返回数值类型

///////////////////////////////////////////////////////////////////////////////////////////
//    This program can determin very accurately the nature of the user input,
//    it detects whether it is an integer, a float, a number in scientific notation
//    or simply an invalid input. To be capable of doing this the program uses a simple FSM
//    (FINITE STATE MACHINE) to represent the possible states of the input.( INT, FLOAT,.. )
//    author: Gonzales Cenelia
///////////////////////////////////////////////////////////////////////////////////////////
#include <iostream>
using std::cin;
using std::endl;
using std::cout;

//===========================================================================
//        the list of all the possible states for the current FSM
//===========================================================================
enum STATE{ START, INT, FLOAT, SCIENTIFIC, EXPONENT, S1, S2, INVALID } state;

STATE Transition( char *str );
void PrintState( STATE state );

int main() {
    // declaring buffer variable
    char buffer[32] = {0}; 
    // geting input from the user
    cout << "\nPlease enter a number: ";
    cin.getline( buffer, 32 );
    // compute final state
    STATE FINAL_STATE = Transition(buffer);
    // prints the final state
    PrintState(FINAL_STATE);
    return 0;
}

//================================================
// makes the transition from one state to another
//================================================
STATE Transition( char *str ) {
    int NEXT_SYMBOL;
    for( ; *str && state != INVALID; str++ ) {
        NEXT_SYMBOL = *str;
        switch(state) {
        case START:
            if(isdigit(NEXT_SYMBOL)) {
                state = INT;
            }
            else if( NEXT_SYMBOL == '+' || NEXT_SYMBOL == '-' ) {
                state = S1;
            }
            else if( NEXT_SYMBOL == '.' ) {
                state = FLOAT;
            }
            else {
                state = INVALID;
            }
            break;
        case S1:
            if(isdigit(NEXT_SYMBOL)) {
                state = INT;
            }
            else if( NEXT_SYMBOL == '.' ) {
                state = FLOAT;
            }
            else if(!isdigit(NEXT_SYMBOL)) {
                state = INVALID;
            }
            break;
        case INT:
            if( NEXT_SYMBOL == '.' ) {
                state = FLOAT;
            }
            else if(!isdigit(NEXT_SYMBOL)) {
                state = INVALID;
            }
            break;
        case FLOAT:
            if( NEXT_SYMBOL == 'E' || NEXT_SYMBOL == 'e' ) {
                state = S2;
            }
            else if(!isdigit(NEXT_SYMBOL)) {
                state = INVALID;
            }
            break;
        case S2:
            if( NEXT_SYMBOL == '+' || NEXT_SYMBOL == '-' ) {
                state = EXPONENT;
            }
            else {
                state = INVALID;
            }
            break;
        case EXPONENT:
            if(isdigit(NEXT_SYMBOL)) {
                state = SCIENTIFIC;
            }
            else {
                state = INVALID;
            }
            break;
        case SCIENTIFIC:
            if(!isdigit(NEXT_SYMBOL)) {
                state = INVALID;
            }
            break;
        }
    }
    return state;
}

//=====================================
// prints the current state of the FSM
//=====================================
void PrintState( STATE state ) {
    cout << "\nFSM state: ";
    switch(state) {
    case INT:
        cout << "INT ";
        break;
    case FLOAT:
        cout << "FLOAT ";
        break;
    case SCIENTIFIC:
        cout << "SCIENTIFIC ";
        break;
    case INVALID:
        cout << "INVALID ";
        break;
    }
}