I need help using the strrep for this code.

Question

Kratos 2015-2-18

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https://ww2.mathworks.cn/matlabcentral/answers/179156-i-need-help-using-the-strrep-for-this-code

评论： Kratos 2015-2-19

here are the three test cases. My question is how do I use strrep to replace the x of that function. The thing is it doesn't have to be x it can be anything but the basic layout is alway going to be functionName(varName) =

 [val1] = ugaFunc('f(x) = 2*x^3', 5)
 [val2] = ugaFunc('veloc(pos) = 2 / pos ^ 2 - 3', 0.5)
 [val3] = ugaFunc('f(var_name) = 4 * (3 / var_name)^var_name-10*var_name',3.14)
 function answer = hat(str, val)
 % replace the input function with the value you want
 a = strrep(str, ' ', 'val');
 [b c] = strtok(a, '=');
 [d e] = strtok(c, '0123456789 ');
  % first number
    [answer rest]= strtok(, '+-*/^');
    answer = str2num(answer);
    % a while loop to get a number and an operator each time
    while (length(rest) ~= 0)
        [operator rest]= strtok(rest, '0123456789 ');
        [num rest] = strtok(rest, '+-*/^ ');
        num = str2num(num);
        % do the math
        switch operator
            case '+'
                answer = answer + num;
            case '-'
                answer = answer - num;
            case '*'
                answer = answer .* num;
            case '/'
                answer = answer ./ num;
            case '^'
                answer = answer .^ num;
        end    
    end
end

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Answer 1

Guillaume 2015-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/179156-i-need-help-using-the-strrep-for-this-code#answer_168502

编辑：Guillaume 2015-2-19

在 MATLAB Online 中打开

You haven't said what you want to replace it with. The value specified in the second argument of your hat function?

In any case, to find the name of the variable, use regular expressions. For example, assuming a simple grammar where the function name is exclusively letters, the following would extract the variable name:

varname = regexp(str, '(?<=[A-Za-z]+\().*?(?=\))', 'match', 'once')

Note that there are many possible expression leading to the same result. The above looks for the shortest (lazy match) sequence of any characters (the .*?) immediately following (the (?<=....) one or more letters (the [A-Za-z]+) plus a bracket (the \(|) and immediately preceding (the |(?=...)) a closing bracket (the \)).

You can then replace the varname in the rest of the expression by the value with strrep or regexprep

newexpr = strrep(str, varname, num2str(val)); %is this what you want?

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Guillaume 2015-2-19

在 MATLAB Online 中打开

Oh, for Pete's sake, try the code. It makes no assumption on what's inside the brackets.

replacevar = @(expr, value) strrep(expr, regexp(expr, '(?<=[A-Za-z]+\().*?(?=\))', 'match', 'once'), num2str(value));
>>replacevar('f(x) = 2*x^3', 5)
  ans = 
  f(3) = 2*3^3
>>replacevar('veloc(pos) = 2 / pos ^ 2 - 3', 0.5)
  ans =
  veloc(0.5) = 2 / 0.5 ^ 2 - 3
>>replacevar('f(var_name) = 4 * (3 / var_name)^var_name-10*var_name',3.14)
  ans =
  f(3.14) = 4 * (3 / 3.14)^3.14-10*3.14