C++ extract polynomial coefficients

Question

So I have a polynomial that looks like this: -4x^0 + x^1 + 4x^3 - 3x^4
I can tokenize this by space and \'+\' into: -4x^0, x^1, 4x^3, -, 3x^4

How could I just get t

Answer 1

Write a simple tokenizer. Define a number token (/[-0123456789][0123456789]+/), an exponent token (/x^(::number::)/). Ignore whitespace and +.

Continually read tokens as you'd expect them until the end of the string. Then spit out the tokens in whatever form you want (e.g. integers).

int readNumber(const char **input) {
    /* Let stdio read it for us. */
    int number;
    int charsRead;
    int itemsRead;

    itemsRead = sscanf(**input, "%d%n", &number, &charsRead);

    if(itemsRead <= 0) {
        // Parse error.
        return -1;
    }

    *input += charsRead;

    return number;
}

int readExponent(const char **input) {
    if(strncmp("x^", *input, 2) != 0) {
        // Parse error.
        return -1;
    }

    *input += 2;

    return readNumber(input);
}

/* aka skipWhitespaceAndPlus */
void readToNextToken(const char **input) {
    while(**input && (isspace(**input) || **input == '+')) {
        ++*input;
    }
}

void readTerm(const char **input. int &coefficient, int &exponent, bool &success) {
    success = false;

    readToNextToken(input);

    if(!**input) {
        return;
    }

    coefficient = readNumber(input);

    readToNextToken(input);

    if(!**input) {
        // Parse error.
        return;
    }

    exponent = readExponent(input);

    success = true;
}

/* Exponent => coefficient. */
std::map readPolynomial(const char *input) {
    std::map ret;

    bool success = true;

    while(success) {
        int coefficient, exponent;

        readTerm(&input, coefficient, exponent, success);

        if(success) {
            ret[exponent] = coefficient;
        }
    }

    return ret;
}

This would probably all go nicely in a class with some abstraction (e.g. read from a stream instead of a plain string).