Basic Calculator II

最新推荐文章于 2021-04-16 11:09:49 发布

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本文介绍了一种基于中缀表达式到后缀表达式转换的方法来实现一个简易计算器。该计算器能够处理包含加减乘除运算的基本算术表达式，并通过设置符号优先级确保正确的运算顺序。

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Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:
“3+2*2” = 7
” 3/2 ” = 1
” 3+5 / 2 ” = 5

说明：先将中缀表达式转化为后缀表达式，再由后缀表达式计算表达式的值。

代码：

struct pri
{
    char op;
    int pri;
};

//设置符号的优先级，便于后续比较符号优先级
struct pri PriArray[5] = {{'=',0}, {'+',1}, {'-',1}, {'*',2}, {'/',2}};

//查找符号的优先级
int findpri(char c)
{
    int i = 0;
    for(i = 0; i < 5; i++)
    {
        if(c == PriArray[i].op)
            return PriArray[i].pri;
    }
    return 0;
}
//比较两个符号的优先级(符号栈栈顶符号和遍历中缀表达式的符号)
int pricom(char top, char tmp)
{
        if(findpri(top) < findpri(tmp))//栈顶符号优先级低，入栈
        return 1;
    else    //栈顶符号优先级高，出栈
        return -1;
}

//中缀表达式转换为后缀表达式
void transfer(char *s, char* postfix)
{
    char *opStack = (char *)malloc(sizeof(char) * strlen(s));  //存放中缀表达式符号的栈
    int top = -1;
    top++;
    char c;
    int i = 0, j = 0;

    opStack[top] = '=';

    while(s[i] != '\0')
    {
        c = s[i];
        if(c >= '0' && c <= '9')
        {
            while(s[i] >= '0' && s[i] <= '9')
            {
                postfix[j++] = s[i++];
            }
            postfix[j++] = '#';
        }
        else if(c == ' ')
        {
            i++;
        }
        else
        {
            switch(pricom(opStack[top], c)){
                case 1:
                    top++;
                    opStack[top] = c;
                    i++;
                    break;
                case -1:
                    while(pricom(opStack[top], c) == -1)
                    {
                        postfix[j++] = opStack[top];
                        top--;
                    }
                    top++;
                    opStack[top] = c;
                    i++;
                    break;
            }

        }
    }
    while(top > 0)    //将符号栈剩余符号全部出栈
    {
        postfix[j++] = opStack[top];
        top--;
    }
    postfix[j] = '\0';
    free(opStack);
}

//计算表达式的值
int calculate(char* s) {
    int *num = (int *)malloc(sizeof(int) * strlen(s)); //存储后缀表达式的数字
    int a = 0, b = 0;
    char *postfix = (char *)malloc(sizeof(char) * 2 * strlen(s)); //存储后缀表达式
    transfer(s, postfix);
    //printf("%s\n",postfix);
    int top = -1, i = 0;
    int tmp;
    //由后缀表达式计算表达式的值
    while(postfix[i] != '\0')  
    {

        switch(postfix[i]){
            case '+':
                a = num[top--];
                b = num[top--];
                num[++top] = a + b;
                i++;
                break;
            case '-':
                a = num[top--];
                b = num[top--];
                num[++top] = b - a;
                i++;
                break;
            case '*':
                a = num[top--];
                b = num[top--];
                num[++top] = a * b;
                i++;
                break;
            case '/':
                a = num[top--];
                b = num[top--];
                num[++top] = b / a;
                i++;
                break;
            default:
                tmp = 0;
                while(postfix[i] >= '0' && postfix[i] <= '9')
                {
                    tmp = 10 * tmp + postfix[i] - '0';
                    i++;
                }
                num[++top] = tmp;
                if(postfix[i] == '#')
                    i++;
                break;
        }
    }
    i = num[top];
    free(num);
    return i;
}