表达式求值 递归实现（c/c++)

include “stdio.h”

include”stdlib.h”

include”queue.h”

/表达式求值，引用了上周发的循环队列/
double factor(Queue exp); /求因子的值*/
double term(Queue exp); /求项的值*/
double expression(Queue exp); /求表达式的值*/

char Peek(Queue *stra) //简化peek函数
{
char a ;
peek(stra,&a);
return a;
}

char Outqueue(Queue *stra) //简化Outqueue函数
{
char a ;
outqueue(stra,&a);
return a;
}

/* （表达式（项（因子（表达式||数字||单目运算符））））
expression term factor*/

double factor(Queue *exp)
{
double result = 0;
if(Peek(exp) == ‘(‘) //因子
{
Outqueue(exp);
result = expression(exp);
if(Outqueue(exp) !=’)’) printf(“Error: lack ‘)’ \n”);
}
else if(Peek(exp) == ‘+’ || Peek(exp) ==’-‘)//单目运算
{
switch(Peek(exp))
{
case ‘+’:
Outqueue(exp);
if(Peek(exp) == ‘(‘)
result = factor(exp);
else
result = term(exp);
break;
case ‘-‘:
Outqueue(exp);
if(Peek(exp) == ‘(‘)
result = -factor(exp);
else
result = -term(exp);
break;

    }}else if(Peek(exp) >= '0' && Peek(exp)  <= '9' ||Peek(exp) == '.') //数字{    char nmb[20]={0};    int i=0;    /*数字 可能有 0~9 和. 组成，可以直接通过C库函数完成字符串转浮点数*/    while(Peek(exp) >= '0' && Peek(exp)  <= '9' ||Peek(exp) == '.')    {        nmb[i++] = Outqueue(exp);    }    nmb[i] = '\0';    result = atof(nmb);}return result;

}

double term(Queue *exp)
{
double result = 0;
result = factor(exp);
while(Peek(exp) ==’*’ || Peek(exp) == ‘/’)
switch(Peek(exp))
{
case ‘*’ :
Outqueue(exp);
result = result * factor(exp);
break;
case ‘/’ :
Outqueue(exp);
result = result / factor(exp);
break;
}

return result;
}

double expression(Queue *exp)
{
char t;
double result = 0;
result = term(exp);
while(Peek(exp) == ‘+’ || Peek(exp) ==’-‘)
switch(Peek(exp))
{
case ‘+’ :
Outqueue(exp);
result = result + term(exp);
break;
case ‘-’ :
Outqueue(exp);
result = result - term(exp);
break;
}

return result;
}

void readme()
{
printf(“support + - * / () quit :quit \n”);
}

int main(int argc, char *argv[])
{
Queue stra;

initqueue(&stra,100);readme();char c;

while(1)
{
do
{
if ((c = getchar()) == ‘q’ ) exit(1);
if(isFull(&stra))
{
printf(“error: full for memery”);
exit(1);
}
else
if(c != ’ ‘)addqueue(&stra,c);
}
while(c != ‘\n’ && c != EOF);

printf("=%lf \n",expression(&stra));Outqueue(&stra);if(!isEmpty(&stra))printf("Error: don't knew sign\n");clearnq(&stra);

}
delq(&stra);
return 0;
}
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