How to fix PATTERN must be a string or cell array of strings error?

Hello! I've got this code:

clear all;

x = [0.0, 6, 9, 12, 15, 18];

% Построить интерполяционный полином по значениям x и y(x) различными методами (прямой метод, Лагранжа, Ньютона)
% Построить интерполяционные сплайны второй степени.
% Методом наименьших квадратов построить аппроксимирующую функцию первой, второй и третьей степени.

yy = y(x);
% lagrange's interpolation
lagrange(x, yy, 1)

% newton's interpolation
newton(x, yy, 1)

% splines 2-order
SplineInterpolation(x, yy, 1)

% min_square_method 1-order
N = length(x);
Mx  = sum(x);
Mx2 = sum(x.^2);
My  = sum(yy);
Mxy = sum(x.*yy);
 
A = [Mx2 Mx
     Mx  N];
B = [Mxy; My];
X=A\B;
 
% ответ при х = 1
X(1)* 1 + X(2)

% рассчитываем результирующую прямую
y2= X(1).*x + X(2);
plot(x,yy,'-or',x,y2,'b'); 
grid on;
legend('Исходная функция','Метод наименьших квадратов');

% min_square_method 2-order
N = length(x);
Mx  = sum(x);
Mx2 = sum(x.^2);
Mx3 = sum(x.^3);
Mx4 = sum(x.^4);
My  = sum(yy);
Mxy = sum(x.*yy);
Mx2y = sum((x.^2).*yy);
 
A = [Mx4 Mx3 Mx2
     Mx3 Mx2 Mx
     Mx2 Mx  N];
B = [Mx2y; Mxy; My];
X=A\B;
 
% ответ при х = 1
X(1)*1 + X(2) * 1 + X(3)

% рассчитываем результирующую прямую
y2= X(1).*(x.^2) + X(2).*x + X(3);
plot(x,yy,'-or',x,y2,'b'); 
grid on;
legend('Исходная функция','Метод наименьших квадратов');


function val = y(x)
val = 1 ./ x;
val(x==0)=0;
end

function yy = lagrange(x,y,xx)
N=length(x);
Nres=length(xx);
yy=zeros(size(xx));
for k=1:N
L=ones(size(xx));
    for j=[1:k-1, k+1:N]
        for i=[1:Nres]
            L(i)=L(i).*(xx(i)-x(j))/(x(k)-x(j));
        end
    end
yy = yy + y(k)*L;
end
end

function yy = newton(x, y, xx)
N = length(x);
y0 = y;
for  k = 1 : N-1
     for i = 1 : N - k
          y0(i) = (y0(i+1) - y0(i)) / (x(i+k) - x(i));
      end
end

yy = y0(1) * ones(size(xx));
for k = 2 : N
     yy = y0(k) + (xx - x(k)) .* yy;
end
end

function yn = SplineInterpolation(x, y, xn)
InputNum = length(x);
An = y;
Bn = zeros(InputNum, 1);
Dn = zeros(InputNum, 1);

A = zeros(InputNum,InputNum);
A(1,1) = 1;
A(InputNum,InputNum) = 1;

for i = 2:InputNum-1
    h0 = x(i)-x(i-1);
    h1 = x(i+1)-x(i);
    a_t = [h0, 2*(h0+h1), h1];
    A(i,i-1:i+1) = a_t;
end

b = zeros(InputNum, 1);
b(1) = 0;
b(InputNum) = 0;

for i = 2:InputNum-1
    h0 = x(i) - x(i-1);
    h1 = x(i+1) - x(i);
    a0 = y(i-1);
    a1 = y(i);
    a2 = y(i+1);
    b(i) = 3/h1*(a2-a1)-3/h0*(a1-a0);
end

% Cn = linsolve(A,b)
Cn = SolveSplineEquations(A, b, InputNum);

for i = 1:InputNum-1
    hi = x(i+1)-x(i);
    Bn(i) = 1/hi*(An(i+1) - An(i)) - hi/3*(2*Cn(i) + Cn(i+1));
    Dn(i) = (Cn(i+1) - Cn(i))/(3*hi);
end

yn = zeros(1,length(xn));

for i = 1:length(xn)
    curx = xn(i);
    pos = curx>=x;
    pos = strfind(pos,1);
    if(isempty(pos))
        PSet=1;
    else
        PSet = pos(end);
    end    
    if(PSet<1)
        PSet=1;
    end
    if(PSet>InputNum-1)
        PSet=InputNum-1;
    end
    xi = x(PSet);
    yn(i) = An(PSet) + Bn(PSet)*(curx-xi) + Cn(PSet)*(curx-xi)^2 + Dn(PSet)*(curx-xi)^3;
end
end

function x = SolveSplineEquations(A, b, InputNum)
y = zeros(InputNum, 1);
x = zeros(InputNum, 1);
Beta = zeros(InputNum, 1);

for i = 1:InputNum-1
    ci = A(i, i+1);
    bi = A(i, i);
    if(i==1)
        Beta(i) = ci/bi;
    else
        ai = A(i, i-1);
        Beta(i) = ci/(bi - ai*Beta(i-1));
    end
end
for i = 1:InputNum
    bi = A(i, i);
    
    if(i==1)
        y(i) = b(i)/bi;
    else
        ai = A(i, i-1);
        y(i) = (b(i) - ai*y(i-1))/(bi-ai*Beta(i-1));
    end
end
for i = InputNum:-1:1
    if(i==InputNum)
        x(i) = y(i);
    else
        x(i) = y(i) - Beta(i)*x(i+1);
    end
end
end

from my friend and tried run it in Octave but it failed with:

error: strfind: PATTERN must be a string or cell array of strings
error: called from
    SplineInterpolation at line 82 column 13
    task1_semester2 at line 147 column 1

Answer 1

[Walter Roberson]

    pos = curx>=x;
    pos = strfind(pos,1);
 

In MATLAB, it is permitted by undocumented to pass in numeric or logical arrays to strfind. For example,

    strfind(logical([1 0 0 0 1 1 1 1 0]), logical([0 1 1 1]))

is legal (but not documented.)

You used a tag of octave which suggests that you might possibly not be MATLAB. octave is a different programming language that is inspired by MATLAB, but which is permitted to have whatever incompatibilities it wants.

You could probably replace

    pos = strfind(pos,1)

with

    pos = find(pos);