1. 1 Week 1: Variables, assignment, expressions READING: 1.2 – 1.4

2. Variables 2 To store a value in a MATLAB program, a variable is used To create a variable, we use an assignment statement: variablename = expression The variable is always on the left, followed by the = symbol, followed by an expression A variable stores a value that can be changed at any time EECS 1541 -- Introduction to Computing for the Physical Sciences

3. Variables 3 Example: The variable “ z ” is on the left, followed by the = symbol >> z = 6 z = 6 This means a value of 6 is assigned to the variable “ z” EECS 1541 -- Introduction to Computing for the Physical Sciences

4. Variables 4 Putting a semicolon at the end of a statement suppresses the output Example: >> z = 6; >> This would assign a value of 6 to the variable z, but the result is not shown. EECS 1541 -- Introduction to Computing for the Physical Sciences

5. Variables 5 1. a variable name must start with a letter 2. the rest of the name can include letters, digits, or underscores 3. names are case sensitive, so “ A ” and “ a ” are two different variables 4. MATLAB has some reserved words called keywords that cannot be used as variable names use the command iskeyword to get a list of keywords EECS 1541 -- Introduction to Computing for the Physical Sciences

6. Variables 6 use short, meaningful names a name that conveys the purpose of the variable is often useful for others who need to read your code, e.g., use massEarth instead of mE massSun instead of mS exceptions to the rule: if you are solving a problem that contains variable names, you should try to use the same names, e.g., in physics the following would likely be common: g, c, v0 ( g = gravity, c = speed of light, v0 = initial velocity) EECS 1541 -- Introduction to Computing for the Physical Sciences

7. Variables 7 Examples: valid variable names invalid variable names reason invalid x$ does not begin with a letter $ is not allowed in variable names x66x does not begin with a letter lastValueIf if is a keyword pi_over_2pi/2 punctuation marks are not allowed in variable names EECS 1541 -- Introduction to Computing for the Physical Sciences

8. Operators 8 Numerical expressions are created using values, variables, operators, built-in functions and parentheses. + addition - negative, subtraction * Multiplication / division ^ exponentiation ()parentheses Common operators used with numerical expressions: EECS 1541 -- Introduction to Computing for the Physical Sciences

9. Operator Precedence Rules 9 Just like normal mathematical computations, some operators have precedence over others in MATLAB. operatornameprecedence ( )Parentheses Highest ^Exponentiation -Negation *, /, \Multiplication and division +, -Addition and subtraction Lowest EECS 1541 -- Introduction to Computing for the Physical Sciences

10. Operators 10 Example: >> y = 6 - 5 * 8 + 9 y = First 5 is multiplied by 8, then the result is subtracted by 6 and added to 9 What about: >> y = (6 – 5 * 8) + 9 y = -25 EECS 1541 -- Introduction to Computing for the Physical Sciences -25

11. Operators 11 What about: >> y = 6 - 5 * 8 + 9; >> y = y + 1 y = The above computation where y = y + 1 is called incrementing, it increases the previous value by 1 -24 EECS 1541 -- Introduction to Computing for the Physical Sciences

12. Constants 12 Recall that variables are used to store values that might change Constants are values that cannot be changed at any time. Some constants that are pre-defined in MATLAB are: Constant namesMeaning/value pi π or 3.1414 i j inf ∞ EECS 1541 -- Introduction to Computing for the Physical Sciences

13. 13 Example: >> pi_over_2 = pi / 2 pi_over_2 = 1.5708 Constants EECS 1541 -- Introduction to Computing for the Physical Sciences

14. Built-in Functions 14 most MATLAB is provided through functions a function in MATLAB accepts a set of inputs and (usually) calculates a set of outputs there can be 0 or more inputs there can be 0 or more outputs the user of the function provides the inputs the input values are called arguments to the function the function provides the outputs the user uses the name of the function to use the function we say that the user calls the function EECS 1541 -- Introduction to Computing for the Physical Sciences

15. Built-in Functions 15 A list of elementary math functions from MATLAB can be found using the following command: >> help elfun EECS 1541 -- Introduction to Computing for the Physical Sciences

16. Built-in Functions 16 This list introduces a long list of functions: EECS 1541 -- Introduction to Computing for the Physical Sciences

17. 17 What does the following expression mean? We can use help to search for the meaning of exp >> exp(1) >> help exp Built-in Functions: Exponential functions EECS 1541 -- Introduction to Computing for the Physical Sciences

18. 18 Built-in Functions: Exponential functions EECS 1541 -- Introduction to Computing for the Physical Sciences >> help exp

19. 19 Since exp is the exponential function, exp(1) means evaluate the exp f unction with an input argument of 1 (i.e. = e 1 ) >> exp(1) ans = 2.7183 Note: MATLAB uses a default variable named “ ans” if an expression is typed at prompt and it is not assigned to a variable Built-in Functions: Exponential functions EECS 1541 -- Introduction to Computing for the Physical Sciences

20. Built-in Functions: Trigonometric functions 20 >> help sin Example: EECS 1541 -- Introduction to Computing for the Physical Sciences

21. 21 Consider the following expressions: y = 2 * sin(pi/2); z = y z = pi What will the output values of the above program? Built-in Functions: Trigonometric functions EECS 1541 -- Introduction to Computing for the Physical Sciences

22. 22 Consider the following expressions: y = 2 * sin(pi/2); z = y z = pi Built-in Functions: Trigonometric functions EECS 1541 -- Introduction to Computing for the Physical Sciences NOTE: “ ; ” is added after the statement, so the final value “2” will not be displayed The first expression will evaluate the sin function with an input argument of pi/2, which is equal to 1, then multiply by 2

23. 23 Consider the following expressions: y = 2 * sin(pi/2); z = y z = pi The second expression simply means whatever is in y is now also assigned to z Built-in Functions: Trigonometric functions EECS 1541 -- Introduction to Computing for the Physical Sciences NOTE: “ ; ” is added after the statement, so the final value “2” will not be displayed The first expression will evaluate the sin function with an input argument of pi/2, which is equal to 1, then multiply by 2

24. 24 Consider the following expressions: y = 2 * sin(pi/2); z = y z = pi The second expression simply means whatever is in y is now also assigned to z Built-in Functions: Trigonometric functions EECS 1541 -- Introduction to Computing for the Physical Sciences The first expression will evaluate the sin function with an input argument of pi/2, which is equal to 1, then multiply by 2 NOTE: “ ; ” is added after the statement, so the final value “2” will not be displayed The last expression means to assign the constant pi (or 3.1416) to variable z

25. 25 Built-in Functions: Trigonometric functions EECS 1541 -- Introduction to Computing for the Physical Sciences NOTE: the new value assigned to z will overwrite the previous value

26. Built-in Functions: Trigonometric functions 26 >> help sin EECS 1541 -- Introduction to Computing for the Physical Sciences

27. 27 Built-in Functions: Trigonometric functions EECS 1541 -- Introduction to Computing for the Physical Sciences >> help sind

28. 28 Built-in Functions: Trigonometric functions Example: >> y = sind(90) y = 1 What do you expect to see from: >> y = cos(sind(360)) y = 1 EECS 1541 -- Introduction to Computing for the Physical Sciences

29. 29 Built-in Functions If you are interested in other built-in functions in MATLAB: EECS 1541 -- Introduction to Computing for the Physical Sciences http://www.mathworks.com/help/matlab/functionlist.html