MATLAB Loops and Branching: For and While Loops

1. Loops and Branching MATLAB

2. Loop types • Counted loops; called for loop • Conditional loops; called while loop

3. Looping! • Scripts and functions also allow the ability to loop using conventional for and while loops. • Note that the interpreter also lets you do it, it is simply less easy to grasp

4. For Loops • Common to other programming languages for variable = expression statement ... statement end

5. For Loops, con’t: 2 • Example: (taken from MATLAB help) • a = zeros(k,k) % Preallocate matrix for m = 1:k for n = 1:k a(m,n) = 1/(m+n -1); end end

6. For Loops, con’t: 3 • The looping variable is defined in much the same way that we defined arrays/vectors. • Ex. m = 1:k • Or m = 1:10

7. For Loops, con’t: 4 • Loops are shown to end by the keyword “end” • Curly braces are not present to subdivide packets of code • Make use of adequate white-space and tabbing to improve code readability

8. While Loops • Similar to while loops in other languages while condition statement … end

9. While Loops, con’t: 2 • Ex. (taken from help while) while (1+eps) > 1 eps = eps/2; end

10. While Loops, con’t: 3 • Same notes apply to while loops. • Code is separated by the keyword “end”

11. Looping conclusion • Some other aspects of looping exist • Use help while and help for to see them

12. LOGICAL CONTROL PROGRAMMING CONSTRUCTS • Syntax of the if statement: if logical expression statements end

13. EXAMPLE x = some given value if x >= 0 y = sqrt (x) end

14. EXAMPLE • x = 10; • y = 20; • if x >= 0 & y >= 0 • z = sqrt(x) + sqrt(y); • w = log(x) – 3*log(y); • end

15. LOGICAL PROGRAMMING CONTRUCTS • Nested “if” statements: if logical expression 1 statement group 1 if logical expression 2 statement group 2 end end • Note the indentions Nested Statement

16. LOGICAL PROGRAMMING CONTRUCTS • THE else STATEMENT: • If two mutually exclusive actions can occur as a result of a decision, use the else statement. • if logical expression • statement group 1 • else • statement group 2 • end

17. In-class Exercise (5 minutes) • Suppose y = x1/2 for x >= 0 and y = ex – 1 for x < 0 • Write a program (.m script file) to calculate y assuming that x already has a scalar value. • Test your program for x = 3 and x = -2.

18. SOLUTION (Script File) • % Solution to In-Class Exercise • if x >= 0 • y = sqrt (x); • else • y = exp (x) -1; • end Did you indent properly?!

19. LOGICAL PROGRAMMING CONSTRUCTS • The elseif statement: • When three actions can occur as a result of a decision, the else and elseifstatements are used along with the if statement. • Remember: ONLY ONE ACTION WILL ACTUALY OCCUR!!!

20. LOGICAL PROGRAMMING CONSTRUCTS • if logical expression 1 • statement group1 • elseif logical expression 2 • statement group 2 • else • statement group 3 • end

21. EXAMPLE • Given: • y = ln x for x > 10 • y = x1/2 for x >= 0 and x <= 10 • y = ex – 1 for x < 0 • Compute y if x has been assigned a scalar value.

22. SOLUTION (Script file) • % Solution to example • if x > 10 • y = log (x) • elseif x >= 0 • y = sqrt (x) • else • y = exp (x) -1 • end Does the order that I check things matter? YES!

23. LOGICAL PROGRAMMING CONSTRUCTS • Write the syntax for the if-elseif-else-end construct if there are more than three alternatives.

24. SOLUTION • if logical expression1 • Statements group1 • elseif logical expression2 • Statements group2 • elseif logical expression3 • Statements group3 • elseif logical expression4 • Statements group4 • … • else • Statement if all other cases are false • end