Parsing strings of equations in c using scanf

Question

I need to parse input from a user that could be any number of variations: 1+1 4( 3-0 ) =x 1*(3)-8

How do I do this using scanf to get the raw_input, then split out all of the different values and tell either if it is a string ie x = - () or a int?

This is what I was thinking

    char * raw_input;
    scanf("%s",raw_input);

It takes a array of char and then I just need to split and convert into a single elements. What is the best way of doing the input and (splitting and converting)

Thanks

Answer 1

If you want to write your own code, the best way is to define your expression in form of a grammar. To easily parse the grammar, it's best to make it simple.

For example, to parse expressions in the form like this (1+(3*4+x)*y)+1, you could write such a grammar:

Expression -> Addition | null
Addition -> Multiplication RestOfAddition
RestOfAddition -> null | + Addition
Multiplication -> Element RestOfMultiplication
RestOfMultiplication -> null | * Element
Element -> number | variable | ( Expression )

Then in your program, for every non-terminal in this grammar (the ones on the left of ->), you write one function, like this:

ExpTree *Expression(char *exp, int *position)
{
    if (exp[*position])
    {
        ExpTree *node = malloc(sizeof(*node));
        node->type = LAMBDA;
        node->value = 0;
        return node;
    }
    else
        return Addition(exp, position);
}

ExpTree *Addition(char *exp, int *position)
{
    ExpTree *node = malloc(sizeof(*node));
    node->type = ADDITION;
    node->left = Multiplication(exp, position);
    node->right = RestOfAddition(exp, position);
    return node;
}

ExpTree *RestOfAddition(char *exp, int *position)
{
    ExpTree *node;
    if (exp[*position] == '+')
    {
        ++*position;
        return Addition(exp, position);
    }
    else
    {
        ExpTree *node = malloc(sizeof(*node));
        node->type = LAMBDA;
        node->value = 0;
        return node;
    }
}

Similarly, Multiplication and RestOfMultiplication would be written as functions.

ExpTree *Element(char *exp, int *position)
{
    if (exp[*position] == '(')
    {
        ExpTree *node;
        ++*position;
        node = Expression(exp, position);
        if (!exp[*position] != ')')
             printf("Expected ) at position %d
", *position);
        else
             ++*position;
        return node;
    }
    else if (exp[*position] == ')')
    {
        printf("Unexpected ) at position %d
", *position);
        return NULL;
    }
    else if (exp[*position] >= '0' && exp[*position] <= '9')
    {
        ExpTree *node = malloc(sizeof(*node));
        node->type = INTEGER;
        node->value = extract_int(exp, position);
        return node;
    }
    else if ((exp[*position] >= 'a' && exp[*position] <= 'z') ||
             (exp[*position] >= 'A' && exp[*position] <= 'Z') ||
             exp[*position] == '_')
    {
        ExpTree *node = malloc(sizeof(*node));
        node->type = VARIABLE;
        node->value = extract_variable(exp, position);
        return node;
    }
    else
    {
        printf("Warning: unexpected character %c in location %d
", exp[*position], *position);
        return NULL;
    }
}

Where extract_int and extract_variable are two functions that take the expression and the position on it, go ahead while they are seeing a number (or a letter in the extract_variable function) they build the number (variable) from the expression string and return it, setting position to after where they finished.

Note: This is not code for copy paste. It is not complete and lacks sufficient error checking. Some details have been omitted and is offered as a solution to teach how simple parsing is done rather than easiest solution.