Are L-- and L=L-1 the same?

I assumed L-- and L=l-1 to be the same but in a code that I am trying, I see different results.
Any help?

If you use L–, the old value will be used for the calculation and the value of L will be decreased by 1 afterwards. where L-- is the postfix decrementation.

For L = L - 1, the opposite is the case: It will first be decremented and only then the calculation will take place.