Asked by dcydhb dcydhb
on 13 May 2019

i want to use a loop of omega and when the omega has diffreent value ,the m0,m1,m2...have the different value,so in the loop i use the 'case' as this

for omega=0.1:0.1:2;

switch omega

case 0.100000;

m0=0.031949;

m1=3.141268;

m2=6.283023;

m3=9.424670;

m4=12.566289;

m5=15.707898;

m6=18.849502;

m7=21.991102;

m8=25.132701;

m9=28.274298;

m10=31.415894;

case 0.200000;

m0=0.063931;

m1=3.140293;

m2=6.282536;

m3=9.424345;

m4=12.566046;

m5=15.707703;

m6=18.849339;

m7=21.990963;

m8=25.132579;

m9=28.274190;

m10=31.415797;

and there are still some codes,but in the result,when omega=0.2,it still use the m0,m1,m2...of case 0.100000;

so when i change it as

case abs(omega-0.200000)<0.01;

it still has the problem,and why,thanks a lot!!!

Answer by Stephen Cobeldick
on 13 May 2019

Edited by Stephen Cobeldick
on 13 May 2019

Accepted Answer

Your first (inadvisable, numerically fragile, should-be-avoided) concept works for me:

for omega = 0.1:0.1:2;

switch omega

case 0.1

disp('one')

case 0.2

disp('two')

end

end

displays this in the command window:

one

two

Because of floating pont error in those numbers, this would be an extremely unreliable way to write code. Do NOT do this!

Your second method can be done quite easily, by simply thinking about the actual case value that you actually want switch to compare (hint: your case conditions are equal to true or false, but omega is not equal to true or false):

for omega = 0.1:0.1:2;

switch true % this is the value for a valid CASE.

case abs(omega-0.1)<0.01;

disp('one')

case abs(omega-0.2)<0.01;

disp('two')

end

end

displays this in the command window:

one

two

dcydhb dcydhb
on 13 May 2019

i have accepted as it is wonderful and i was curious that

switch true % this is the value for a valid CASE.

case abs(omega-0.1)<0.01;

disp('one')

you must use the

switch true % this is the value for a valid CASE.

rather than

switch omega

why?

Walter Roberson
on 13 May 2019

When you use

switch numeric_value

case another_numeric_value

then the test is

if numeric_value == another_numeric_value

So let us follow that through with your proposed version with switch omega:

if omega == (abs(omega-0.1) < 0.01)

The right hand side is a logical value, false or true, convertable to numeric 0 or 1, and you want to compare that to omega. That can only possibly succeed if omega is 0 or 1 . In the case where omega is 1 then abs(1-0.1) is 0.9 and < 0.01 of that is false. In the case where omega is 0, then abs(0-0.1) is 0.1 and < 0.01 of that is false. Therefore switch omega cannot work.

Let us instead follow through with switch true:

if true == (abs(omega-0.1) < 0.01)

which can be true if omega is in the range approximately 0.1-0.01 to 0.1+0.01

dcydhb dcydhb
on 13 May 2019

great explanation and thank you all a lot!!!

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## Stephen Cobeldick (view profile)

Direct link to this comment:https://de.mathworks.com/matlabcentral/answers/461708-the-problem-of-using-case#comment_704501

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