ENGR 1320 Final Review - Programming

• Major Topics:– Functions and Scripts– Vector and Matrix Operations in Matlab

• Dot product• Cross product

– Plotting in Matlab– Decision Making– Loops

• For• While

– Input methods• Menu• input

– Output methods• Disp• Fprintf

Functions and Scripts• What is the difference between a function and a script file in

MATLAB?– A script is simply a file that contains a series of commands– A function has inputs and outputs.

• To call the function from the command window, you must supply the input values, and the function will return the output values.

• For example the ‘sqrt()’ function will take the square root of the input, and return the value as the output.

– Example problem: Write a function in MATLAB that returns the roots of a quadratic equation.• Inputs: a,b,c of the quadratic formula • Outputs: roots • How would you make this a script?

– See Function Example and Functions in MATLAB for proper coding of functions

Vector operations in MATLAB• Vectors and Matrices are handled in the same way in MATLAB.

– To create a vector or matrix, use square brackets• A = [1 2 3; 4 5 6; 7 8 9]• Spaces or commas separate columns• Semicolons separate rows• You must enter data row-by-row

– To retrieve elements of a matrix, you can specify the value you want with row-column notation• A(3,2) • A(:,1)

– To take dot or cross products in MATLAB, use the cross() and dot() functions. (Type help cross or help dot for correct usage)

– To multiply matrices, use the * operator. • Matrices must be correctly sized, or the operation will return an error

– To find an inverse of a matrix, use the inv() function.– Example problem: write a function to find the angle between 2 vectors

• Hint: the norm() function finds the magnitude of a vector

– Example 2: solve the matrix equation:

Plotting in MATLAB• Use the plot() command to plot data in an x vector against data

in a y vector• Additional plot commands:

– title– xlabel– ylabel– legend

• Example problem: plot the two vectors– x = [0:1:10]– y = x.^2

• What does the period before the ^ operator do?

• See Matlab Basics

User input and output

• MATLAB has 2 main ways of prompting a user for information– input()– menu()

• To output data, you will either plot results or display information to the command window– fprintf()– disp()– use fprintf() to display formatted results and numerical

values• See input examples and output examples

Decision Making• MATLAB has several logical operators

– ==– ~=– &&– ||– >– <– <=– >=

• These operators compare two things and return either a ‘1’ if the statement is true or a ‘0’ if the statement is false– Examples:– a+b == b+a returns a ‘1’– 3+3 == 4+3 returns a ‘0’– a+b == b+a && 3+3 == 4+3 returns ?

• MATLAB can make decisions based on the truth of an expression – If/then statement– Code will execute if the logical expression is true, or be skipped if the expression is false

• Example problem: Write a code that has the user input a number between 1 and 10 and returns a statement to the screen that tells the user whether the number greater or less than 5.

• See Decision Making Screencast

Loops• We studied two methods for making code execute multiple times: for loop and while loop

– the for structure will loop a set number of timesfor i=1:n

code to execute n times

end

– the while structure will loop until a condition is metwhile (logical expression)

code to execute until the logical expression is false

end

• See loops for the proper syntax• Example problem: Write a function that has an input of a single number n and returns a vector [1 2 3 4 …

n] using a ‘for’ loop.• Example problem 2: What is the value of ‘factorial’ at the end of the execution of this code:counter = 5; factorial = 1;

while (counter > 0) factorial = factorial * counter; %Multiply counter = counter - 1; %Decrement end

factorial

Study Strategy

• Exam problems will be similar to homeworks• Several problems have been revisited in this

class:– Electric circuits, truss equations, etc…

• Look over the first 2 exams for representative problems (particularly the 2st for matlab-related problems)

• Refer to the MATLAB book for help