//Simple intger arithmetic calculator according to the EBNF:
// <exp> -> <term>{<addop><term>}
// <addop> -> +|-
// <term> -> <factor>{<mulop><factor>}
// <mulop> -> *
// <factor> -> (<exp>)|Number
//
// Inputs a line of text from stdin
// Outputs "ERROR" or the result.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
char token; //global token variable
//function prototypes for recursive call;
int exp(void);
int term(void);
int factor(void);
void error(void)
{
fprintf(stderr, "Error\n");
exit(1);
}
void match(char expectedToken)
{
if(token == expectedToken)
token = getchar();
else
error();
}
int main()
{
while(1)
{
int result;
printf("请输入算式（可以包含整数的 \'+\' \'-\' \'*\' 和 \')\' \'(\'）运算:\n");
token = getchar();//load token with first character for lookahead
result = exp();
if(token == '\n')//check for end of line
printf("Result = %d\n", result);
else
error();//extraneous chars on line
}
return 0;
}
int exp(void)
{
int temp = term();
while((token == '+')||(token == '-'))
{
switch(token)
{
case '+':
match('+');
temp += term();
break;
case '-':
match('-');
temp -= term();
break;
}
}
return temp;
}
int term(void)
{
int temp = factor();
while(token == '*')
{
match('*');
temp *= factor();
}
return temp;
}
int factor(void)
{
int temp;
if(token == '(')
{
match('(');
temp = exp();
match(')');
}
else if(isdigit(token))
{
ungetc(token, stdin);
scanf("%d", &temp);
token = getchar();
}
else
error();
return temp;
}
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