将字符串迭代器传递给函数

Question

I have a function I want to use in other function which gets number from string. In base funcion I create std::string::iterator and I want to pass it to other function that gets this number, skips spaces, and then outer function continues to search through string.

When I am using iterator I do something like *strPtr to get value, and *++strPtr to get value and move to another character. When I pass pointer to that I need to use **strPtr to get value and I hove no idea how to get value and increment iterator. :(

I did *++*strPtr and it kinda worked but it crashes when it quits internal function.

double calculate( char* x ){
std::string formula = x;
ValueStack valueStack;
OperatorStack operatorStack;

std::string::iterator strPtr = formula.begin();

while( *strPtr == ' ' )
{
    strPtr++;
}

if( *strPtr == '-' )
{
    std::string temp;
    temp += *strPtr;

    while( *++strPtr == ' ' );

    while( ((*strPtr <= '9' ) && (*strPtr >= '0' )) || (*strPtr == '.') )
    {
        temp += *strPtr;
        while( *++strPtr == ' ' );
    }

    char** x;
    valueStack.push( strtod( temp.c_str(), x ) );
}

while( strPtr < formula.end() )
{

    if( *strPtr == ' ' )
    {
        while( *++strPtr == ' ' );
    }

    if( operatorStack.isOper( *strPtr ) )
    {   
        if( *strPtr == ')' )
        {
            evaluate( &valueStack, &operatorStack );
            strPtr++;
        }

        else if( *strPtr == '(' )
        {
            operatorStack.push( *strPtr );

            while( *++strPtr == ' ' );

            if( *strPtr == '-' )
            {
                std::string temp;
                temp += *strPtr;

                while( *++strPtr == ' ' );

                while( ((*strPtr <= '9' ) && (*strPtr >= '0' )) || (*strPtr == '.') )
                {
                    temp += *strPtr;
                    while( *++strPtr == ' ' );
                }

                char** x;
                valueStack.push( strtod( temp.c_str(), x ) );
            }
        }

        else if( operatorStack.prior( *strPtr ) <= operatorStack.prior( operatorStack.top() ) )
        {
            evaluate( &valueStack, &operatorStack );
            operatorStack.push( *strPtr++ );
        }

        else operatorStack.push( *strPtr++ );
    }

    else
    {
        std::string temp;
        temp += *strPtr;

        while( *++strPtr == ' ' );

        while( ((*strPtr <= '9' ) && (*strPtr >= '0' )) || (*strPtr == '.') )
        {
            temp += *strPtr;
            while( *++strPtr == ' ' );
        }

        char** x;
        valueStack.push( strtod( temp.c_str(), x ) );
    }
}

evaluate( &valueStack, &operatorStack );

double ret = valueStack.pop();
if( pow(ret, 2) < 0.000000001 ) return 0;
return ret; 
}

To clarify, I want make function like this, and use it above:

void getValue( std::string::iterator strPtr, ValueStack* valueStack )
{
std::string temp;
    temp += *strPtr;

    while( *++strPtr == ' ' );

    while( ((*strPtr <= '9' ) && (*strPtr >= '0' )) || (*strPtr == '.') )
    {
        temp += *strPtr;
        while( *++strPtr == ' ' );
    }

    char** x;
    valueStack->push( strtod( temp.c_str(), x ) );
}

Answer 1

There are an easier way for iterating over a string you can, for instance:

    string str("3+5-4   -  3 / 4");

    for (string::iterator it = str.begin(); it!=str.end();++it)
    {
        if (!(*it == ' ')) {  // Skip spaces.

           if(isdigit(*it))
           {
             // Do your thing with ValueStack.
           }
           else
           {
               // Handle operators here.
               switch(*it)
               {
                   case('-'): break;
                   case('+'): break;
                   case('/'): break;
                   case('*'): break;
               }
           }
        }
    }

On the other hand, I recommend you take a look at Boost.Spirit . It will help you to achive what you're trying to do (evaluating mathematical expressions) in a more elegant way. Also in that link there are an example of a "calculator" just like you want but using spirit .

Your algorithm only supports one digit numbers. And its very hard to get multi-digit numbers supported without using some external library (such as Boost.Spirit for instance ;) ).

Answer 2

One thing I see in your code is that you assume that std::string is null terminated. Most of your loops are not bounded properly. For example:

while( *strPtr == ' ' )
{
    strPtr++;
}

What if the entire string is spaces? What stops this loop from going into uncharted territory?

A std::string does not have to be null terminated, so there is no guarantee this loop will stop when it reaches the end of the string. You do the same sort of loop in other places in your code.

If you're going to loop this way, it would need to be written:

while( *strPtr == ' ' && strPtr != formula.end())
    strPtr++;