![Matlab Functions
• In our script, type %%
• This creates a new cell – a part of a script that can be run separately
• Now at the BOTTOM of the script …
– function [seconds_in_n_years] = compute_seconds_in_years(n_years)
– seconds_in_minute = 60
– minutes_in_hour = 60
– hours_in_day = 24
– days_in_year = 365
– seconds_in_year = seconds_in_hour *hours_in_day*days_in_year
– seconds_in_n_years = n_years * seconds_in_year
– end
• Type another %% above the function, then
– compute_seconds_in_years(2)](https://image.slidesharecdn.com/programmingtutorialcourselesson1-231009092357-0ff702f7/85/programming_tutorial_course_-lesson_1-pptx-36-320.jpg)
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programming_tutorial_course_ lesson_1.pptx
- 1. Programming tutorial course Lesson 1: Why learn to program? Python vs Matlab; The Basics: variables, scripts and functions Dr Michael Berks michael.berks@manchester.ac.uk
- 2. Why do I need to learn ‘it’? What is Matlab? Because the course leader said so… What is python?
- 3. What is Matlab/python? What does computer programming mean? What is a programming language? What is code? What is a computer program?
- 4. What is a computer program? Something you use to achieve some task on a computer (or phone, tablet etc)… Word Write a letter, essay, lab report etc… Excel Analyse results, produce graphs for your report etc… Angry Birds Kill time when you’re sitting in boring programming lectures…
- 5. What is a computer program? In medical imaging… Image(s) Lab results Patient data DATA Do something clever… PROGRAM Hopefully clinically useful! RESULT
- 6. What is a computer program? In medical imaging… DATA Measure volume of grey/white matter… PROGRAM Predict whether the patient has Alzheimer’s Disease RESULT MR images of the brain
- 7. What is a computer program? In medical imaging… DATA Measure amount dense breast tissue… PROGRAM Predict risk of developing breast cancer RESULT Mammogram acquired during routine screening + patient data
- 8. What is a computer program? In medical imaging… DATA Measure change in size of tumour… PROGRAM Tell doctor/drugs company if the drug is working RESULT MRI liver tumours before and after drug treatment
- 9. What does computer programming mean? Writing your own computer programs... … telling the computer to do exactly what you want. What is a programming language? A way of translating human logic into commands a computer understands… … like human languages, there a lots of different languages (often similar to each other), each with a specific set of rules (syntax, grammar) to obey. What is ‘code’? A chunk of commands in a specific programming language… A program consists of bits of code put together in a logical way … … by using other people’s code, you can incorporate bits of their program in your own (as long as you’re using the same language!).
- 10. Why do I need to learn how to write my own computer code? Think of it like learning to cook….. Now the big question….
- 11. You’re hungry and want something good to eat….. Get parents to cook Go to a restaurant Get a microwave meal x x x
- 12. Vs Heating someone else’s food in the microwave Ok if they’ve cooked exactly what you want Cooking your own food Can eat exactly as you like it Vs Using a computer program Ok if it does exactly what you want Writing your own computer program Make it do exactly what you want Research is likely to be here
- 14. How good a cook/programmer do I need to be? Do I need to write all my programs from scratch? No! Just like in cooking, you don’t need make everything from raw ingredients, can use pre-made pasta, sauces, wine etc… Remember you can use other people’s code to include bits of their programs in yours … … but you do need to know the basics to put those bits together (how to chop an onion, when to add seasoning etc.) Vs
- 15. And finally…. What is Matlab? Matlab is both a program and a programming language ... … it has lots of tools you can just use to analyse data and images. But you can also write code to extend it to do any analysis you like (although unlike a ‘pure’ language, it will only work if the Matlab program is installed on the computer). Excel Photoshop Writing your own code Using other people’s code + + + = A really powerful tool for imaging research!
- 16. • General purpose programming language • Packages provide support for scientific computing • Open-source But what about…. Python?
- 17. How does python differ from Matlab? • It’s free – No need for an expensive license* – Widely used, both in academia and industry – Often installed as part of OS • No single development environment – This can make it a little trickier get started – But, also more flexible – We’ll use Spyder on this course *Currently free because of COVID, but usually you have to pay for Matlab!
- 18. Course aims • To show you how to use Matlab and/or python • To introduce some basic programming ideas common to all programming languages • To give examples of some tasks in medical imaging processing • To provide code you can reuse later in your projects • To make some pretty pictures!
- 19. I’m a bit confused… Should I use Matlab… ..or Python?
- 20. For this course, consider Matlab and python as interchangeable… you can use either or both. I will demonstrate mainly in Matlab, but everything we do will be very similar in both… just watch out for the subtle differences!
