Error with equation using sym/subsindex
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emma
el 6 de Abr. de 2021
Comentada: emma
el 7 de Abr. de 2021
When I try to run my code, I get "Error using sym/subsindex...Invalid indexing or function definition..." and "Error in line with f(A) = x.*tan(A)-(g./(2.*V.^2.*((cos(A)).^2))).* x.^2-y". Can someone please show me how I can fix my code below?
% Variables
V = 1500;
g = 32.2;
x = 500;
y = 50;
tol = 0.000001; % Error tolerance
% Function
syms A
f(A) = x.*tan(A)-(g./(2.*V.^2.*((cos(A)).^2))).* x.^2-y;
x_coor = linspace(0, 0.2, 1000);
y_coor = f(x_coor);
figure
plot(x_coor, y_val)
xline(0.103247833251953)
yline(0)
root = bisection(f,0.1,0.2,tol); % Root from Bisection Method
function [c] = bisection(f,a,b,tol) % Function file
% Inputs: f = function; a,b = range or interval for roots to be found where a>b; tol = error tolerance
err = 1/tol;
% Bisection method
while err>tol
c = (a+b)/2;
if(abs(f(c))== 0) %if f(c) is equal to 0, then stop
break
end
if f(a)*f(c)<0
b=c;
else
a=c;
end
err=abs(a-b);
end
end
0 comentarios
Respuesta aceptada
Walter Roberson
el 6 de Abr. de 2021
Editada: Walter Roberson
el 6 de Abr. de 2021
% Variables
V = 1500;
g = 32.2;
x = 500;
y = 50;
tol = 0.000001; % Error tolerance
% Function
syms A
f(A) = x.*tan(A)-(g./(2.*V.^2.*((cos(A)).^2))).* x.^2-y;
x_coor = linspace(0, 0.2, 1000);
y_coor = f(x_coor);
figure
plot(x_coor, y_coor) %you used undefined variable y_val
xline(0.103247833251953)
yline(0)
[root, iters] = bisection(f,0.1,0.2,tol); % Root from Bisection Method
root
iters
function [c, iters] = bisection(f,a,b,tol) % Function file
% Inputs: f = function; a,b = range or interval for roots to be found where a>b; tol = error tolerance
err = 1/tol;
% Bisection method
iters = 0;
while err>tol
iters = iters + 1;
c = (a+b)/2;
if(abs(f(c))== 0) %if f(c) is equal to 0, then stop
break
end
if f(a)*f(c)<0
b=c;
else
a=c;
end
err=abs(a-b);
end
end
3 comentarios
Walter Roberson
el 7 de Abr. de 2021
% Variables
V = 1500;
g = 32.2;
x = 500;
y = 50;
tol = 0.000001; % Error tolerance
% Function
syms A
f(A) = x.*tand(A)-(g./(2.*V.^2.*((cosd(A)).^2))).* x.^2-y; %DEGREES
x_coor = linspace(0, 0.2, 1000);
y_coor = f(x_coor);
figure
plot(x_coor, y_coor) %you used undefined variable y_val
xline(0.103247833251953)
yline(0)
[root, iters] = bisection(f,0.1,10,tol); % Root from Bisection Method %CHANGE UPPER BOUND
root
iters
function [c, iters] = bisection(f,a,b,tol) % Function file
% Inputs: f = function; a,b = range or interval for roots to be found where a>b; tol = error tolerance
err = 1/tol;
% Bisection method
iters = 0;
while err>tol
iters = iters + 1;
c = (a+b)/2;
if(abs(f(c))== 0) %if f(c) is equal to 0, then stop
break
end
if f(a)*f(c)<0
b=c;
else
a=c;
end
err=abs(a-b);
end
end
Más respuestas (1)
Sulaymon Eshkabilov
el 6 de Abr. de 2021
In your code, there are a few confusions and flaws. You've define: f(A) = x.*tan(A)-(g./(2.*V.^2.*((cos(A)).^2))).* x.^2-y; f as a function of A. At the same time, y_coor = f(x_coor). The syntax of y_coor = f(x_coor) is not correct as defined above.
Some confusions need to be fixed, y_coor vs. y_val, x vs. x_coor.
Moreover, without the value of A, y_coor can't be computed.
Here is a corrected code:
% Variables
V = 1500;
g = 32.2;
x = 500;
y = 50;
tol = 0.000001; % Error tolerance
% Function
syms A
f(A)= x.*tan(A)-(g./(2.*V.^2.*((cos(A)).^2))).* x.^2-y;
root = bisection(f,0.1,0.2,tol); % Root from Bisection Method
A = root;
fprintf('Computed root is: %f \n ', root);
% Plot:
x = linspace(0, 0.2, 1000);
Y = @(x)(x.*tan(A)-(g./(2.*V.^2.*((cos(A)).^2))).* x.^2-y);
y_val=Y(x);
figure
plot(x, y_val)
xline(0.103247833251953)
yline(0)
function [c] = bisection(f,a,b,tol) % Function file
% Inputs: f = function; a,b = range or interval for roots to be found where a>b; tol = error tolerance
err = 1/tol;
% Bisection method
while err>tol
c = (a+b)/2;
if(abs(f(c))== 0) %if f(c) is equal to 0, then stop
break
end
if f(a)*f(c)<0
b=c;
else
a=c;
end
err=abs(a-b);
end
end
2 comentarios
Walter Roberson
el 6 de Abr. de 2021
The syntax of y_coor = f(x_coor) is not correct as defined above.
That is not correct. The poster defined a symbolic function. Just like regular functions and anonymous functions, the named parameters will be substituted with whatever is passed to the function when it is invoked, so it is fine for numeric values or a variable of a different name to be passed to f.
Perhaps the user edited the original code after you posted, but for the code currently posted at least, there are almost no changes needed to get the plot -- only one misnamed variable.
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