asking for a polynomial expression in an input

I am building a calculator app for an assignment, and got stuck on the fact that the equation I am calculating evaluates the gradient of the inputs. when the inputs are constant the gradent is 0 (obviously). For the calculator to work I would like the user to be able to input a polynomial expression. Any help or hints are apprecited, I am very stuck and confused.

Answer 1

[John D'Errico]

Can you type in a polynomial? Why not? You type it anyway. Just use input, which can return the result as a string. Or, use a dialog box.

help input

  Copyright 2015-2020 The MathWorks, Inc.

    Documentation for input
       doc input

    Folders named input

       shadows/input

help inputdlg

 INPUTDLG Input dialog box.
   ANSWER = INPUTDLG(PROMPT) creates a modal dialog box that returns user
   input for multiple prompts in the cell array ANSWER. PROMPT is a cell
   array containing the PROMPT strings.
 
   INPUTDLG uses UIWAIT to suspend execution until the user responds.
 
   ANSWER = INPUTDLG(PROMPT,NAME) specifies the title for the dialog.
 
   ANSWER = INPUTDLG(PROMPT,NAME,NUMLINES) specifies the number of lines for
   each answer in NUMLINES. NUMLINES may be a constant value or a column
   vector having one element per PROMPT that specifies how many lines per
   input field. NUMLINES may also be a matrix where the first column
   specifies how many rows for the input field and the second column
   specifies how many columns wide the input field should be.
 
   ANSWER = INPUTDLG(PROMPT,NAME,NUMLINES,DEFAULTANSWER) specifies the
   default answer to display for each PROMPT. DEFAULTANSWER must contain
   the same number of elements as PROMPT and must be a cell array of
   strings.
 
   ANSWER = INPUTDLG(PROMPT,NAME,NUMLINES,DEFAULTANSWER,OPTIONS) specifies
   additional options. If OPTIONS is the string 'on', the dialog is made
   resizable. If OPTIONS is a structure, the fields Resize, WindowStyle, and
   Interpreter are recognized. Resize can be either 'on' or
   'off'. WindowStyle can be either 'normal' or 'modal'. Interpreter can be
   either 'none' or 'tex'. If Interpreter is 'tex', the prompt strings are
   rendered using LaTeX.
 
   Examples:
 
   prompt={'Enter the matrix size for x^2:','Enter the colormap name:'};
   name='Input for Peaks function';
   numlines=1;
   defaultanswer={'20','hsv'};
 
   answer=inputdlg(prompt,name,numlines,defaultanswer);
 
   options.Resize='on';
   options.WindowStyle='normal';
   options.Interpreter='tex';
 
   answer=inputdlg(prompt,name,numlines,defaultanswer,options);
 
   See also DIALOG, ERRORDLG, HELPDLG, LISTDLG, MSGBOX,
     QUESTDLG, TEXTWRAP, UIWAIT, WARNDLG .

    Documentation for inputdlg
       doc inputdlg

Next, how can you convert the string into a symbolix expression?

help str2sym

 STR2SYM Evaluate string representing symbolic expression.
    str2sym(S) evaluates S as a symbolic expression, where S can be
    a string, character vector, or cell array of character vectors.
 
    str2sym(S) behavior:
      Only executes functions on path when converting the string.
      Does not consider variables and values from workspace.
      Does not add variables in the input to the workspace.
      Does not support programming statements like IF.
      Treats assignments ('=') as equations ('==').
 
    For numeric strings, use SYM for exact symbolic numbers and
    VPA for floating-point numbers.
 
    See also SUBS, SYM, SYMFUN, SYMS, SYMVAR, VPA.
 
    Examples:
 
       >> str2sym("b^2 + pi/2 + sin(x) = x^3 - 2")
       ans =
       b^2 + pi/2 + sin(x) == x^3 - 2
 
       >> str2sym('int(x^3,x)')
       ans =
       x^4/4
 
       >> str2sym('y(0) = 42')
       ans =
       y(0) == 42
 
       >> str2sym({'42', 'a', 'y(0)'})
       ans =
       [ 42, a, y(0)]
 
       >> str2sym('[42, a, y(0)]')
       ans =
       [ 42, a, y(0)]

    Documentation for str2sym
       doc str2sym

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