表达式求值

来源：互联网发布：巨人网络2018校招面试编辑：程序博客网时间：2024/06/05 16:42
#include <stdio.h>#include <stdlib.h>#include <string.h>#include <iostream>using namespace std;#define MaxSize 20#define MaxOp 7struct    //设定运算符的优先级{    char ch;    //运算符    int pri;    //优先级}lpri[] = {{'=',0},{'(',1},{'*',5},{'/',5},{'+',3},{'-',3},{')',6}}, rpri[] = {{'=',0},{'(',6},{'*',4},{'/',4},{'+',2},{'-',2},{')',1}};int leftpri(char op)    //求左运算符op的优先级{    int i;    for(i = 0; i<MaxOp; i++)        if(lpri[i].ch == op)            return lpri[i].pri;}int rightpri(char op)    //求右运算符op的优先级{    int i;    for(i = 0; i<MaxOp; i++)        if(rpri[i].ch == op)            return rpri[i].pri;}bool InOp(char ch)    //判断ch是否为运算符{    if(ch == '(' || ch == ')' || ch == '+' || ch == '-' || ch == '*' || ch == '/')        return true;    else        return false;}int Precede(char op1, char op2)    //op1 和 op2运算符优先级的比较结果{    if(leftpri(op1) == rightpri(op2))        return 0;    else if(leftpri(op1) < rightpri(op2))        return -1;    else        return 1;}void trans(char *exp, char postexp[]){    struct    {        char data[MaxSize];        int top;    }op;    int i = 0;    op.top = -1;    op.top++;    op.data[op.top] = '=';    while(*exp != '\0')    {        if(!InOp(*exp))        {            while(*exp >= '0' && *exp <='9')            {                postexp[i++] = *exp;                exp++;            }            postexp[i++] = '#';        }        else        {            switch(Precede(op.data[op.top],*exp))            {                case -1:                    op.top++;                    op.data[op.top] = *exp;                    exp++;                    break;                case 0:                    op.top--;                    exp++;                    break;                case 1:                    postexp[i++] = op.data[op.top];                    op.top--;                    break;            }        }    }    while(op.data[op.top] != '=')    {        postexp[i++] = op.data[op.top];        op.top--;    }    postexp[i] = '\0';}float compvalue(char * postexp){    struct    {        float data[MaxSize];        int top;    }st;    float d,a,b,c;    st.top = -1;    while (*postexp != '\0')    {        switch(*postexp)        {            case '+':                a = st.data[st.top];                st.top--;                b = st.data[st.top];                st.top--;                c = a+b;                st.top++;                st.data[st.top] = c;                break;            case '-':                a = st.data[st.top];                st.top--;                b = st.data[st.top];                st.top--;                c = b-a;                st.top++;                st.data[st.top] = c;                break;            case '*':                a = st.data[st.top];                st.top--;                b = st.data[st.top];                st.top--;                c = a*b;                st.top++;                st.data[st.top] = c;                break;            case '/':                a = st.data[st.top];                st.top--;                b = st.data[st.top];                st.top--;                if(a != 0)                {                    c = b/a;                    st.top++;                    st.data[st.top] = c;                }                else                {                    printf("\n\t除零错误！\n");                    exit(0);                }                break;            default:                d = 0;                while(*postexp >= '0' && *postexp <='9')                {                    d = 10 * d + *postexp - '0';                    postexp++;                }                st.top++;                st.data[st.top] = d;                break;        }        postexp++;    }    return (st.data[st.top]);}int main(){    char exp[MaxSize];    char postexp[MaxSize];    while(true)    {        printf("请输入表达式:");        gets(exp);        trans(exp, postexp);        printf("表达式的值：%g\n",compvalue(postexp));    }    return 0;}
心得体会：栈的东西，一定要注意指针的移动，指向的位置。
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