MATLAB Basics 9 loops

Transcription

1 1 MATLAB Basics 9 loops Anthony Rossiter University of Sheffield For a neat organisation of all videos and resources

2 Introduction 1. We have established that using script files for sequences of operations is useful. 2. Sometimes, it is useful to repeat the same operation over and over again, but with slightly different numbers each time. 3. MATLAB provides a convenient mechanism for this by way of loop structures. 4. These resources cover an introduction to basic loop structures (for loops and while loops). The coding is largely intuitive and follows reasoning common to humans and thus is easy and transparent to code. 2

3 Loops are essential Code allows repeated numerical computations. We do not want to write a line of code multiple times to do the same thing with different numbers. We want to tell the computer to use the same line, but with different numbers.

4 Common loop structures 4 For loops Specify exactly which values change during the repeated operations and how many times the operation is repeated. While loops. Use a conditional to decide whether the loop action should be repeated or not. This was covered in the previous videos with conditionals.

5 matlab_basics9e.m Open the file. 1. Two ways to compute 6 values 2. First look at lines Or try lines Now imagine you needed to do this for 100 different values, or a Using a loop is simple to code and efficient.

6 How does a for loop work? equivalently k=[1,2,3,4,5,6] Back to start of loop for k=1:6; end y2(k)=sin(xvalues(k)); more code First k=1 hence do y2(1)=sin(xvalues(1)); Now k=2 hence do y2(2)=sin(xvalues(2)); Now k=3 k=4 hence do do Y2(3)=sin(xvalues(3)); y2(4)=sin(xvalues(4)); Now k=5 hence do Now y2(5)=sin(xvalues(5)); k=6 hence do y2(6)=sin(xvalues(6)); Now k=6 is equal to max of allowed k, continue to next line of code

7 How to write the statement for k=3:8 code end for k=4:3:13 code end k takes the values 3,4,5,6,7,8 k takes the values 8,6,4,2 k takes the values 4,7,10,13 for k=8:-2:2 end code

8 Generalisation 8 One can write a statement like: for k=[a,b,c,d,e,,f]; CODE that depends on k end Equivalently for k=vec; vec a vector Where it is noted the values k takes in the loop do not need to be equi-spaced.

9 General examples of for loops Open the file matlab_basics9a.m Note the for loop statement has 3 variables. Run in the command window Note values of k are: start, start+step, start+2*step,,start+ 6*step

10 Nested loops Open matlab_basics9b.m Note that one loop is inside another loop. First k=1 and then l goes from 1 to 5. Next k=2 and then l goes from 1 to 5.. Finally k=10 and then l goes from 1 to 5. Inner loop Outer loop Run the file and experiment with changing the loop for statements

11 Loop counters can be variables 11 It may be that the number of times a loop is needed, or indeed the variables to be used in the loop are unknown during coding and supplied as a user input, or as a consequence of earlier code. This is illustrated with a simple example in matlab_basics9f.m In general, for the statement for counter=a:b:c; CODE; end All of a,b,c can be determined as a consequence of earlier code and/or USER Input.

12 12 LIVE DEMONSTRATIONS WITH MATLAB

13 Files to look at 13 matlab_basics9a.m matlab_basics9b.m matlab_basics9c.m matlab_basics9d.m matlab_basics9e.m matlab_basics9f.m

14 Conclusions 14 Demonstrated the usefulness of for loops for executing the same command multiple times, with slightly different numerical values. Allows complex computations to be tested and saved for easy use later. In general terms, for loops are one of the most common structures used in coding and thus it is essential students become skilled in these.

15 For a neat organisation of all videos and resources Anthony Rossiter Department of Automatic Control and Systems Engineering University of Sheffield University of Sheffield This work is licensed under the Creative Commons Attribution 2.0 UK: England & Wales Licence. To view a copy of this licence, visit or send a letter to: Creative Commons, 171 Second Street, Suite 300, San Francisco, California 94105, USA. It should be noted that some of the materials contained within this resource are subject to third party rights and any copyright notices must remain with these materials in the event of reuse or repurposing. If there are third party images within the resource please do not remove or alter any of the copyright notices or website details shown below the image. (Please list details of the third party rights contained within this work. If you include your institutions logo on the cover please include reference to the fact that it is a trade mark and all copyright in that image is reserved.)