loops cs 103 february 13, 2009 author: nate hamm

LoopsLoops

CS 103CS 103

February 13, 2009February 13, 2009

Author: Nate Hamm

What is a lWhat is a loooop?p?

A control construct that allows for a block A control construct that allows for a block of code to be repeatedof code to be repeated

In Matlab, loops come in two forms:In Matlab, loops come in two forms:• FOR loopsFOR loops• WHILE loopsWHILE loops

FOR LoopsFOR Loops

Syntax:Syntax:

FOR variable = expressionFOR variable = expression

statementsstatements

ENDEND

FOR LoopsFOR Loops

FOR FOR variablevariable = expression = expressionstatementsstatements

ENDEND

The The variablevariable is known as an is known as an indexindex, and can be , and can be treated as a variable by the program.treated as a variable by the program.

Traditionally, the letter i is used as the index, Traditionally, the letter i is used as the index, but in Matlab, i also is used to represent but in Matlab, i also is used to represent imaginary numbers, so we use ii or another imaginary numbers, so we use ii or another letter.letter.

FOR LoopsFOR Loops

FOR variable = FOR variable = expressionexpression


ENDEND

In FOR loops, the In FOR loops, the expressionexpression is a counting is a counting mechanism. We typically use the colon mechanism. We typically use the colon operator to define how many times the operator to define how many times the loop should run. For example, 1:10 means loop should run. For example, 1:10 means to run the loop 10 times (from one to ten)to run the loop 10 times (from one to ten)

FOR LoopsFOR Loops

FOR variable = expressionFOR variable = expression


ENDEND

The The statementsstatements are the set of code which is are the set of code which is to be executed over and over again during to be executed over and over again during the loop.the loop.

Every loop needs to conclude with an Every loop needs to conclude with an ENDEND

Example: Count to 10Example: Count to 10

FOR ii = 1:10FOR ii = 1:10

fprintf(‘\n Count: %d’, ii)fprintf(‘\n Count: %d’, ii)

ENDEND


FOR ii = 1:10FOR ii = 1:10


ENDEND

Before the loop, ii doesn’t exist


FOR ii = 1:10FOR ii = 1:10


ENDEND

First time through,

ii = 1


FOR ii = 1:10FOR ii = 1:10


ENDEND

First time through, ii = 1

Now we print Count: 1


FOR ii = 1:10FOR ii = 1:10


ENDEND

First time through, ii = 1


The loop ends, so we’ll return to the top of the loop


FOR ii = 1:10FOR ii = 1:10


ENDEND

Second time through, so ii is incremented

ii = 2


FOR ii = 1:10FOR ii = 1:10


ENDEND

Second time through, ii = 2



FOR ii = 1:10FOR ii = 1:10


ENDEND

Third time through,

ii = 3


FOR ii = 1:10FOR ii = 1:10


ENDEND

Third time through, ii = 3


This will continue for awhile…


FOR ii = 1:10FOR ii = 1:10


ENDEND

Imagine we’re now on the 9th iteration

Ninth time through, ii = 9




FOR ii = 1:10FOR ii = 1:10


ENDEND

Tenth time through,

ii = 10

This will be the last time we go through the loop!


FOR ii = 1:10FOR ii = 1:10


ENDEND

Tenth time through, ii = 10



FOR ii = 1:10FOR ii = 1:10


ENDEND

Tenth time through, ii = 10


We’ve run the loop 10 times….because that’s as far as we have been asked to go, the loop ends and we’ll go on to whatever code exists after the end statement.

Notes About FOR LoopsNotes About FOR Loops

Used when the required values of Used when the required values of your index variable are known before your index variable are known before the loop beginsthe loop begins

Though not necessarily before the Though not necessarily before the program begins.program begins.

Example: count to 10, add 12 Example: count to 10, add 12 numbers, calculate pi to 30 decimal numbers, calculate pi to 30 decimal places, etc.places, etc.

FOR Loop ExampleFOR Loop Example Suppose we wanted to write a program that Suppose we wanted to write a program that

prints out all the positive numbers in an array, xprints out all the positive numbers in an array, x X = [-2 4 6 -7 3 5 -45 1 -.5 2.6]X = [-2 4 6 -7 3 5 -45 1 -.5 2.6]

for ii = 1:10for ii = 1:10if x(ii) > 0if x(ii) > 0

fprintf(‘%f \n’, x(ii))fprintf(‘%f \n’, x(ii))endend

endend

But what happens if instead, we ask the user to But what happens if instead, we ask the user to input x first? Will we know how many elements input x first? Will we know how many elements are in x?are in x?

Working Around the Number Working Around the Number LimitationLimitation

When working with an array of When working with an array of numbers that may change, you can numbers that may change, you can use the length function.use the length function.

FOR ii = 1:length(x) FOR ii = 1:length(x)

FOR Loop ExampleFOR Loop Example

Ask the user to input an array, x, and then Ask the user to input an array, x, and then print out all the positive numbers in the print out all the positive numbers in the array.array.

X = input(‘Please give me an array: ‘)X = input(‘Please give me an array: ‘)FOR ii = 1:length(x)FOR ii = 1:length(x)

if x(ii) > 0if x(ii) > 0fprintf(‘%f \n’, x(ii))fprintf(‘%f \n’, x(ii))

endendENDEND

Questions?Questions?

loops cs 103 february 13, 2009 author: nate hamm

Documents