递归实现表达式运算（C语言）

通过递归的方法实现简单表达式的运算，支持加减乘除以及多层括号运算。

//eval.h

//
//eval.h

#if !defined(__EVAL__H__)
#define __EVAL__H__

//  evaluate the value of the str.
//  It support +-*/ with non-nagetive integers only.
//  @param str The string to be evaluated. This string is to be modified.
//  @return the value evaluated.
int eval(char* str);

#endif 

//eval.c

//
//eval.c 

#include "eval.h"
#include <string.h>
#include <stdio.h>

#define CH2DEC(x) ((x) - '0')

//  find the most right and lowest operator in the str
//  @return -1 if no any operators in the str; or
//  the index of the operator sepeicified in the str
static int findop(const char *str);

//  return the rank of the operator
//  @return -1 if the char ch is not an operator; or
//  the rank listed as 1 for * and /, 2 for + and -
static int oprank(char ch);

//  convert a string into a number.
//  The string should consist of all digits.
//  @return the converted number
static int str2int(const char *str);

//  calculate a op b.
//  It supports +-*/ as the op.
//  @return the result of a op b
static int calc(int a, char op, int b);

int eval(char *str)
{
	int ret = 0;
	char str1[200];
	int loc = findop(str);
	if (loc == -1) ret = str2int(str);
	else {
		char op = str[loc];
		str[loc] = '\0';
		if (op == ')') {
			int k;
			for (k = 0; k < loc; k++) {
				if (str[k] == '(') {
					str[k] = '\0';
					break;
				}
			}
			int temp = eval(str + k + 1);
			itoa(temp, str1, 5);
			strcpy(str1, strcat(str, str1));
			strcpy(str, strcat(str1, str + loc + 1));
			ret = eval(str);
		}
		else {
			int left = eval(str);
			int right = eval(str + loc + 1);
			ret = calc(left, op, right);
		}
	}
	return ret;
}

static int findop(const char *str)
{
	int i;
	int ret = -1;
	int lowestoprank = -1;
	for (i = strlen(str) - 1; i >= 0; i--) {
		int rank = oprank(str[i]);
		if (rank == 0) {
			ret = i;
			break;
		}
		if (rank == 3 && oprank(str[i - 1]) > 0) rank = 1;
		else if (rank > lowestoprank) {
			ret = i;
			lowestoprank = rank;
			if (str[i - 1] == ')') break;
		}
	}
	return ret;
}

static int oprank(char ch)
{
	if (ch == '+' || ch == '-') return 3;
	else if (ch == '*' || ch == '/') return 2;
	else if (ch == ')') return 0;
	else return -1;
}

static int str2int(const char *str)
{
	int i;
	int ret = 0;
	for (i = 0; i < strlen(str); i++) ret = ret * 10 + CH2DEC(str[i]);
	return ret;
}

static int calc(int a, char op, int b)
{
	switch (op) {
	case '+': return a + b;
	case '-': return a - b;
	case '*': return a * b;
	case '/': return a / b;
	}
}

void meau()
{
	printf(">> Welcome to the intelligent computing system!\n");
	printf(">> Enter 1 for the operation and 0 for the exit!\n");
	printf("\n>> Your choice:");
}

//main.c

//
//main.c

#include <stdio.h>
#include <assert.h>
#include <conio.h>
#include "eval.h"

int main()
{
	char line[100];
	int flag = 1;
	while (flag)
	{
		meau();
		int a;
		scanf("%d", &a);
		if (a == 1) {
			printf("\n>> Please enter the string:");
			scanf("%s", line);
			printf(">> The result is:%s=", line);
			printf("%d\n\n", eval(line));
		}
		else {
			flag = 0;
			break;
		}
		printf(">> Press any key to continue!");
		getch();
		system("cls");
	}
	printf("\n>> Welcome to use next time!");
}