Chapter-5-6Logical functions, Selection structures and Repetition

structures

Faculty of Chemical & Natural Resources Engineering

Content

Flow diagram while loop and for loop. Addition, subtraction and other mathematical

functions. Break and continue statement. Branches if else, switch/case. Debugging tools

Execution Control

This chapter discusses techniques for changing the flow of control of a program, which may be necessary for two reasons.

You may want to execute some parts of the code under certain circumstances only.

You may want to repeat a section of code a certain number of times.

Logical functions A sequence is a list of commands that are

executed one after another. A selection structure allows the programmer

to execute one command if some criterion is true and second if the criterion is false.

A repetition structure, or loop, causes a group of statements to be executed multiple times.

Structures

Sequence Selection Repetition

Sequence Selection Repetition (Loop)

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?

Block of code to excute if the

comparison is true

ComparisonTrue False

Block of code to excute if the

comparison is false

Flow chart of an if/else structure

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

If it is, calculate the natural logIf 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

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

End

elseif

elseif

else

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

As a general rule…

If structures work well for scalars For vectors or arrays use a find function or.. Combine if structures with a repetition

structure Repetition structures are introduced in the next

chapter

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

When to use switch/case

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

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

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 n

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

end

The structure of switch/case

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

Remember…You tell the input command to expect a string by adding ‘s’ in the second field.

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.

input = menu(‘Message to the user’,’text for button 1’,’text for button 2’, etc)

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

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

If I select Honolulu…

Summary

Sections of computer code can be categorized assequencesselection structuresrepetition structures

Summary – Sequence

Sequences are lists of instructions that are executed in order

Summary – Selection Structure

Selection structures allow the programmer to define criteria (conditional statements) which the program uses to choose execution paths

Summary – Repetition Structures

Repetition structures define loops where a sequence of instructions is repeated until some criterion is met (also defined by conditional statements).

Summary – Relational Operators

MATLAB uses the standard mathematical relational operators<<=>>===~=

Recall that = is the assignment operator, and can not be used for comparisons

Summary – Logical Operators

MATLAB uses the standard logical operators && and || or~ notxor exclusive or

Summary – if family

The family of if structures allows the programmer to identify alternate computing paths dependent upon the results of conditional statements. ifelseelseif

Summary switch/case

Similar to the if/elseif/else structure Commonly used with menu

X=5 Y=1 X<Y another example X=[1, 2, 3, 4,5] Y=[-2,0,2,4,6] Use same sign

Selection structuresIf comparison statementsEndif G<50disp(‘G is a small value equal to :’)disp(G);end

if /elseif/ else The elseif function allows you to check

multiple criteria while keeping the code easy to read.

eg

Example for “If”, command% example for if statementday = 1if day== 7 % saturday state = 'weekend'elseif day ==1 % sunday state = 'weekend'else state= 'weekday'end

Switch and Case The above structure is often used when a series of programming

path options exists for a given variable, depending on its value. The above structure can be used either a scalar ( a number) or a

string.switch variablecase option 1 code to be executed if variable equal to option 1case option 2 code executed if variable equal to option 2OtherwiseIf none of the optionend

For loop

% To understand For Loop% For loop will do iterations only the fixed number of timesclcclear SumNumbers =0 ;for I = 1:10 SumNumbers = SumNumbers+ Iend

Exercise

Convert the values from degree to radian from 10 degree to 360 degree with an increment of 10.

Develop the table for the results. Formula for Radian = (Value in Degree X Pi)/180

Example for loop

for k= 1:5a(k)=k^2end

while loop% To understand while loop% while loop is an iteration loop% It will keep on going round and round until the condition is satisfied.% This loop is used, when we dont know the exact number of iterations.

clcclearNumber =50 while Number > 20 Number = Number -5 end

Some Useful MATLAB commands what List all m-files in current directory dir List all files in current directory ls Same as dir type test Display test.m in command window delete test Delete test.m cd a: Change directory to a: chdir a: Same as cd pwd Show current directory which test Display directory path to ‘closest’

test.m

MATLAB Logical Operators

MATLAB supports three logical operators.

not ~ % highest precedenceand & % equal precedence with oror | % equal precedence with and

break and continue

break causes the loop to terminate prematurely

continue causes MATLAB to skip a pass through the loop, but continue on until the criteria for ending is met

both are used in conjunction with an if statement

This program prompts the user 10 times to enter a value, and computes the natural log.

If a negative value is entered, the break command causes MATLAB to exit the loop

Notice that n had a value of 3 when the program terminated – if it had run to completion it would have had a value of 10

The continue command is similar to break, however instead of terminating the loop, the program just skips to the next pass.

Matlab Selection Structures An if - elseif - else structure in MATLAB. Note that elseif is one word.

if expression1 % is true% execute these commands

elseif expression2 % is true% execute these commands

else % the default% execute these commands

end

MATLAB Repetition Structures

A for loop in MATLAB for x = array for X = 1:100 disp X

end

A while loop in MATLAB while expression while x <= 10

% execute these commands end