- 21. vs If you see this I’m referring to Matlab only If you see this I’m referring to differences in python
- 22. First tasks • Open Matlab (if you have it installed) • Open Spyder (if you have it installed) • Open a windows explorer window – Go to the MSc folder you created for Professor’s Cootes, maths course – Make a new folder Programming – Inside Programming, make new folders: • Matlab • Python • Data • TutorialºÝºÝߣs
- 23. First tasks • Open an internet explorer/Google chrome window • Go to the following webpage: – http://personalpages.manchester.ac.uk/staff/michael.berks – Click on the Matlab Tutorial tab – Bookmark this page for future weeks • Save the week 1 powerpoint slides and tutorial exercise into the TutorialºÝºÝߣs folder – These will also be added to the ‘Course content’ section of blackboard • Leave the window open (we’ll need it later) • Open the ‘lesson 1’ powerpoint file you’ve just saved – As I’m going through the slides, follow them on your computer
- 24. Lesson Overview • Using the Matlab/Spyder interface • Doing maths in the Command Window • Assigning variables and suppressing visual output • Collecting sequences of commands: scripts • An introduction to functions • Basic programming tips: expressive variable names and comments
- 25. Command window
- 26. Command window • A fancy scientific calculator – 2+3 – 2*3 – 2/3 – 2^3 • Our first python difference! It’s 2**3 in python – 2^0.5 – 2+3*6/2 – (2+3)*6/2 • Try using ↑ and ↓ – Select earlier expressions and edit them Any of the text in navy blue is code you can run in the command window. Try copy and pasting, then hitting return
- 28. Assigning Variables • Use ‘=‘ to assign variables – Keeps data in memory to use again – a = 4 – b = 3 + 6 – y = (4*a^2 +3*b^3) / 5 • Can also self-assign: – b = b*2 • Check the work space
- 29. Workspace Shows you what variables are currently stored in memory
- 30. Workspace Shows you what variables are currently stored in memory
- 31. Suppressing visual output • Try a = rand(100) • This creates a 100 x 100 matrix of random numbers • Use clc to clear the Command Window • Try a = rand(100);
- 32. Let's compute the number of seconds in a year… • Let's create some variables – seconds_in_minute = 60 – minutes_in_hour = 60 – hours_in_day = 24 – days_in_year = 365 • Then compute: – seconds_in_year = seconds_in_minute* minutes_in_hour*hours_in_day*days_in_year
- 33. Scripts • Use clear to wipe the current memory – Check that the workspace is now empty • Click the ‘New script’ button (top left of main menu) – Opens a blank page in the editor • Copy the previous commands to compute the number of seconds in a year from the Command History • Paste them into the blank document • Save the document as – Matlab: programming_tutorial_lesson1_script.m – Python: programming_tutorial_lesson1_script.py
- 34. Calling a script from the command-line • Type: programming_tutorial_lesson1_script at the command line • Note the lack of ‘.m’ • Check your work-space • We'll look at different ways of calling python scripts in future lessons
- 35. How many seconds in… • 2 years? • 10 years? • N-years? • Scripts are useful… but if we want to re-run our script with different inputs, we should use a function
- 36. Matlab Functions • In our script, type %% • This creates a new cell – a part of a script that can be run separately • Now at the BOTTOM of the script … – function [seconds_in_n_years] = compute_seconds_in_years(n_years) – seconds_in_minute = 60 – minutes_in_hour = 60 – hours_in_day = 24 – days_in_year = 365 – seconds_in_year = seconds_in_hour *hours_in_day*days_in_year – seconds_in_n_years = n_years * seconds_in_year – end • Type another %% above the function, then – compute_seconds_in_years(2)
- 37. Python Functions • Again type %% • Now type … – def compute_seconds_in_years(n_years) seconds_in_minute = 60 minutes_in_hour = 60 hours_in_day = 24 days_in_year = 365 seconds_in_year = seconds_in_hour *hours_in_day*days_in_year seconds_in_n_years = n_years * seconds_in_year return seconds_in_n_years • Run this cell, no output is generated but the function is now available to use… • Type another %%, and then: – compute_seconds_in_years(10)
- 38. What differences did you spot between the Matlab and python functions? vs
- 39. Inputs and output • The variable n_years is an input to our function • You will often see this described as an argument • The variable seconds_in_n_years is the output of our function • We look next at using multiple inputs • We can also have multiple outputs but will see those in future lessons
- 40. Exercises • Lookup ‘Newton's law of universal gravitation’ (yes, you can use wikipedia) • In Matlab, use the command prompt to compute the force of attraction between the moon and the earth – You can assume • Earth’s mass: 5.97x1024 kg • Moon’s mass: 7.35x1022 kg • Distance from earth to moon: 380,000km • Gravitational constant: 6.674×10−11 N m2 kg−2 • Rewrite this as a function that takes as input, M1, M2 and r • Use commas to separate multiple inputs to your function • Can we write this in python?
- 41. Variable, script and function names • Must start with a letter • Followed by any number of letters, digits, or underscores. • Matlab and python are case sensitive – A and a, my_fun and My_fun, etc are not the same name • Certain keywords cannot be used – if, for, end, etc • Be expressive: try and use names that – Describe what functions do – Describe what variables are
- 42. Scripts summary • Scripts allow us to run sequences of commands • All data is stored in the main workspace, as if we typed the commands in the command window • We can run parts of scripts by – Selecting text and hitting F9 – Using %% to create ‘cells’ Even when ‘hacking around’ use scripts, date tagged (e.g. work2013_02_11) to run commands – That way you have a record of your work – Think of them as your Matlab/Python lab book
- 43. Functions summary • Functions group commands so that we can run them with different inputs • We have looked at defining and calling functions inside scripts • Next week we will look at how we can call functions from other places (hint: you've already done this!) • Matlab and python have 1,000s of in-built functions, by learning how to call those we can create really powerful programs
Editor's Notes
- #17: Image source: http://nbviewer.jupyter.org/github/jrjohansson/qutip-lectures/blob/master/Lecture-1-Jaynes-Cumming-model.ipynb