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8/9/2019 If the Logical Expression Evaluates to a Nonscalar Value, All the Elements of the Argument Must Be Nonzero. for Ex

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his group of control statements enables you to select at run-time which block of code is executed. To

make this selection based on whether a condition is true or false, use the if statement (which may

include else or elseif). To select from a number of possible options depending on the value of an

expression, use the switch and case statements (which may include otherwise).

You cannot define nested functions within a conditional control block. Nested functions must always be

defined at the top level of a function.if, else, and elseif

if evaluates a logical expression and executes a group of statements based on the value of theexpression. In its simplest form, its syntax is

if logical_expression

statementsend

If the logical expression is true (that is, if it evaluates to logical 1), the MATLAB software executes allthe statements between the if and end lines. It resumes execution at the line following the end

statement. If the condition is false (evaluates to logical 0), MATLAB skips all the statements between

the if and end lines, and resumes execution at the line following the end statement.

For example,

if rem(a, 2) == 0

disp('a is even')

b = a/2;

end

You can nest any number of if statements.

If the logical expression evaluates to a nonscalar value, all the elements of the argument must be

nonzero. For example, assume X is a matrix. Then the statement

if X

statements

end

is equivalent to

if all(X(:))statements

end

The else and elseif statements further conditionalize the if statement:

*

The else statement has no logical condition. The statements associated with it execute if the

preceding if (and possibly elseif condition) evaluates to logical 0 (false).


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*

The elseif statement has a logical condition that it evaluates if the preceding if (and possibly elseif

condition) is false. The statements associated with it execute if its logical condition evaluates to logical1 (true). You can have multiple elseif statements within an if block.

if n < 0 % If n negative, display error message.disp('Input must be positive');

elseif rem(n,2) == 0 % If n positive and even, divide by 2.

A = n/2;else

A = (n+1)/2; % If n positive and odd, increment and divide.

end

if Statements and Empty Arrays. An if condition that reduces to an empty array represents a false

condition. That is,

if A

S1

elseS0

end

executes statement S0 when A is an empty array.

switch, case, and otherwise

switch executes certain statements based on the value of a variable or expression. Its basic form is

switch expression (scalar or string)

case value1statements % Executes if expression is value1

case value2

statements % Executes if expression is value2.

.

.

otherwisestatements % Executes if expression does not

% match any case

end

This block consists of

*

The word switch followed by an expression to evaluate.*

Any number of case groups. These groups consist of the word case followed by a possible value for


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the expression, all on a single line. Subsequent lines contain the statements to execute for the given

value of the expression. These can be any valid MATLAB statement including another switch block.

Execution of a case group ends when MATLAB encounters the next case statement or the otherwise

statement. Only the first matching case is executed.*

An optional otherwise group. This consists of the word otherwise, followed by the statements toexecute if the expression's value is not handled by any of the preceding case groups. Execution of the

otherwise group ends at the end statement.

*

An end statement.

switch works by comparing the input expression to each case value. For numeric expressions, a casestatement is true if (value==expression). For string expressions, a case statement is true if

strcmp(value,expression).

The code below shows a simple example of the switch statement. It checks the variable input_num for

certain values. If input_num is -1, 0, or 1, the case statements display the value as text. If input_num is

none of these values, execution drops to the otherwise statement and the code displays the text 'othervalue'.

Abram

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anggiswitch input_numcase -1disp('negative one');case 0

disp('zero');

case 1

disp('positive one');

otherwisedisp('other value');

end

Note For C programmers, unlike the C language switch construct, the MATLAB switch does not

"fall through." That is, if the first case statement is true, other case statements do not execute.Therefore, break statements are not used.

switch can handle multiple conditions in a single case statement by enclosing the case expression in a

cell array.

switch varcase 1disp('1')

case {2,3,4}

disp('2 or 3 or 4')case 5

disp('5')

otherwisedisp('something else')

end

Back to Top of Page Back to TopLoop Control for, while, continue, break

With loop control statements, you can repeatedly execute a block of code, looping back through theblock while keeping track of each iteration with an incrementing index variable. Use the for statement

to loop a specific number of times. The while statement is more suitable for basing the loop execution

on how long a condition continues to be true or false. The continue and break statements give you morecontrol on exiting the loop.

You cannot define nested functions within a loop control block. Nested functions must always be


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defined at the top level of a function.

