Regula–Falsi Method

I keep getting an error saying:

Error in solution: Line: 1 Column: 34

Invalid expression. Check for missing multiplication operator, missing or unbalanced delimiters, or other syntax error. To construct matrices, use brackets instead of parentheses.

My work is below

function rootArray=RegulaFalsi(f,0.5,1)
f = @(x) x^2-1;
i = 0;
tol = 1e-6;
g = 1;
while(g > tol)
    i = i + 1;
    c = a - (f(a)*(b-a))/(f(b)-f(a));
    if (f(c)*f(a) > 0)
        b = c;
        g = f(b);
    else 
        a = c;
        g = f(a);
    end
end
end

Answer 1

[Walter Roberson]

function rootArray=RegulaFalsi(f,0.5,1)

In MATLAB, function must have one of the following syntaxes:

function FunctionName
function FunctionName()
function OutputVariable = FunctionName
function OutputVariable = FunctionName()
function [OutputVariable] = FunctionName
function [OutputVariable] = FunctionName()
function [OutputVariable1, OutputVariable2, ... OutputVariableN] = FunctionName
function [OutputVariable1, OutputVariable2, ... OutputVariableN] = FunctionName()
function [OutputVariable1 OutputVariable2 ... OutputVariableN] = FunctionName
function [OutputVariable1 OutputVariable2 ... OutputVariableN] = FunctionName()
function FunctionName(InputVariable1, InputVariable2, ... InputVariableM)
function OutputVariable = FunctionName(InputVariable1, InputVariable2, ... InputVariableM)
function [OutputVariable] = FunctionName(InputVariable1, InputVariable2, ... InputVariableM)
function [OutputVariable1, OutputVariable2, ... OutputVariableN] = FunctionName(InputVariable1, InputVariable2, ... InputVariableM)
function [OutputVariable1 OutputVariable2 ... OutputVariableN] = FunctionName(InputVariable1, InputVariable2, ... InputVariableM)

Each InputVariable listed may be replaced with ~ such as

function z = sq(~, y)
    

You will observe that the output variables can be separated by space or comma. The same is not true for the input variables: commas must be used for them.

In each case, where I indicate an input or output variable, a simple variable name must be used, within any () or {} indexing and without any . referencing.

In the input variable list, the ~ mentioned earlier is the only thing permitted instead of a variable name. In particular, constant values such as 0.5 are not permitted.

The function line is a function definition. The names listed in () on the line after the function name are the "dummy" names that will "stand in" for whatever is passed in the corresponding parameter position. In your attempt, whatever was passed in the first position would be known under the name f inside the function. Whatever was passed in the second position... has no way of being associated with any variable inside the function because you did not give a name to the parameter.

Meanwhile, might I point out that inside your function, you use variables a and b before you assign values to them ?

[Yogesh Bhambhwani]

I fixed it i meant to change it to this:

function rootArray=RegulaFalsi(f,a,b)

and I get the error:

Output argument "rootArray" (and maybe others) not assigned during call to "RegulaFalsi".

[Walter Roberson]

The message appears to be correctl

function rootArray=RegulaFalsi(f,a,b)

means that when RegulaFalsi is called, if the context of the call expects an output value, then the output value will be taken as whatever was assigned to the variable named rootArray . Outputs are positional, not named, so the fact that you named the variable rootArray is irrelevant in the calling environment; the calling environment works entirely by position.

Anyhow, in your code, you do not assign anything to rootArray so MATLAB does not know which of your values to return the the caller.

My guess is that rootArray should be assigned either a or c but possibly rootArray = [a, g]

[Yogesh Bhambhwani]

rootArray is already assigned to: rootArray=RegulaFalsi(f,0.5,1)

[Walter Roberson]

No, you had the line

function rootArray=RegulaFalsi(f,0.5,1)

as an invalid function definition.

When you corrected that to

function rootArray=RegulaFalsi(f,a,b)

then you do not assign to rootArray inside the function . Any assignment to rootArray outside the function is not going to have an effect on the meaning of the variable inside the function.

You are calculating several variables. Which of the variables do you want to return from the function ? For example it got down to a=0.75 b=0.750001 then what would you want the outcome of the function to be?