How do I turn string "C = A*B" to "data.C = data.A.*data.B " while the equation and variables may change?

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Shao-Yu Wang
Shao-Yu Wang 2022-6-27
评论： Voss 2022-6-28
I'm building a GUI that reads simple formulas from an Excel file, then evaluate them as Matlab code. For example, the user may write "C=A*B" in an Excel cell, then Matlab will import the table and calculate a new variable C with the function eval. The problem is, the variables A and B are actually variables in a table, so the correct code should have been "data.C = data.A.*data.B ". The goal is to let the user create new variables without building a complicated calculator GUI.
The difficult part is, the names (A, B, C) are dynamic and there can be all sort of equations like "A=B+C/D" or "A=B^2-C". The name of the table itself is however fixed. How can I achieve that?

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Voss
Voss 2022-6-27
编辑：Voss 2022-6-27
Ran in:
Maybe this is worth a try:
% example usage
fix_formula("C=A*B")
ans = "data.C=data.A.*data.B"
fix_formula("A=B+C/D")
ans = "data.A=data.B+data.C./data.D"
fix_formula("A=B^2-C")
ans = "data.A=data.B.^2-data.C"
% running it on a table
A = [1;2;3];
B = [4;5;6];
C = [7;8;9];
D = [10;11;12];
data = table(A,B,C,D)
data = 3×4 table
A B C D _ _ _ __ 1 4 7 10 2 5 8 11 3 6 9 12
eval(fix_formula("C=A*B"))
data = 3×4 table
A B C D _ _ __ __ 1 4 4 10 2 5 10 11 3 6 18 12
eval(fix_formula("A=B+C/D"))
data = 3×4 table
A B C D ______ _ __ __ 4.4 4 4 10 5.9091 5 10 11 7.5 6 18 12
eval(fix_formula("A=B^2-C"))
data = 3×4 table
A B C D __ _ __ __ 12 4 4 10 15 5 10 11 18 6 18 12
function str = fix_formula(str)
str = regexprep(str,'\<([A-Za-z_]+)\>','data.$1'); % prepend variable names with "data."
str = regexprep(str,'([*/^\\])','.$1'); % replace operators *, /, ^, \ with .*, ./, .^, .\ (element-wise)
end
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dpb
dpb 2022-6-27
If the table name is fixed, then it's easy -- use dynamic field names as explained in accessing data in tables -- and specifically using variables for table variable names at <<Access-data-in-a-table>
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Steven Lord
Steven Lord 2022-6-27
Ran in:
There are a few functions in MATLAB that may be able to help you like symvar, vectorize, and str2func.
s = 'A=B+C/D';
sides = split(s, '=');
lhs = sides{1}
lhs = 'A'
rhs = sides{2}
rhs = 'B+C/D'
listOfVariablesOnRight = symvar(rhs);
str = "@(" + join(listOfVariablesOnRight, ', ') + ") " + vectorize(rhs)
str = "@(B, C, D) B+C./D"
fh = str2func(str)
fh = function_handle with value:
@(B,C,D)B+C./D
Now call the function handle with the appropriate inputs and store the output in a field of a struct array.
result.(lhs) = fh(1, 2, 3)
result = struct with fields:
A: 1.6667
check = 1 + (2/3)
check = 1.6667

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