explain computation of P2(1:L/2+1)

What exacly does P2(1:L/2+1) this

where L is length of signal and P2 is two sided spectrum

P1 = P2(1:L/2+1)

Answer 1

[Omer Yasin Birey]

It takes one more element than half of it. Let's say your array is

P2 = [1 2 5 6 7 8 9];
L = length(P2);
P2(1:L/2+1)

the output of this will be

[1 2 5 6]

Because the length is 7 and (7/2+1) will be 4 (it is 4 because for odd numbers it will round down the result of dividing operation). And it will take until 4th element.

[madhan ravi]

Suspect OP wants to know what it does in signal processing , not what the syntax does.

[Omer Yasin Birey]

Hard to tell with a single line code. Also after his edit it still seems like he wants to know what syntax does

[P Sun]

Well, I know neither and it is great to know the syntax part.

This line from intro of fft https://uk.mathworks.com/help/matlab/ref/fft.html