Software tutorial/Scripts and functions

Scripts

• Scripts are a series of commands, saved in a .m or .py file.
• When you run a script, it is as if you typed each line, in sequence, into MATLAB or Python.
• It is good programming practice, and is required for this course, that all all your programs should run from a script.
  • It prevents you retyping in the commands if you need to change a variable's value
  • You must use scripts if you need to debug your program
  • Is helpful when you come back weeks or months later to figure out what you did (like revising for exams)
  • Can be shared with your colleagues for collaborative development.
• All variables resulting from calculations performed in the script are available to you in the command window after the script is finished

A simple example of a script:

MATLAB: example.m	Python: example.py
rho = 1200; % [kg/m^3] Cp = 4180; % [J/(kg.K)] f = 0.1; % [m^3/s] T = 341; % [K] energy_in = rho * Cp * f * T; % [J] disp(['The energy in = ', num2str(energy_in/1E6), ' MJ/s']) Now at the MATLAB command prompt you can type: >> example The energy in = 171.0456 MJ/s	rho = 1200 # [kg/m^3] Cp = 4180 # [J/(kg.K)] f = 0.1 # [m^3/s] T = 341 # [K] energy_in = rho * Cp * f * T # [J] print('The energy in = %f MJ/s' % energy_in/1E6) From the Python command prompt: >> execfile('example.py') The energy in = 171.0456 MJ/s

Functions

• Functions are more versatile than scripts because they can accept input arguments and return output values
• An example of a simple function:

function d = distance(x,y)
% calculates distance between the point (x,y) and the origin
z = x2 + y2;
d = sqrt(z);
end

• The output variable here is d and the input variables are x and y
• The name of the function is distance
• The function .m file MUST HAVE THE SAME NAME AS THE FUNCTION
• The % sign signifies the start of a comment (text that is ignored by MATLAB)
• The above function may be called (both at the command line and in a script) by the following command:

D = distance(2.0,3.5)

• The function will calculate the distance between (2.0,3.5) and the origin and put the result in the variable D
• Note that the function allows us to perform the distance calculation without previously assigning values to the variables x and y in the workspace
• A function can assign multiple output variables (consult MATLAB help to find out how)
• Functions can call other functions