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Control Statements

ELEC 206

Prof. Siripong Potisuk

Objectives

Learn how to change the flow of execution of a

MATLAB program through some kind of a

decision-making process within that program

The program makes these decisions by comparing

values of some variables

The comparison is done by using a combination of

relational & logical operators

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Controlling Flow of Execution

Constructs that allow MATLAB to decide whether

or not to execute some code that follows the

statement or to repeat certain groups of statements

Repetitions or loops:

1) for-end 2) while-end

Decision-making:

1) if-end 2) if-else-end

3) if-elseif-else-end 4) Switch case

Jumping or Skipping:

1) break 2) continue

Flow Chart

A diagram that shows the step-by-step flow of

execution.

It is particularly useful for showing how control

statements work.

Some common flowchart symbols are

represents a sequence of commands

represents an if-statement

shows the direction of code execution4

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Repetitions or Loops

A loop executes one set of commands repeatedly.

MATLAB has two ways to control number of

times loop executes commands

- Method 1: for..end loop

executes commands a specified number of times

- Method 2: while..end loop

executes commands as long as a specified

expression is true

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Loop index variable can have any variable name (usually

i, j, k, m, and n are used)

Avoid using i and j when working with complex numbers.

In general, loop body should not change value of k

Each for command must have an end command

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1. The loop sets k to f, and executes the loop body, i.e.,

commands between the for and the end commands,

2. The loop then sets k to f+s and re-executes the body

3. The process repeats itself until k>t,at which point

the program continues with commands that follow the end command

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f and t are usually integers while s usually omitted.

If omitted, loop uses an increment of 1

The increment s can be negative

e.g., k=25:–5:10

produces four passes with k = 25, 20, 15, 10

If f=t, the loop executes once

If f>t and s>0, or if f<t and s<0,loop not

executed

Value of k not displayed automatically. To display

the value in each pass, type k as one of commands in

loop body (sometimes useful for debugging)

When loop ends, k has the value last assigned to it

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If k, s, and t are such that the last value of k

cannot be equal to t, then

- for s positive, the last pass is one where k has

largest value smaller than t.

For example, k=8:10:50

produces five passes (k = 8,18,28,38,48)

- for s negative, last pass is one where k has

smallest value larger than t

k can also be assigned specific values (typed as

a row vector)For example: for k = [7 9 –1 3 3 5]

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EXAMPLE: Show the output of the following script

for k=1:3:10

k

x = k^2

end

fprintf('After loop k = %d\n', k);

Output: k = 1

x = 1

k = 4

x = 16

k = 7

x = 49

k = 10

x = 100

After loop k = 10

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EXAMPLE: Write a script file using a for-end loop

to calculate the sum of the first n terms of the series:

for n = 4 and 20.

n = input(‘Enter the # of terms: ’)

sum = 0;

for k=1:n

sum = sum+(-1)^k*k/2^k;

end

fprintf(‘The total sum is %f \n', sum)

Output: Enter the # of terms: 4

The total sum is -0.125000

n

kk

k k

1 2

(-1)

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The while-end loop used when- number of loop iterations not known

- a testable condition to stop looping when it is false.

Examples of test condition:- keep reading data from a file until the end is reached

- keep adding terms to a sum until the difference of the

last two terms is less than a certain amount

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1. Loop starts by evaluating conditional expression

2. If true, executes code in the loop body, then goes

back to step 1

3. If false, skips code in the loop body and continue program execution with the code after the end

statement

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The conditional expression must have a variable

which changes its value after each pass

There must be some value of the variable that

makes the conditional expression false

EXAMPLE: The script x = 1

while x <= 15

x = 2*x

end

outputs x = 1

x = 2

x = 4

x = 8

x = 16

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EXAMPLE: The sequence of Fibonacci numbers F [n] is a

sequence of integers starting with 0 and 1, i.e., F [1] = 0 and

F [2] = 1. The next number in the sequence is the sum of the

previous two numbers, i.e., 0 1 1 2 3 5, 8, 13, …..

(a) Derive a recursive equation that can be used to compute

the sequence.

(b) Write a script file using a while-end loop to compute the

first 25 numbers in the sequence.

