MATLAB Loops and Branching

Loop types Counted loops; called for loop Conditional loops; called while loop

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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.

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!

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

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