MATLAB POLYSPACE RELEASE NOTES Manual de usuario descargar pdf (Pagina 141)

Flow Control

It is important to unde rstand how relational operators and if statements

work with matrices. When you want to check for equality betwe en two

variables, you might use

ifA==B,...

This is valid MATLAB code, and does w hat you ex pe ct when A and B are

scalars. But when

A and B are matrices, A==Bdoes not test if they are

equal, it tests where they are equal; the result is another m atrix of 0’s and

1’s showing element-by-element equality. (In fact, if

A and B are not the same

size, then

A==Bis an error.)

A = magic(4); B = A; B(1,1) = 0;

A==B

ans =

0111

1111

The proper way to check for equality between two variables is to use the

isequal function:

if isequal(A, B), ...

isequal

returns a scalar logical value of 1 (representing true)or0 (false),

instead of a matrix, as the expression to be evaluated by the

if function.

Using the

A and B matrices from above, you get

isequal(A, B)

ans =

Here is another example to emphasize this point. If A an d B are scalars, the

following program will never reach the “unexpected situation”. But for most

pairs of matrices, including our magi c squares with interchanged columns,

none of the matrix conditions

A>B, A<B,orA==Bis true for all elemen ts

and so the

else clause is executed:

if A > B

'greater'

4-3

1 2 ... 136 137 138 139 140 141 142 143 144 145 146 ... 239 240

Sin comentarios

MATLAB POLYSPACE RELEASE NOTES Manual de usuario Pagina 141