First input must be function handle error using arrayfun()

1
votes

I'm trying to use arrayfun() to map a function over a cell array. The following is happening:

>> arrayfun(solveFunc, equArray)
Error using arrayfun
First input must be a function handle.

Error in solve>genGuess (line 33)
funcVals = abs(arrayfun(inFunc, xValues));

Error in solve (line 8)
        x = genGuess(inFunc, varargin{1}, varargin{2});

Error in makeSolveFunc>@(func)solve(func,start,stop) (line 3)
sFunc = @(func) solve(func, start, stop);

But, the first input IS a function handle. Also... if I manually apply the function to each element of the provided cell array, everything works fine:

>> solveFunc(equArray{1})
ans =
    4.7335
>> solveFunc(equArray{2})
ans =
    4.7356

Does anyone know why this would be happening? I assumed that if I could manually apply the function to each element of my array, and the return type of the function was consistent and one of the allowed types (you can't for example have arrayfun return an array of function handles... I already tried doing that), it should work. Perhaps that is not the only requirement.

Here is some code that generates this error:

solve.m

function solution = solve(inFunc, start, stop)
%SOLVE solve an equation using Newton's Method

x = genGuess(inFunc, start, stop);

for i = 1:100
    m = getSlope(inFunc, x);
    x = (m*x - inFunc(x))/m;
end

solution = x;
end

function slope = getSlope(inFunc, x)
%SLOPE calculate the slope at a given point
inc = 1e-5;
if x ~= 0
    inc = inc * x;
end

slope = (inFunc(x + inc) - inFunc(x - inc))/(2*inc);
end

function guess = genGuess(inFunc, start, stop)
%GENGUESS get an initial guess to the solution
xValues = linspace(start, stop, 101);
funcVals = abs(arrayfun(inFunc, xValues));
[~, minIndex] = min(funcVals);
guess = xValues(minIndex);
end

charEqu.m

function equ = charEqu(a)
%CHAREQU create a KP model characteristic equation with provided p
equ = @(x) x + a;
end

makeSolveFunc.m

function sFunc = makeSolveFunc(start, stop)
%MAKESOLVEFUNC return a function that solves an equation
sFunc = @(func) solve(func, start, stop);
end

test.m

pArray = 1:5;
equArray = cell(1,arrayLen);
for i = 1:5
    equArray{i} = charEqu(pArray(i));
end
solveFunc = makeSolveFunc(1.1*pi, 2*pi);
alphaAArray = arrayfun(solveFunc, equArray);

I have narrowed down the error to something in genGuess(). For some reason, in the line funcVals = abs(arrayfun(inFunc, xValues)); the variable inFunc is a 1x1 cell array containing a function handle. I have no idea why that would be the case. I traced this back to the anonymous function call @(func) solve(func, start, stop); in the makeSolveFunc() function. There it is still a 1x1 cell array containing a function handle. I'm not really sure where that cell array is coming from as that function is getting called from arrayfun().

Background information on what I'm trying to do in case someone wants to suggest a better way: I'm trying to solve equations using Newton's method. I have written a function that can solve an equation given an initial guess range. This function is the solve() function you can see in the first error message. It takes a function, and the guess range and returns a function that I'm calling solveFunc(). solveFunc() takes a function and solves it using the initial guess range previously provided.

Maybe I'm just too used to functional programming and should just use a loop.

2
matlab
Please provide some code to reproduce your problem. - Daniel
How did you define solveFunc? I'm guessing that it's not a function handle. Does ishandle(solveFunc) return true? A handle is not just the name of the function. - horchler
@horchler No, ishandle(solveFunc) does not return true BUT, according to the matlab documentation ishandle tells you if the parameter is a graphics handle, not a function handle. However, isa(solveFunc, 'function_handle') does return true. Ill work on a minimal working example. - Matt
@DanielR example added - Matt

2 Answers

2
votes

If the arguments passed to the function handle are contents of elements of a cell array, you need to use cellfun instead of arrayfun:

  cellfun(solveFunc, equArray)

This is equivalent to

for i=1:length(equArray)
    out(i) = solveFunc(equArray{i});
end

since solveFunc is already a function handle.

1
votes

Check where the error comes from. This line causes the error:

funcVals = abs(arrayfun(inFunc, xValues));

The first input argument is a 1x1 cell containing one function handle. This is caused because equArray is a cell, thus use cellfun as Jonas already mentioned:

pArray = 1:5;
equArray = cell(1,arrayLen);
for i = 1:5
    equArray{i} = charEqu(pArray(i));
end
solveFunc = makeSolveFunc(1.1*pi, 2*pi);
alphaAArray = cellfun(solveFunc, equArray);