On line 53: error = abs(-15 - Theta(i)); . I'm getting an error that says, "Index exceeds the number of array elements. Index must not exceed 17901". what am I doing wrong?

clc
clear all
%Given:
M1 = 3;
G = 1.4;
tot = 180;
Theta= zeros(1,tot);
p2_p1= zeros(1,tot);


n=1;


for i = 1 : 0.01 : tot

    Theta(n)  = atand(2*cotd(i)*((M1^(2)*sind(i))^2-1)/(M1^(2)*(G+cosd(2*i)+2)));
    p2_p1(n) = 1+(2*G)/((G+1)*(M1*sind(i))^2-1);

    n = n+1;
end

error = 1;

i = 0;

while error > 0.001
    i=i+1;
    error = 20 - Theta(i);

end

d=i;

b2 = 38;
Mn1 = M1*sind(b2);
Mn2 = sqrt((Mn1^(2)+(2/(G-1)))/(2*G/(G-1)*Mn1^(2)-1));    
M2 = Mn2/(sind(b2-20))

n = 1;

for i = 30 : 0.01 : 149

    theta2(n)  = 20+atand(2*cotd(i)*((M2*(sind(i)))^2-1)/(2+M2^(2)*(G+cosd(2*i)+2)));
    p3_p1(n) = p2_p1(d)*(1+(2*G)/(G+1)*(M2*sind(i))^2-1);

    n = n+1;
end

error = 1;
i=0;
while error > 0.001
    i=i+1;
    error = abs(-15 - Theta(i));

end

a=i;

b3 = 32;
Mn1 = M1*sind(b3);
Mn3 = sqrt((Mn1^(2)+(2/(G-1)))/(2*G/(G-1)*Mn1^(2)-1));    
M3 = Mn3/(sind(a-15))

Answer 1

[Voss]

First, let me run the code almost up to where the error happens:

clc
clear all
%Given:
M1 = 3;
G = 1.4;
tot = 180;
Theta= zeros(1,tot);
p2_p1= zeros(1,tot);


n=1;


for i = 1 : 0.01 : tot

    Theta(n)  = atand(2*cotd(i)*((M1^(2)*sind(i))^2-1)/(M1^(2)*(G+cosd(2*i)+2)));
    p2_p1(n) = 1+(2*G)/((G+1)*(M1*sind(i))^2-1);

    n = n+1;
end

error = 1;

i = 0;

while error > 0.001
    i=i+1;
    error = 20 - Theta(i);

end

d=i;

b2 = 38;
Mn1 = M1*sind(b2);
Mn2 = sqrt((Mn1^(2)+(2/(G-1)))/(2*G/(G-1)*Mn1^(2)-1));    
M2 = Mn2/(sind(b2-20))

M2 = 1.9621

n = 1;

for i = 30 : 0.01 : 149

    theta2(n)  = 20+atand(2*cotd(i)*((M2*(sind(i)))^2-1)/(2+M2^(2)*(G+cosd(2*i)+2)));
    p3_p1(n) = p2_p1(d)*(1+(2*G)/(G+1)*(M2*sind(i))^2-1);

    n = n+1;
end

Now, at this point, Theta is a 1x17901 row vector:

whos Theta

  Name       Size                Bytes  Class     Attributes

  Theta      1x17901            143208  double              

The minimum value of abs(-15-Theta), which you call error and calculate element-by-element, is approximately 0.0139:

min(abs(-15 - Theta))

ans = 0.0139

Therefore, no element of abs(-15-Theta) is less than or equal to 0.001, so your while loop iterates through all elements of Theta and never terminates, until the error occurs when trying to access element 17902 (i.e., one beyond the end of Theta).

error = 1;
i=0;
while error > 0.001
    i=i+1;
    error = abs(-15 - Theta(i));

end

Index exceeds the number of array elements. Index must not exceed 17901.

a=i;

b3 = 32;
Mn1 = M1*sind(b3);
Mn3 = sqrt((Mn1^(2)+(2/(G-1)))/(2*G/(G-1)*Mn1^(2)-1));    
M3 = Mn3/(sind(a-15))