You could just write this yourself. It's easier than it looks. First start with an evaluator for rpn (reverse polish notation) expressions. "x + sin(y)" in rpn would be "y sin x +". First you split that string and make it into a list. Then you can evaluate that with a few lines of code using a stack. Go through the list and if an element is a number or a variable push it on the stack. If it's an operator pop the appropriate number of elements from the stack, apply the operator and push the result back on the stack. At the end the stack should contain only one element, which is the result. Evaluating an infix expression is more complicated. One way to do it is by first converting the infix expression to rpn. Notice that the order of the operands is the same in infix and rpn. Only the operators change. To do this conversion you need one list to hold the result and a stack. Go through all the elements of the infix expression. If it's an operand just add it to the result list. If it's an operator pop operators from the stack and add them to the result for as long as the top element on the stack is an operator with a higher precedence then the current one. Then push the current operator on the stack. Opening parenthesis are also pushed and when there is a closing parenthesis you pop operators and add them to the result until you find an opening one. There also is another method of evaluating infix expressions using recursion. This works by first searching for the operator with the lowest precedence and then recursively evaluating the expressions left and right from that operator.
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