Using logically indexed vector in a loop to create another vector

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AtoZ
AtoZ 2018-11-28
编辑： Andrei Bobrov 2018-11-28
I have a vector `p=[-3 -2 -1 0 1 2 3]`, and an expression for and for ; and c have values. I want to create a vector . How to write a `for` loop to implement the above statement? The code I wrote to implement the first part of the statement is
k=2; p=[-3 -2 -1 0 1 2 3]; c=1; a=5; b=9;
L=logical(p);
p(L);
for K = 1:numel(L)
if L(K) < 1
L(K) = a*exp(1i*K*k)+b*exp(-1i*K*k)
end
end
However, it returns error "Complex values cannot be converted to logicals". Secondly, the output for p(L) doesn't have the value 0 in it. I did it with a direct way which is not convenient for large vectors.
psi_33 = a*exp(-1i*3*k)+b*exp(1i*3*k);
psi_22 = a*exp(-1i*2*k)+b*exp(1i*2*k);
psi_11 = a*exp(-1i*1*k)+b*exp(1i*1*k);
psi_0 = a+b;
psi_1 = c*exp(1i*k);
psi_2 = c*exp(1i*2*k);
psi_3 = c*exp(1i*3*k);
phi11=[psi_33,psi_22,psi_11,psi_0,psi_1,psi_2,psi_3];

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回答（2 个）

KSSV
KSSV 2018-11-28
p=[-3 -2 -1 0 1 2 3] ;
phi = zeros(size(p)) ;
for i = 1:length(p)
if -3<p(i)<1
% phi(i) = your expression
elseif 1<=p(i)<=3
% phi(i) = your expression
end
end
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AtoZ
AtoZ 2018-11-28
@KSSV For example, for p=-2 using your following code, and comparing the results of phi(2) (from your code) with the manually calculated phi(2) do not match?
Manual
phi_22 = a*exp(-1i*2*k)+b*exp(1i*2*k)
your code:
p=[-3 -2 -1 0 1 2 3] ;
phi = zeros(size(p)) ;
for i = 1:length(p)
if -3<p(i)<1
phi(i) = a*exp(1i*p(i)*k)+b*exp(-1i*p(i)*k);
elseif 1<=p(i)<=3
phi(i) = c*exp(p(i)*k);
end
end
phi(2)

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Andrei Bobrov
Andrei Bobrov 2018-11-28
编辑：Andrei Bobrov 2018-11-28
a = 5;
b = 9;
c = 1;
p = (-3:3)';
k = 2;
out = psifun(a,b,c,k,p);
here psifun - function:
function out = psifun(a,b,c,k,p)
a = [a;c];
b = [b;0];
ii = (p >= 1) + 1;
x = 1i*k.*p;
out = a(ii).*exp(x) + b(ii).*exp(-x);
end

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