CMPS 1371Introduction to

Computing for Engineers

CONDITIONAL STATEMENTS

ARRAYS

The important use of arrays as indices has to do with picking out the elements of a vector (or array) that have a certain property or characteristic. To do this requires to things: definition of the characteristic "Finding" the indices of the vector with that

property

Boolean test on Vectors

Consider the vector A = [ 5 3 1 7 -3 -1 9 27 6 1]

Suppose want non-negative elements. Use a logic comparison to locate: Need notion of true/false – called

Boolean Only two possible values (true or false) In Matlab false is 0, true is 1

Relational Operators

< Less than

<= Less than or equal to

> Greater than

>= Greater than or equal to

== Equal to

~= Not equal to

Comparisons

Comparisons are either true or false Most computer programs use the number 1

for true and 0 for false

The results of a comparison are used in selection structures and repetition structures to make choices

MATLAB compares corresponding elements and determines if the result is true or false for each

Comparisons

In order for MATLAB to decide a comparison is true for an entire matrix, it must be true for every element in the matrix  

Logical Operators

& and

~ not

| or

xor exclusive or

Simple “if” statement

if comparison statementsend

For example….

if G<50 count = count +1; disp(G);end

IF statements

Each branch can include multiple statements Statements can also include other if statements (can

nest if statements inside if statements)

if A>10

% computations;

end

ifcondition

statements

true

false

if A>10 % computations;else % computations;end

ifcondition

statements (1)

true false

statements (2)

The if/else structure

The simple if triggers the execution of a block of code if a condition is true

If it is false that block of code is skipped, and the program continues without doing anything

What if instead you want to execute an alternate set of code if the condition is false?

Flow chart of an if/else structure

Block of code to execute if the

comparison is true

ComparisonTrue False

Block of code to execute if the

comparison is false

Calculation Example

Use an if structure to calculate a natural log Check to see if the input is positive

If it is, calculate the natural log If it isn’t, send an error message to the screen

M-file Program

Interactions in the Command Window

The if/else/elseif structure Use the elseif for multiple selection criteria For example

if (grade >= 90) Letter = ‘A’Disp(‘You did great’) % note disp function

Elseif (grade >= 80) % didn’t need <90Letter = ‘B’

Elseif (grade >= 70)Letter = ‘C’

Elseif (grade >= 60)Letter = ‘D’

Else Letter = ‘F’

End

Examples

• Write a program to determine if an applicant is eligible to drive

Start

if age<16 TrueSorry – You’ll have to wait

age<18You may have a youth license

age<70

True

You may have a standard license

True

Drivers over 70 require a special license

elseif

elseif

else

M-file Program

Always test your programs – making sure that you’ve covered all the possible calculational paths

switch/case

This structure is an alternative to the if/else/elseif structure

The code is generally easier to read This structure allows you to choose between multiple

outcomes, based on some criterion, which must be exactly true

The criterion can be either a scalar (a number) or a string.

In practice, it is used more with strings than with numbers.

The structure of switch/case

switch variablecase option1

code to be executed if variable is exactly equal to option 1

case option2code to be executed if variable is exactly equal to option 2…

case option_ncode to be executed if variable is exactly

equal to option notherwise

code to be executed if variable is not equal to any of the options

end

Suppose you want to determine what the airfare is to one of three cities

Airfare Example

Menu

The menu function is often used in conjunction with a switch/case structure.

This function causes a menu box to appear on the screen with a series of buttons defined by the programmer.

Because the input is controlled by a menu box, the user can’t accidentally enter a bad choice

This means you don’t need the otherwise portion of the switch/case structure

Menu

Note that the otherwise portion of the switch/case structure wasn’t used

When you run this code a menu box appears

Instead of entering your choice from the command window, you select one of the buttons from the menu