四则运算算法

通过输入四则运算表达是如1+2*3+4/5转换为1 2 3 * + 4 5 / +并求值。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define SIZE 100

/*符号优先级*/
int pri(char c)
{
	switch(c)
	{
		case '*':
		case '/':
			return 3;
		case '+':
		case '-':
			return 2;
		case '(':
		case ')':
			return 4;
		case '=':
			return 1;
		default:
			return -1;
	}
}

/*通过输入的算术式得到该算术式对应的后缀表达式*/
void change(char* src, char* output)
{
	int i = 0, optSiz = 0, symSTop = -1;
        /*用于存放算术符号的栈*/
	char symS[SIZE] = {'\0'};
        /*遍历算术表达式*/
	while(i < strlen(src))
	{	/* 如果是数字，不需要压入到符号栈，直接写入到输出字符串*/
		if( src[i] >= '0' && src[i] <= '9')
		{
			while(src[i] >= '0' && src[i] <= '9')
			{
				output[optSiz++] = src[i++];
			}
                        /*12+34 -> 1234+ 加上空格-> 12 34 +  */
			output[optSiz++] = ' ';
		}
                /*当遇上非右括号的符号，进行压栈和出栈*/
		if( ')' != src[i])
		{	/* + - * / and = ( */
			if( (SIZE > symSTop + 1)&& (-1 == symSTop 
				|| pri(src[i]) > pri(symS[symSTop])) )
			{	/*栈未满并且 栈空或者待处理符号的优先级大于栈顶符号的优先级 入栈*/
				symS[++symSTop] = src[i++];
			}else{
				while( -1 != symSTop && '(' != symS[symSTop]
					&& pri(src[i]) <= pri(symS[symSTop]) )
					{       /* 栈不为空 栈顶符号不为'(' 并且 待处理符号的优先级小于栈顶符号时 出栈*/
						output[optSiz++] = symS[symSTop];
						output[optSiz++] = ' ';
						symS[symSTop--] = '\0';
					}
				/* 优先级不小于当前处理符号优先级的符号全部出栈后，将当前符号入栈*/
				symS[++symSTop] = src[i++];
			}
		}
                /*遇上右括号时，进行括号的处理*/
		if( ')' == src[i] )
		{	/* 出栈，直到遇到左括号*/
			while( -1 != symSTop )
			{       /*遇到左括号，进入下一轮处理*/
				if( '(' == symS[symSTop] )
				{
					++i;
					break;
				}
                                /*非左括号，进行出栈处理*/
				output[optSiz++] = symS[symSTop];
				output[optSiz++] = ' ';
				symS[symSTop--] = '\0';
			}
			if( -1 == symSTop ) 
			{
				printf("'(' is missed!\n");
				exit(0);
			}else symS[symSTop--] = '\0';
		}
	}	
	while( -1 != symSTop )
	{	/*将剩余的符号进行出栈处理*/
		if('('== symS[symSTop]){
			printf("')' is missed!\n");
			exit(0);
		}
		output[optSiz++] = symS[symSTop];
		output[optSiz++] = ' ';
		symS[symSTop--] = '\0';
	}
}

int calculate(int m,int n, char com)
{
	switch(com)
	{
		case '+':
			return m + n;
		case '-':
			return m - n;
		case '*':
			return m * n;
		case '/':
			return m / n;
		default:	
			return -1;
	}
}

int cal(char* output)
{
	int i = 0, rslSTop = -1;
	int rslS[SIZE] = {0};
	while(i < strlen(output))
	{
		if(' ' == output[i] || '=' == output[i]){ 
			++i;
			continue;
		}else if('0' <= output[i] && '9' >= output[i])
		{
                        /*获取数据*/
			int val = 0;
			while('0' <= output[i] && '9' >= output[i])
			{
				val = 10*val + (output[i++]-'0');/*in ASCII '0' is 48 in decimal*/
			}
			rslS[++rslSTop] = val;
			continue;
		}
		else if( 1 <= rslSTop )
		{       /*遇上符号时，将栈顶两数与该符号进行四则运算，将计算结果存于栈顶的下一个位置*/
			rslS[rslSTop-1] = calculate(rslS[rslSTop-1], rslS[rslSTop], output[i++]);
                        /*是计算得到的结果成为栈顶*/
			rslS[rslSTop--] = '\0';
		}else{
			printf("error\n");
			return -1;
		}
	}
        /*获取最终的计算结果 即rslS[0]*/
	return rslS[rslSTop];
}

int main()
{
	char input[SIZE] = {'\0'};
	char output[2*SIZE] = {'\0'};
	int iptSiz = 0;
	char gc;
	while(iptSiz < SIZE-1)
	{
		gc = getchar();
		if(' ' == gc) continue;
		if('\n' == gc) break;
		input[iptSiz++] = gc;
		//if('=' == gc) break;
	}
	change(input, output);
	printf("%s\n", output);
	printf("%d\n", cal(output));
	return 0;
}