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(a) F[n] = F [n 1] + F [n 2], n 3

(b) N = input(‘Enter the # of terms: ’)

F(1) = 0; F(2) = 1;

k = 3;

while k <= N

F(k) = F(k-1)+F(k-2);

k = k+1;

end

fprintf(‘The first %d terms of the

Fibonacci sequence is \n',N)

fprintf(‘%d \n’, F)

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Output:Enter the # of terms: 25

The first 25 terms of the Fibonacci

sequence is

0 1 1 2 3

5 8 13 21 34

55 89 144 233 377

610 987 1597 2584 4181

6765 10946 17711 28657 46368

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An infinite loop (also known as indefinite

loop) happens if the conditional expression

never becomes false, the loop will keep

executing... forever!

If there is no output from the loop (as if often

the case), it will look like MATLAB has

stopped responding

If your program gets caught in an indefinite

loop, activate (i.e., put the cursor in) the

Command Window and press CTRL+C

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Common Causes of Indefinite Loops

1. No variable in conditional expression

d1 = 1; d2 = 10; d3 = 0;

while d1 < d2

fprintf('Distance = %d\n', d3);

end

2. Variable in conditional expression never changes

min_D = 42; D_Increment = 0;

D = 0;

while D < min_D

D = D + D_Increment;

end

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d1 and d2 never change

Typo – should be 10

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3. Wrong variable in conditional expression changed

min_D = 42; delta = 10;

D = 0;

while D < min_D

min_D = min_D + delta;

end

4. Conditional expression never becomes false

Min_D = 42; x = 0; y = 0;

while -sqrt(x^2+y^2) < min_D

x = x + 1;

y = y + x;

end

Typo – should be D

Typo – shouldn't be any negative

sign in front of sqrt

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Conditional Statements

A construct that allows MATLAB to decide

whether or not to execute some code that follows

the statement

Four general forms

– if-end

– if-else-end

– if-elseif-else-end

– switch case

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Conditional expression true MATLAB execute code segment between the if and end statement

conditional expression false MATLAB skips the code segment between the if and end statement

Then, it continues executing the code after the end-line

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This construct executes one section of code if a

condition is true and a different section of code if

it is false.

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if-elseif-else-end structure chooses

one of three or more sections of code to execute

Can have as many elseif statements as

necessary

Can omit else statement

If omitted and no match to if- or elseif-

statements, no code in structure gets executed

An easier-to-read alternative structure to theif-elseif-else-end structure

Choose code to execute based on value of scalar

or string, not just true/false

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The switch-case structure

When the switch-expression is evaluated,

– If value is equal to value1, executes Group 1

commands then executes code after end

statement

– If value is equal to value2, same as above

but Group 2 commands only

– Etc.

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The switch-case structure

If switch-expression not equal to any of values in case statement, commands after otherwise

executed.

If otherwise not present, no commands

executed

If switch expression matches more than one case value, only first matching case executed

Comparisons of text strings (within single quotes)

are case-sensitive.

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Nesting

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If a loop or conditional statement is placed inside

another loop or conditional statement, the former

are said to be nested in the latter

Most common to hear of a nested loop, i.e., a loop

within a loop

– Often occur when working with two-

dimensional problems

Each loop and conditional statement must have an end statement

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The break Command

When inside a loop (for and while), break

terminates execution of loop

- MATLAB jumps from break to end

command of loop, then continues with next command (does not go back to the for or

while command of that loop).

- break ends whole loop, not just last pass

If break inside nested loop, only nested loop

terminated (not any outer loops)

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The break Command

break command in script or function file but

not in a loop terminates execution of file

break command usually used within a

conditional statement.

If in loops, it provides way to end looping if

some condition is met

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while( 1 )

name = input( 'Type name or q to quit:

', 's' );

if length(name) == 1 && name(1) == 'q'

break;

else

fprintf('Your name is %s\n', name);

end

end

Only way to exit loop!

Trick – "1" is always true so it makes loop iterate forever!

If user entered only one letter and it is a "q",

jump out of loop; Otherwise print name

The continue command

Use continue inside a loop (for- and while-) to

stop current iteration and start next iteration

continue usually part of a conditional statement

When MATLAB reaches continue it does not

execute remaining commands in loop but skips to the end command of loop and then starts a new

iteration

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for ii=1:100

if rem( ii, 8 ) == 0

count = 0;

fprintf('ii=%d\n',ii);

continue;

end

% code

% more code

end

Every eight iteration reset count to zero, print the

iteration number, and skip

the remaining computations

in the loop