四则运算c++实现

// Ex6_10Extended.cpp代码源自《Beginning Visual C++ 2010》Ivor horton著// A program to implement a calculator accepting parentheses

#include// For stream input/output

#include// For the exit() function

#include// For the isdigit() function

#include// For the strcpy() function

using std::cin;

using std::cout;

using std::endl;

void eatspaces(char* str); // Function to eliminate blanks double expr(char* str); // Function evaluating an expression double term(char* str, int& index); // Function analyzing a term double number(char* str, int& index); // Function to recognize a number char* extract(char* str, int& index); // Function to extract a substring const int MAX(80); // Maximum expression length,

// including '\0'

int main()

{

char buffer[MAX] = {0}; // Input area for expression to be evaluated

cout << endl

<< "Welcome to your friendly calculator."

<< endl

<< "Enter an expression, or an empty line to quit."

<< endl;

for(;;)

{

cin.getline(buffer, sizeof buffer); // Read an input line

eatspaces(buffer); // Remove blanks from input

if(!buffer[0]) // Empty line ends calculator return 0;

cout << "\t= " << expr(buffer) // Output value of expression << endl << endl;

}

}

// Function to eliminate spaces from a string

void eatspaces(char* str)

{

int i(0); // 'Copy to' index to string int j(0); // 'Copy from' index to string

while((*(str + i) = *(str + j++)) != '\0') // Loop while character

// copied is not \0 if(*(str + i) != ' ') // Increment i as long as i++; // character is not a space return;

}

// Function to evaluate an arithmetic expression

double expr(char* str)

{

double value(0.0); // Store result here

int index(0); // Keeps track of current character position

value = term(str, index); // Get first term

for(;;) // Indefinite loop, all exits inside

{

switch(*(str + index++)) // Choose action based on current character

{

case'\0': // We're at the end of the string return value; // so return what we have got

case'+': // + found so add in the

value += term(str, index); // next term

break;

case'-': // - found so subtract

value -= term(str, index); // the next term

break;

default: // If we reach here the string

cout << endl // is junk

<< "Arrrgh!*#!! There's an error"

<< endl;

exit(1);

}

}

}

// Function to get the value of a term

double term(char* str, int& index)

{

double value(0.0); // Somewhere to accumulate

// the result

value = number(str, index); // Get the first number in the term

// Loop as long as we have a good operator

while(true)

{

if(*(str + index) == '*') // If it's multiply,

value *= number(str, ++index); // multiply by next number

else if(*(str + index) == '/') // If it's divide,

value /= number(str, ++index); // divide by next number else

break;

}

return value; // We've finished, so return what

// we've got

}

// Function to recognize a number in a string

double number(char* str, int& index)

{

double value(0.0); // Store the resulting value

if(*(str + index) == '(') // Start of parentheses

{

char* psubstr(nullptr); // Pointer for substring

psubstr = extract(str, ++index); // 将两个括弧间的子字符串拷贝给