About rectangular pulse wave function code.

function y = rect_pulse(t_vec,tau)
%   This function generates a rectangular pulse of pulse width tau.
%   Generated pulse is centered about t=0.  
%   By default, the generated pulse has width 1.  
%   The signal has value 1 in interval [-Tw/2,+Tw/2)
%
if nargin<2, tau=1;   end

y = double(abs(t_vec)<tau/2-eps);
y(abs(t_vec-(-tau/2))<eps) = 1.0;

This code is generating the rectangular pulse wave.

but i can't understand the last two lines.

First, where the 0? fundamentally rectangular pulse has 0 except for the width range.

how can the zero make in this code?

Next, is there conditional code beside 'if' or 'elseif'?

because in 'y=double(~~)' '~~' looks like conditional code.

Thank you for your help.