MatLab

1
MatLab Palm Chapter 4, Part 3For and While
Loops

• Class 11.1 Palm Ch 4.5

2
RAT 11.1

• As in INDIVIDUAL you have 2 minutes to answer the
  following question.
• What is the FINAL value assigned to y in the
  following segment of MATLAB code?
• x 0
• while x lt 5
• y sqrt(x)
• x x 1
• end
• When asked, you have 30-seconds to submit your
  paper.
• Answer y 2

3
Learning Objectives

• Students should be able to
• Understand Matlab iteration structuresfor end
  Loops
• while end Loops
• Identify Implied loops
• Recognize these structures as repetition (or
  iteration) structures.
• Develop flow (logic) diagrams for combinations of
  branching and looping structures

4
LOOPS

• A LOOP is a structure for repeating (iterating) a
  block of code.
• MATLAB uses two types of loops.
• for loop number of passes known a priori (p.
  210).
• while loop loop terminates when a specified
  condition is satisfied (p. 221).

5
for LOOPS

• SYNTAX
• for k startstepstop
• block
• end
• start initial value
• step incremental value
• stop terminating value
• See p. 212 for flowchart of for loop.

Indentation Mandatory!
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for LOOP FLOW CHART
Loop structure
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EXAMPLE

• for k 51035
• x k2
• end

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for end Loops (p. 211)

• for k 51035
• x k2
• end
• Means
• starting with k 5, execute the statements
  between for end
• then increment k by 10 and execute the statements
  with k 15 etc. until k gt 35.
• Then exit the loop (k? at this point)

9
for end Loops contd.

• Step value may be negative (default is positive
  1)
• For step gt 0, loop does not execute if start gt
  stop
• For step lt 0, loop does not execute if start lt
  stop
• If start stop, loop executes once
• Non-integer step values give unpredictable
  results (dont use them!)

10
INDIVIDUAL EXERCISE (5 min.)

• Evaluate xn sin (np/10) for n between 1 and 10
  (in increments of 1).
• Write a Matlab PROGRAM.
• (Plot x versus n)
• Save file on R\ and name file after an
  historical figure.

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SOLUTION

• for n 110
• x(n) sin(npi/10)
• end
• plot(110,x) or
• plot(x)
• title(In-class
• example)
• xlabel(n)
• ylabel(x(n))

12
for Loops

• for loops can be nested (one inside each other).
• Each loop is closed with an end.
• EXAMPLE Proper indention is a MUST!!
• N 10
• for I 1N
• for J 1N
• A(I,J) 1/(I J 1)
• end
• end
• Why is I OK and i not OK?

13
for Loops

• for loops can be nested (one inside each other).
• Each loop is closed with an end.
• EXAMPLE Proper indention is a MUST!!
• N 10
• for r 1N
• for c 1N
• A(r,c) 1/(r c 1)
• end
• end
• Using r (for row) and c (for col) is MUCH
  better!!!

14
In-class Assignment 11.1 (20 min.)

• Work through the nested loop and if example on
  page 212.
• Write an program file to solve T4.5-1 (p. 213)
• Save your solutions on the R\ drive
• Name your files after an animal.

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Solution

• PROGRAM example1.m
• Solution to T4.5-1 on p. 213 of the Palm
• Matlab book.
• S. Socolofsky
• ENGR 111A 501-503
• for r 14
• for c 13
• A(r,c) 4 (r-1)6 (c-1)4
• end
• end

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The break Statement

• You can EXIT out of any looping structure with a
  break (Both for and while)
• y0
• x1 -2 3 -4 5
• for k 1length(x)
• if x(k) lt 0
• break
• end
• y y sqrt(x(k))
• end
• -- Jump to here on break

17
The continue Statement

• You can skip bad values in any looping
  structure with a continue
• y0
• x1 -2 3 -4 5
• for k 1length(x)
• if x(k) lt 0
• continue
• end
• y y sqrt(x(k))
• end Jump to here on continue

18
Implied Loops (p. 216)

• You can use built-in loops instead of for end
  loops in most cases. Sometimes it is better,
  sometimes not.
• Note the matrix example with simple logic
• Note the for end example with more complex
  logic
• Note the find function example for a very normal
  situation that you once upon a time had to write
  code for every time.

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Implied Loops (cont.)

• Many MatLab commands contain implied loops.
• Example
• x 05100
• y cos(x)
• Example
• y find(xgt0)
• See p. 216 for equivalent loops for these.

20
while LOOPS

• The while loop terminates because a specified
  condition is satisfied.
• The number of passes is NOT known a priori
• SYNTAX
• while logical expression
• statements
• end
• See p. 223 for flowchart of while loop.

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while LOOP FLOW CHART
Loop structure
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while end Loops

• Look at the examples on page 221.
• x 5
• while x lt 25
• disp(x)
• x 2x-1 -make sure x changes
• end
• Means
• starting with x 5, execute the statements
  between the while end this displays x and
  calculates a new value of x and loops back.
• Then exit the loop when x exceeds or equals 25.

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while end Loops (p.222)

• THE LOOP VARIABLE MUST HAVE A VALUE BEFORE THE
  while STATEMENT IS EXECUTED
• THE LOOP VARIABLE MUST BE CHANGED SOMEHOW BY THE
  STATEMENTS
• Note the usual mistakes toward the bottom of the
  page.
• Make sure that you save your code before testing
  any while end loop.

24
INDIVUDAL EXERCISE (5 min.)

• Write an equivalent while loop for the implied
  loop shown
• x 05100
• y sin(x)
• disp (y)
• Save on the R\ drive
• Name file after a city outside of Texas

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SOLUTION

• x 0
• while x lt 100
• y sin(x)
• x x5
• disp(y)
• end

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NESTED while LOOPS

• while loops can also be nested.
• Each loop must be closed with an end.
• break statements may be used to jump out of loops.

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EXAMPLE

• r 1 N 10
• while r lt N
• c 1
• while c lt N
• A(r,c) 1/(r c -1)
• c c 1
• end
• r r 1
• end - note use of r,c instead of i,j.

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Assignment 11.1

• Individual assignment.
• Due Nov 15, 2004
• Palm MATLAB
• Chapter 4 22 (use for-loop in part b), 28 (use
  a while-loop), T4.5-4 (p. 225)
• Extra Problem (see assignments web page)!
• Extra Credit (see assignments web page)!!
• Read Palm section 4.7.