MATLAB: Help on if statement

How can I use a if loop to display where X4(n,1)>X4(n,2)>X4(n,3)>X4(n,4)>X4(n,5) == 1 n = 1:31 I want it to show where this is true in the matrix or should i use a while loop?

Best Answer

You can vectorize this in one command:

idx = 1:31;
find( (X4(idx,1)>X4(idx,2))&(X4(idx,2)>X4(idx,3))&(X4(idx,3)>X4(idx,4))&(X4(idx,4)>X4(idx,5))&(X4(idx,5)==1))

HTH

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MATLAB: If statement

When I uncomment the lines as you mentioned, I still do not get an error:

phi_an=0.5;
phi_ca=0.80;
am=(phi_ca+phi_an)/2;
if (am>0) && (am<=1)
  lambda_m=0.043+17.81*am-39.85*am^2+36*am^3;
elseif (am>1) && (am<3)
  lambda_m=14+1.4*(am-1);
end
if (lambda_m<2)
  Dy=1E-6;
elseif (lambda_m>=2) && (lambda_m<=3)
  Dy=1E-6*(1+2*(lambda_m-2));
elseif (lambda_m>3) && (lambda_m<4.5)
  Dy=1E-6*(3-1.67*(lambda_m-3));
elseif (lambda_m>=4.5)
  Dy=1.25E-6;
end

All is fine.

MATLAB: If statement

There are two problems with your code:

"==" doesn't work well with floating point precision (EDIT: understatement)
"if (final(1,:) == final(2,:))" should read "if all(final(1,:) == final(2,:))"

--- EDIT ---

Make sure you read the link provided by Walter and Floating points by Cleve Moler. The simple rule is: never test if two floating point values are equal.

--- EDIT ---

The internal representation and what is displayed differ. With Matlab the display format can be controlled with the function, FORMAT. With format('hex') the full internal precision is shown. Try

>> v1 = 1/3;
>> v2 = 0.333;
>> v3 = 0.3333;
>> format('hex')
>> v1
v1 =
   3fd5555555555555
>> v2
v2 =
   3fd54fdf3b645a1d
>> v3
v3 =
   3fd554c985f06f69
>> 
>> format('short')

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MATLAB: If statement

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