Matlab error: ??? Error using ==> interp1 at 259 The values of X should be distinct.

I'm trying to find the values of z that correspond to the x values in xib but I keep getting this error and I don't know what it means. I am not very good at programming so please answer in layman's terms as much as possible. Below is my code:

clear

clc

%Part 1: Airfoil Geometry

    %Plot x vs. z for a 15% thick airfoil

e = 0.04725;

a = 0.563;

k = 1.889;

theta = 0:1:360;

r1 = sqrt((a*cosd(theta)-a).^2+(a*sind(theta)).^2);

r2 = sqrt((a*cosd(theta)-e*a).^2+(a*sind(theta)).^2);

b = size(theta);

p = a*cosd(theta)-a;

theta1 = atan(a*sind(theta)./(p))+pi;

for d = 1:b(2)

    c = a*cosd(theta(d))-e*a;

    if c > 0 && theta(d) < 100

        n1 = 0;

    elseif c < 0

        n1 = 1;

    else n1 = 2;

    end

    theta2(d) = atan(a*sind(theta(d))./(a*cosd(theta(d))-e*a))+n1*pi;

end

x = ((r1.^k)./(r2.^(k-1))).*(cos(k*theta1).*(cos((k-1)*(theta2)))+sin(k*theta1).*(sin((k-1)*theta2)))+1;

z = ((r1.^k)./(r2.^(k-1))).*(sin(k*theta1).*(cos((k-1)*(theta2)))-cos(k*theta1).*(sin((k-1)*theta2)));

figure(1), plot(x,z), grid

axis equal;

title('Airfoil Geometry at 15% Thickness'), xlabel('X'), ylabel('Z')

%Part 2: Airfoil Pressure Distribution

alpha = 0;

A = cos((k-1)*theta1).*cos(k*theta2)+sin((k-1)*theta1).*sin(k*theta2);

B = sin((k-1)*theta1).*cos(k*theta2)-cos((k-1)*theta1).*sin(k*theta2);

D0 = a*(1-k+k*e);

D1 = A.*(a*cosd(theta)-D0)-B*a.*sind(theta);

D2 = A.*(a*sind(theta))+B.*(a*cosd(theta)-D0);

u = 2*((r2.^k)./(r1.^(k-1))).*(sind(alpha)-sind(alpha-theta))./(D1.^2+D2.^2).*(D1.*sind(theta)+D2.*cosd(theta));

w = -2*((r2.^k)./(r1.^(k-1))).*(sind(alpha)-sind(alpha-theta))./(D1.^2+D2.^2).*(D1.*cosd(theta)-D2.*sind(theta));

Cp = 1-(u.^2+w.^2);

figure(2), plot(x,Cp), grid

axis equal;

title('Airfoil Pressure Distribution on a 15% Thick Airfoil at Zero AOA')

xlabel('X'), ylabel('Cp(X)')

%Part 3: Discretizing the Airfoil Geometry and Linear Interpolation

N = 11;

dx = 4/N;

xib = 1:-dx:-1;

r = size(xib);

xit = xib(r(2)):dx:1;

xi = horzcat(xib, xit);

X = x(1,361:-1:180);

X(1)=1;

Z = z(1,361:-1:180);

Z(1)=0;

zi = interp1(X,Z,xib);

Answer 1

[Wayne King]

The problem is your last X value is -1.0012, which is repeated from the 3rd to the last

 X(end-2:end)

So note that

zi = interp1(X(1:end-1),Z(1:end-1),xib);

Does not error.

[Nicole]

thank you!