Change a variable till a condition is met

Hi, how can i say to matlab to change the variable x till the condition varptf=0 is met? Note that y is (1-x). I did it on excel by analysis tool solver (the solution is rougly x=0.65), idk how to do this on matlab (and for an assignment I have to do this on matlab). Thank you for your time

% point 1
SDa = 0.08 % std of A

SDa = 0.0800

SDb = 0.15 % std of B

SDb = 0.1500

x = 0 % weight of A

x = 0

y = 1-x % weight of B

y = 1

p = -1 % correlation coefficient

p = -1

varptf = x^2*SDa^2+y^2*SDb^2+2*x*y*p*SDa*SDb

varptf = 0.0225

while varptf == 0
    x = x+0.01
end
x

x = 0

Answer 1

[Voss]

% point 1
SDa = 0.08 % std of A

SDa = 0.0800

SDb = 0.15 % std of B

SDb = 0.1500

x = 0 % weight of A

x = 0

y = 1-x % weight of B

y = 1

p = -1 % correlation coefficient

p = -1

varptf = x^2*SDa^2+y^2*SDb^2+2*x*y*p*SDa*SDb

varptf = 0.0225

while abs(varptf) >= 1e-6
    x = x+0.01;
    y = 1-x;
    varptf = x^2*SDa^2+y^2*SDb^2+2*x*y*p*SDa*SDb;
end
x

x = 0.6500

[Luca]

Thank you very much !!

[Voss]

You're welcome!

Answer 2

[David Hill]

SDa = 0.08; 
SDb = 0.15;
x = 0; 
y = 1-x; 
p = -1;
varptf =@(x,y)x^2*SDa^2+y^2*SDb^2+2*x*y*p*SDa*SDb;
v=varptf(x,y);
while v>1e-10%~=0 is problematic (unlikely it is going to exactly equal zero)
    x=x+.0001;%change additional amount for more accuracy
    y=1-x;
    v=varptf(x,y);
end
x

x = 0.6522