math - Using atan2 to find angle between two vectors -

i understand that:

atan2(vector.y, vector.x) = angle between vector , x axis.

but wanted know how angle between two vectors using atan2. came across solution:

atan2(vector1.y - vector2.y, vector1.x - vector2.x) 

my question simple:

will 2 following formulas produce same number?

• atan2(vector1.y - vector2.y, vector1.x - vector2.x)

• atan2(vector2.y - vector1.y, vector2.x - vector1.x)

if not: how know vector comes first in subtractions?

thanks

 atan2(vector1.y - vector2.y, vector1.x - vector2.x) 

is angle between difference vector (connecting vector2 , vector1) , x-axis, problably not meant.

the (directed) angle vector1 vector2 can computed as

angle = atan2(vector2.y, vector2.x) - atan2(vector1.y, vector1.x); 

and may want normalize range 0 .. 2 * pi:

if (angle < 0) angle += 2 * m_pi;