Say I have a function
a = b / c
and I ask the user to input two of these variables, either b and a or c and a and I want it to calculate the unknown variable without needing to write a function for every variable
In this case I would use:
pseudo-code
if input is a & b
then c = b / a
if input is a & c
then b = a * c
if input is b & c
then a = b / c
I know this is a function with only three variables so it is easy to put it in an if-statement and voilà BUT I want to apply it to a system containing lots of equations (a jet engine for example). I used TkSolver before and it was really great, you throw as many equations as you want at it (a jet engine was an example!), you only need to give a number of known quantities and in a few seconds, all the unknowns are calculated (even if I had a known quantity in one side of the equation and unknown on another side mixed with known ones, it will do the maths!)
So, is there a way to do this in MatLab or perhaps python which I'm learning?
Edit to the question, thanks for directing me to use the Symbolic toolbox, it's great, I have another problem:
I couldn't think of a way to let the program know which of the variables is entered. I can do this:
syms f a b c
f = a * c - b %(I want to write a = b / c)
c = 10; a = 5;
X = eval(solve(f,b))
What I want now is a way of letting the user enter two knowns (e.g c & a) and the code will recognise them and solve to the unknown variable (e.g b).
Edit 2: I managed to get what I want, it's a bit long and there might be another way of achieving the same thing.
clear, clc
syms a b c
var = [{'a'}, {'b'}, {'c'}];
var1n = 0;
var2n = 0;
while isvarname(var1n) == 0
var1n = input('Which variable is known: ','s');
while ( any( strcmpi(var1n,var) ) )== 0
fprintf('\nThe variable entered is wrong, please enter a, b, or c')
var1n = input('\nWhich variable is known: ', 's');
end
end
fprintf('\nPlease enter the value of %s', var1n)
var1v = input(': ');
eval([var1n '=' num2str(var1v)]);
while isvarname(var2n) == 0
var2n = input('What is the second known variable: ', 's');
while ( any( strcmpi(var2n,var) ) ) == 0
fprintf('\nThe variable entered is wrong, please enter a, b, or c')
var2n = input('\nWhat is the second known variable: ', 's');
end
end
fprintf('\nPlease enter the value of %s', var2n)
var2v = input(': ');
eval([var2n '=' num2str(var2v)]);
var3n = char ( var ( find( strcmpi(var1n, var) == strcmpi(var2n, var) ) ) );
var3v = solve(a - b / c);
eval([var3n '=' char(var3v)]);
You could use this: http://www.mathworks.de/help/toolbox/symbolic/solve.html but you have to have the symbolic math toolbox (:
EDIT: On the documentation page of solve there's a sentence:
If the right side of an equation is 0, specify the left side as a
symbolic expression or a string:
That means, if you want to solve a = b/c for the value which is NOT set, simply rewrite the equation so that there is a zero on the right hand side:, i.e. a - b/c = 0, than you can use:
syms a b c
% E.g. assign values to a and c and automatically let matlab chose the 'free' variable (in this case, it's b:
a = input('Input var a: ')
a = input('Input var c: ')
solve(a - b/c)
which than gives you b (e.g. entering a = 10 and c = 40 gives you b = a * c = 400). Input function is explained here: http://www.mathworks.de/help/techdoc/ref/input.html! Hope that helps!
Related
I'm trying to write a simple script that request a equation as input and I like to calculate with this function more than once without always requesting user input.
My current script defined f as function handle and executed the function 2 times, so I'm always asking for a new equation, which is not desirable.
f = #(x) input('f(x) = ');
a = f(2); % requests user input
b = f(3); % requests user input again
And it should look more like this (not working).
func = input('f(x) = ');
f = #(x) func;
a = f(2);
b = f(3);
And here without user input to get an idea what I try to achieve.
f = #(x) x^2;
a = f(2);
b = f(3);
I think I found a solution with Symbolic Math Toolbox, but I do not have this addon, so I cannot use/test it.
Is there another solution?
There's no need for the Symbolic Mathematics Toolbox here. You can still use input. Bear in mind that the default method of input is to directly take the input and assign it as a variable where the input is assumed to be syntactically correct according to MATLAB rules. That's not what you want. You'll want to take the input as a string by using the 's' option as the second parameter then use str2func to convert the string into an anonymous function:
func = input('f(x) = ', 's');
f = str2func(['#(x) ' func]);
a = f(2);
b = f(3);
Take note that I had to concatenate the #(x) anonymous function string with the inputted function you provided with input.
Example Run
Let's say I want to create a function that squares every element in the input:
>> func = input('f(x) = ', 's');
f(x) = x.^2
>> f = str2func(['#(x) ' func])
f =
#(x)x.^2
>> a = f(2)
a =
4
>> b = f(3)
b =
9
Take special note that I assumed the function will element-wise square each elements in the input. The . operator in front of the exponentiation operator (i.e. ^) is very important.
I got one question in matlab:
I get a value of c from other function, then I want to put c value 1 into dsolve to solve this differential equation. but the value cannot put into dsolve, what should I do to transfer value?
eg:
c = 1;
u = dsolve('Du = 1+u^c','t') % c is c, but is not 1!
