a = b<0

What is the meaning of this line?

...
a = b<0;
c = 3*a;
...

Is a IF condition?

Answer 1

[Voss]

a = b<0;

checks if b is less than 0 and stores the result in a.

If b is a scalar: if b is less than 0, then a will be true; otherwise a will be false.

If b is a non-scalar array: a will be a logical array the same size as b, with each element being true or false, depending on whether the corresponding element in b is less than 0 or not.

Examples:

b = 2;
a = b<0 % false

a = logical
   0

b = -2;
a = b<0 % true

a = logical
   1

b = 0;
a = b<0 % false

a = logical
   0

b = randn(3) % non-scalar array

b = 3×3

0.2296 2.5457 -0.5802 -0.3745 -0.5770 0.2825 -0.8004 0.7450 -1.0347

a = b<0 % 3-by-3 logical array, true where b<0 and false elsewhere

a = 3×3 logical array
   0   0   1
   1   1   0
   1   0   1

Then the next line

c = 3*a;

multiplies a by 3 and stores it in c, so c will be an array the same size as a, with the value 3 where a is true and the value 0 where a is false.

Examples:

b = 2;
a = b<0; % false
c = 3*a

c = 0

b = -2;
a = b<0; % true
c = 3*a

c = 3

b = 0;
a = b<0; % false
c = 3*a

c = 0

b = randn(3) % non-scalar array

b = 3×3

0.0840 0.2037 -0.9633 -0.7413 -1.2592 0.3485 -2.1917 0.0558 -2.0495

a = b<0 % 3-by-3 logical array, true where b<0 and false elsewhere

a = 3×3 logical array
   0   0   1
   1   1   0
   1   0   1

c = 3*a

c = 3×3

0 0 3 3 3 0 3 0 3

Answer 2

[Torsten]

"a" is a logical variable.

It is set to "true" (=1) if b<0, else it is set to "false" (=0).

The next operation c = 3*a works as if "a" was a double variable.

Thus c = 3*1 = 3 if "a" was "true", else c = 3*0 = 0 if "a" was "false".