WSTP simplifying expression from C

Question

I am looking to simplify arbitrary algebraic expressions by invoking the mathematica kernel from C using WSTP. My initial attempt was the following, which was based on the online documentation and the example "factor.c"

algebra = "(a + a + a + a - b)";

WSPutFunction(lp, "EvaluatePacket", 1L);
WSPutFunction(lp, "FullSimplify", 1L);
WSPutString(lp, algebra);
WSEndPacket(lp);

WSFlush(lp);

while( (pkt = WSNextPacket(lp), pkt) && pkt != RETURNPKT ) {
    WSNewPacket(lp);
        if (WSError(lp)) {
            error(lp);
        }
}

const char *output;
if(! WSGetString(lp, &output)) {
    printf("Unable to read from link\n");
    return -1;
} else {
    printf("%s\n", output);
}
WSReleaseString(lp, symbol);

WSPutFunction(lp, "Exit", 0);

However, WSTP returns the still unsimplified expression from the WSGetString() call. I expected to see "4 a - b" as the result of "FullSimplify[(a + a + a + a - b)]".

I then attempted using EnterTextPacket and ReturnTextPacket but it isn't clear how these work.

My goal is to simplify algebraic expressions through WSTP. What is a working simple or common way to do this?

Answer 1

You are not evaluating

FullSimplify[(a + a + a + a - b)]

You are evaluating

FullSimplify["(a + a + a + a - b)"]

You used the PutString function. It does exactly that: it puts a string on the link.

There are three options:

Either put the correct expression structure, which is

Plus[a, a, a, a, Times[-1, b]]

and would involve something like this (untested):

MLPutFunction(lp, "Plus", 5);
  MLPutSymbol(lp, "a");
  MLPutSymbol(lp, "a");
  MLPutSymbol(lp, "a");
  MLPutSymbol(lp, "a");
  MLPutFunction(lp, "Times", 2);
    MLPutInteger(lp, -1);
    MLPutSymbol(lp, "b");

Or you can precede putting the string by MLPutFunction(lp, "ToExpression", 1). This will send ToExpression["(a + a + a + a - b)"] instead of "(a + a + a + a - b)".

Finally, you could use an EnterTextPacket instead of an EvaluatePacket.

Please read this tutorial, especially section 2. This document explains things much better than the documentation.

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Additional comments:

• FullSimplify is not necessary to compute a+a+a+a-b. The result would be the same as without FullSimplify

• You used 1L in WSPutFunction(). Its third argument is in fact of type int (and not long), thus using 1L is not appropriate.