Note You can often speed up the execution of MATLAB code by replacing for and while loops

with vectorized code. See Techniques for Improving Performance for more information on this.

for

The for loop executes a statement or group of statements a predetermined number of times. Its syntax is

for index = start:increment:endstatements

end

The default increment is 1. You can specify any increment, including a negative one. For positiveindices, execution terminates when the value of the index exceeds the end value; for negative

increments, it terminates when the index is less than the end value.

For example, this loop executes five times.

for n = 2:6x(n) = 2 * x(n - 1);

end

You can nest multiple for loops.

for m = 1:5

for n = 1:100A(m, n) = 1/(m + n - 1);

end

end

Note You can often speed up the execution of MATLAB code by replacing for and while loops

with vectorized code. See Vectorizing Loops for details.

Using Arrays as Indices. The index of a for loop can be an array. For example, consider an m-by-n

array A. The statement

for k = A

statements

end

sets k equal to the vector A(:,i), where i is the iteration number of the loop. For the first loop iteration, k

is equal to A(:,1); for the second, k is equal to A(:,2); and so on until k equals A(:,n). That is, the loopiterates for a number of times equal to the number of columns in A. For each iteration, k is a vector

containing one of the columns of A.

while

The while loop executes a statement or group of statements repeatedly as long as the controlling

expression is true (1). Its syntax is


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while expression

statements

end

If the expression evaluates to a matrix, all its elements must be 1 for execution to continue. To reduce a

matrix to a scalar value, use the all and any functions.

For example, this while loop finds the first integer n for which n! (n factorial) is a 100-digit number.

n = 1;

while prod(1:n) < 1e100

n = n + 1;

end

Exit a while loop at any time using the break statement.

while Statements and Empty Arrays. A while condition that reduces to an empty array represents a

false condition. That is,

while A, S1, end

never executes statement S1 when A is an empty array.continue

The continue statement passes control to the next iteration of the for or while loop in which it appears,

skipping any remaining statements in the body of the loop. In for loops, the loop counter is incrementedby the appropriate value (either 1 or the specified step value) at the start of the next iteration.

continue works the same way in nested loops. That is, execution continues at the beginning of the loopin which the continue statement was encountered.

The example below shows a continue loop that counts the lines of code in the file, magic.m, skippingall blank lines and comments. A continue

statement

Abram


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angga

anggiis used to advance to the next line in magic.m

without incrementing the count whenever a blank line or comment line is encountered.

fid = fopen('magic.m','r');count = 0;

while ~feof(fid)

line = fgetl(fid);if isempty(line) || strncmp(line,'%',1) || ~ischar(line)

continue

endcount = count + 1;

end

fprintf('%d lines\n',count);

fclose(fid);

break

The break statement terminates the execution of a for loop or while loop. When a break statement is

encountered, execution continues with the next statement outside of the loop. In nested loops, break

exits from the innermost loop only.

The example below shows a while loop that reads the contents of the file fft.m into a MATLAB

character array. A break statement is used to exit the while loop when the first empty line is

encountered. The resulting character array contains the help for the fft program.

fid = fopen('fft.m','r');

s = '';

while ~feof(fid)


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line = fgetl(fid);

if isempty(line) || ~ischar(line)

break

ends = sprintf('%s%s\n', s, line);

end

disp(s);

fclose(fid);

Back to Top of Page Back to Top

Error Control try, catch

Error control statements provide a way for you to take certain actions in the event of an error. Use thetry statement to test whether certain commands in your code generates an error. If an error does occur

within the try block, MATLAB immediately jumps to the corresponding catch block. Use the catch part

of the statement to respond in some way to the error.

You cannot define nested functions within a try-catch block. Nested functions must always be defined

at the top level of a function.try and catch

The general form of a try-catch statement sequence is

try

statement

...statement

catch meObj

statement...

statement

end

In this sequence, the statements in the try block (that part of the try-catch that follows the word try

statement, and precedes catch) between try and catch execute just like any other program code. If an

error occurs within the try section The statements between catch and end are then executed. Examinethe contents of the MException object meObj to see the cause of the error. If an error occurs between

catch and end, MATLAB terminates execution unless another try-catch sequence has been established.


Program Termination return

Program termination control enables you to exit from your program at some point prior to its normal

termination point.

return

After a MATLAB function runs to completion, it terminates and returns control either to the function

that called it, or to the keyboard. If you need to exit a function prior to the point of normal completion,


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you can force an early termination using the return

funAbram

angga

anggiction. return immediately terminates the

current sequence of commands and exits the currently running function.

return is also used to terminate keyboard mode.


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