IF statements with zero

Does matlab have a problem with the number zero? I am learning the basics and trying out an if statement in an m-file called sorttemp to sort ones and zeros in a vector to become either 14 or 19 , but why are all my answers 19...?

EDU>> clear
EDU>> a = [ 0 0 1 1 0 0 ]

a =

      0    0    1    1    0    0

EDU>> sorttemp

b =

      19    19    19    19    19    19

EDU>> 

%sorttemp
n = 1:6;

if a(1,n) <= 0.5;
    b(1,n) = 14;
else b(1,n) = 19

[Fangjun Jiang]

You can use {}Code format next time.

[Lisa Collins]

Thanks to all who answered, I appreciate it! I now realise the if statement looks at the whole thing not individual elements... I am going to make this more complicated now and develop this so when the sequence goes 0 to 1 the answer is 16. I assumed that b is unknown but accept that it could be set up to have a default value of all 19's.

Answer 1

[Wayne King]

Hi Lisa,

   a = [ 0 0 1 1 0 0 ];
   b = [ 19    19    19    19    19    19];

   for n = 1:6
   if (a(n) <=0.5)
       b(n) = 14;
   else 
       b(n) = 19;
   end
   end

but more efficient in MATLAB is simply:

b(a<=0.5) = 14;

[Wayne King]

in other words, try

b = [19 19 19 19 19 19];

a = [ 0 0 1 1 0 0 ];

b(a<=0.5) = 14;

Answer 2

[Jan]

The actual question is: Why are all b set to 19.

n = 1:6;
if a(1,n) <= 0.5

This means explicitely:

if [0, 0, 1, 1, 0, 0] <= 0.5

Now Matlab performs the comparison elementwise:

if [true, true, false, false, true, true]

Finally if needs a single value to decide for a branch, therefore in includes an all automatically:

if all([true, true, false, false, true, true])

No, not all of these values are true, therefore if uses the else branch:

   b(1, 1:6) = 19

All b are set to 19.

NOTE: To be exact if inserts: all(condition) && ~isempty(condition).