 0 and slope:
                return float((y - float(yintercept))) / float(slope)
        else:
                raise Exception('Can not solve on a horizontal line')

And we can do the reverse of above to verify they are both working, when X is equal to 5 our Y should be 3:

>>> line.solve_for_x(5, m, b)
3.0

Put it all together

class Line(object):

    def __init__(self, data):
            self.first, self.second = data

    def slope(self):
            '''Get the slope of a line segment'''
            (x1, y1), (x2, y2) = self.first, self.second
            try:
                    return (float(y2)-y1)/(float(x2)-x1)
            except ZeroDivisionError:
                    # line is vertical
                    return None

    def yintercept(self, slope):
            '''Get the y intercept of a line segment'''
            if slope != None:
                    x, y = self.first
                    return y - slope * x
            else:
                    return None

    def solve_for_y(self, x, slope, yintercept):
            '''Solve for Y cord using line equation'''
            if slope != None and yintercept != None:
                    return float(slope) * x + float(yintercept)
            else:
                    raise Exception('Can not solve on a vertical line')

    def solve_for_x(self, y, slope, yintercept):
            '''Solve for X cord using line equatio'''
            if slope != 0 and slope:
                    return float((y - float(yintercept))) / float(slope)
            else:
                    raise Exception('Can not solve on a horizontal line')

7 thoughts on “Line Equations in Python

  1. nkinga

    Dear Ness!

    I would like to thank you, your blog is very useful! I did nont find how to get the slope and the Y Intercept through the web, so I was very happy when I found your blog.
    Although the code does not work for me.. It says at the very beginning (after “print m”) that “‘Line’ object has no attribute ‘slope'”.
    I am a real beginer with python, so I am pretty sure that I made some silly mistake, but could you help me with this?

    Thank you very much,
    And again, congrats for your blog.
    Bye
    Kinga

    • nessy

      Hello Kinga,

      Thank you for your kind words.

      I’m going to assume the issue is that the code above was written in Python 2.x, and you are using Python 3.x.

      In Python 3.x print becomes a method and needs to be called like “print(m)” rather than “print m”:

      https://docs.python.org/3/whatsnew/3.0.html#print-is-a-function

  2. Mike

    Also,
    self.slope in solve for y should just be slope as you are passing it to the function

    • nessy

      Ah nice catch!

  3. r

    Hey, your blog is awesome! Thank you for this 🙂

    • nessy

      Thank you very much!

  4. You could calculate the slope in the constructor and save it as a member variable. Then you do not need to pass it as an argument to the member functions, they can just use : self.slope
    Rename the slope() function to slopecalc() and in the constructor add:
    self.slope = self.slopecalc()

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