Error with equation using sym/subsindex

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emma
emma 2021-4-6
评论： emma 2021-4-7
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

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采纳的回答

Walter Roberson
Walter Roberson 2021-4-6
编辑：Walter Roberson 2021-4-6
Ran in:
% 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
root = 0.1032
iters
iters = 17
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 个评论
显示 1更早的评论
Walter Roberson
Walter Roberson 2021-4-7
Ran in:
% 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
root = 5.9157
iters
iters = 24
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
emma
emma 2021-4-7
Thank you so much! You are a godsend.

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更多回答（1 个）

Sulaymon Eshkabilov
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 个评论
emma
emma 2021-4-6
Thank you! This was a big help. Is there any way I can count the number of iterations for the plot?
Walter Roberson
Walter Roberson 2021-4-6
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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