Line Segment - equation of a straight line in Matlab

m is the slope of the line (y₂ – y₁)/(x₂ – x₁)
b is the y-intercept of the line
x is the independent variable of the function y = f(x) 

Our first function assumes you enter points (x1, y1) and (x2, y2), and the function returns the slope m and the intersect b. 

function [m, b] = str_lin1(x1, y1, x2, y2)
m = (y2-y1) / (x2-x1);
b = y1 - m*x1;

Now, the following video shows you how to use the concepts of point and slope to find linear equations. After the video, we show you how to implement a function to easily achieve the equivalent results.

Our second function in this task, assumes you enter point (x, y) and the slope. It returns the intercept. 

function b = str_lin2(x1, y1, m)
b = y1 - m*x1;
 

The third function interpolates an entered value. You enter the slope, the intercept and the value. The function returns the interpolated values.

function [x, y] = str_lin3(m, b, val)
y = m*val + b;
x = (val - b) / m;
 

Now, we’re going to wrap those three functions using a menu and a switch structure:
 

% Functions on straight lines
clear all, clc, format compact, close all 

option = menu('Choose your case: y = mx + b',...
    'Enter (x1, y1) and (x2, y2); get m and b',...
    'Enter (x1, y1) and m; get b',...
    'Enter m, b and a value; get interpolated (val, y) and (x, val)'); 

  switch option
      case 1
          x1 = input('Enter x1 = ');
          y1 = input('Enter y1 = ');
          x2 = input('Enter x2 = ');
          y2 = input('Enter y2 = ');
          [m, b] = str_lin1(x1, y1, x2, y2)         

      case 2
          x1 = input('Enter x1 = ');
          y1 = input('Enter y1 = ');
          m = input('Enter m = ');
          b = str_lin2(x1, y1, m)          

      case 3
          m = input('Enter m = ');
          b = input('Enter b = ');
          v = input('Enter val = ');
          [x, y] = str_lin3(m, b, v);
          str = ['(' num2str(v) ', ' num2str(y) ')'];
          disp(str)
          str = ['(' num2str(x) ', ' num2str(v) ')'];
          disp(str)
  end       

Let’s assume that the script above is called line_segment. We can run some examples from the command window, like this... 

>>line_segment

Example 1: enter two points

Matlab displays the following menu:

 
If we click on the first option, the flow is like this: 

Enter x1 = 0
Enter y1 = -2
Enter x2 = 2/3
Enter y2 = 0

and the results are

m =  3
b = -2
 

Example 2: enter one point and slope

Let's run again our code, but this time we select the second button. The scirpt works like this...

Enter x1 = 2/3
Enter y1 = 0
Enter m = 3

and the result is

b = -2
 

Example 3: enter slope, intercept and one value to be interpolated

Finally, the third option can be used for a simple linear interpolation. Let's run it, and we get...

Enter m = -4
Enter b = 1
Enter val = 0

(0, 1)
(0.25, 0)