The reason for your error is that when you use 'c', c is interpreted as a char (or string) and not a variable. What you could do is use num2str to combine strings and variables, like this:
u = dsolve(['Du = 1+u^' num2str(c)],'t') % c is c, but is not 1!
u =
C16*exp(t) - 1
If you have several variables, you can do something like this:
u = dsolve(['Du = ' num2str(1/(m*n)) '*1 + u^' num2str(c)])
u =
C16*exp(t) - 1
I was given a simple exercise by my professor, which is to define a cerrtain function. Here is my function
function [a,b,c] = PHUN([x,y],[z,t,w])
a = x
b = ((y+z)^2)*t
c = z/w + x
However, matlab states that I am using invalid syntax in the first line. So, I figured, perhaps there is a particular way in which vector inputs are supposed to be typed. I have attempt several searches on defining functions with vector inputs (or arguments), but have not been successful. I was wondering if someone could possibly help me.
You can pass vectors as arguments in the same way you pass variables. Then access them in the function body appropriately. Your function could be re-written as follows:
function [a,b,c] = PHUN(X,Y)
a = X(1)
b = ((X(2)+Y(1))^2)*Y(2)
c = Y(1)/Y(3) + X(1)
Or if you want to keep the original variables:
function [a,b,c] = PHUN(X,Y)
Z = num2cell([X,Y]);
[x,y,z,t,w] = Z{:};
a = x
b = ((y+z)^2)*t
c = z/w + x
I am a novice in matlab and currently searching, how to solve a mathematical function in matlab.
It's what i want, i want to solve a function f(x)=x^2+2x+1, where x=2.
I want to take x^2+2x+1 and 2 as input and show the output.
from this link, i saw how to solve a mathematical function.
Procedure available in the link is:
>>f = #(x) 2*x^2-3*x+4;
>>f(3)
ans = 13
So, i wrote the following part in my script:
F=input('enter function: ','s');
v = input('parameter: ');
f=#(x)F;
disp(f(v));
when i provide x+1 and 3 as input, the ans i am getting is x+1. How to solve this problem?
Thanks in advance.
Your F variable is a string that simply says "x + 1". You'll have to ask matlab to interpret this string as a function in order to get a numerical value.
One way of doing so would be using eval
F = input('enter function: ', 's' );
v = input('parameter: ');
myfun = sprintf('#( %s ) %s', v, F ); % a string
f = eval( myFun ); % interpret string as a command
f( 3 ), % do the math - evaulate F(3)
EDIT, a clarification based on comments below:
The above solution assumes F, the input string representing the mathematical formula may have user-chosen variable (i.e., the unknown does not have to be 'x', it may be 'y', 'a' etc.) In order to comunicate this, v is a char storing this information.
In case where F always depends on 'x', and the desired output is F(v) for a numeric value v the following modification is needed:
myfun = sprintf('#(x) %s', F );
f = eval(myfun);
F_of_v = f( v )
I've created a function (name it MyFunction) which, given a matrix A, outputs two matrices B and C i.e. [B C] = MyFunction(A).
I'm trying to create another function which, when given a matrix A, will calculate MyFunction(A) and then calculate MyFunction(B) = [D E] and MyFunction(C) = [F G], and then calculate MyFunction(D), MyFunction(E), MyFunction(F) and MyFunction(G), and so on, until the matrices it outputs start to repeat. I know this process necessarily terminates.
I'm really having difficulty constructing this code. Any advice would be really appreciated.
I think what you're trying to do is binary tree recursion. It's hard to give a good solution without knowing more about the problem, especially without knowing what you want as the output of this process.
I whipped this up to give an example of how you could do this. It's not necessarily the most efficient because it stores all of the results at every step. Given an input matrix A, it calculates a 2-output function [B, C] = MyFunction(A) and looks for either isequal(A, B) or isequal(A, C). When that occurs, it outputs the depth of the tree at that point, i.e. how many iterations had to occur before there was a repetition. The bit with global variables is just so that I could do a simple example with an easy fixed point (the k'th iteration is just A^k). It will iterate a maximum of 10 times.
function depth = myRecursor(A)
global A_orig;
A_orig = A;
depth = 1;
max_depth = 10;
pvs_level = cell(1);
pvs_level{1} = A;
while depth < max_depth,
this_level = cell(2*length(pvs_level), 1);
for ix = 1 : length(pvs_level),
[B, C] = MyFunction(pvs_level{ix})
if isequal(B, A) || isequal(C, A),
return;
end
this_level{2*ix - 1} = B;
this_level{2*ix} = C;
end
depth = depth + 1;
pvs_level = this_level;
end
function [B, C] = MyFunction(A)
global A_orig;
B = A_orig*A;
C = 2*A;
So, for example myRecursor(eye(2)) gives 1 (duh) and myRecursor([0 1; 1 0]) gives 2.
EDIT: rewritten wrong alg
function [list] = newFunction(A) % returns all matrix generated matrix
list{1}=A;
done=0;
ii = 1;
while(~done)
[B C] = myFunction(list{ii});
list = list{list{:}, B, C};
for jj=1:numel(list)-2
if(all(all(list{jj}==B)) || all(all(list{jj}==C)))
done = 1;
end
end
ii=ii+1;
end
end
UPDATE: a more general way to handle an unknown number of outputs is to modify myFunction so that it outputs all matrices within a single cell vector. This way you can concatenate list this way:
[newMat] = myFunctions(list{ii}); % where newMat={B,C,...}
list = {list{:}, newMat{:}}; % or list=cat(2,list,newMat)
for jj=1:numel(list)-numel(newMat)
for nn=1:numel(newMat) % checking for repetitions
if(all(all(list{jj}==newMat{nn})))
done=1;
end
end
end
I think you should use cells:
function [B]=Myfunction(A)
B=cell(1,numel(A)*2);
for n=1:numel(A)
B{n}=func1(A{n}) %%% put some processing here
B{numel(A)+n}=func2(A{n}) %%% put some other processing here
end
end