前缀中缀后缀表达式求值

前缀表达式
前缀表达式求值方式是从后向前算数，遇到符号就把栈中的上面两个数拿出来计算，然后再压栈。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
using namespace std;

double a[1005];
int top;
char str[105];
int is_op(char ch){
  if (ch=='+' || ch=='-' || ch=='*' || ch=='/'){
    return 1;
  }else{
    return 0;
  }
}
int main(){
  while(cin.getline(str, 105)){
    memset(a, 0, sizeof(0));
    top=0;
    int len=strlen(str);
    int pow=1;
    double tsum=0;
    int flagErr=0;  //err->flag=1
    for (int i=len-1; i>=0; i--){
      if (str[i]>='0' && str[i]<='9'){
        tsum+=(str[i]-'0')*pow;
        pow*=10;
      }else if (str[i]=='.'){
        tsum=tsum/(pow*1.0);
        pow=1;
      }else if (str[i]==' ' && tsum!=0){
        a[++top]=tsum;
        tsum=0, pow=1;
      }else if (str[i]=='-' && tsum!=0){
        a[++top]=-tsum;
        tsum=0;
        pow=1;
        i--;
      }else if (str[i]=='+' && tsum!=0){
        a[++top]=tsum;
        tsum=0;
        pow=1;
        i--;
      }else if (is_op(str[i])){
        double x=a[top]; top--;
        double y=a[top]; top--;
        double z=0;
        if (str[i]=='+'){
          z=x+y;
        }else if (str[i]=='-'){
          z=x-y;
        }else if (str[i]=='*'){
          z=x*y;
        }else if (str[i]=='/'){
          if (y==0){
            flagErr=1;
            break;
          }else{
            z=x/y;
          }
        }
        a[++top]=z;
      }
    }
    if (flagErr==1){
      printf("err\n");
    }else{
      printf("%.1f\n", a[top]);
    } 
  }
  return 0;
}

中缀表达式

#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
#include <algorithm>
using namespace std;

double a[205];
int top, flag;
string str;

int main()
{
  while (getline(cin, str)){
    memset(a, 0, sizeof(a));
    int len=str.length();
    top=0, flag=1;
    if (str[0]=='0' && len==1)
      break;
    double ans=0, sum=0;
    for (int i=0; i<len; i++){
      if (str[i]==' ')  continue;
      if (str[i]>='0' && str[i]<='9')
        ans=ans*10+str[i]-'0';
      else{
        if (flag==1){a[++top]=ans; ans=0;
        }
        if (flag==2){a[++top]=-ans; ans=0;
        }
        if (flag==3){double tmp=a[top];
          a[top]=tmp*ans;
          ans=0;
        }
        if (flag==4){double tmp=a[top];
          a[top]=tmp/ans;
          ans=0;
        }
        if (str[i]=='+')  flag=1;
        if (str[i]=='-')  flag=2;
        if (str[i]=='*')  flag=3;
        if (str[i]=='/')  flag=4;
      }
      if (i==len-1){
        if (flag==1){a[++top]=ans; ans=0;
        }
        if (flag==2){a[++top]=-ans; ans=0;
        }
        if (flag==3){double tmp=a[top];
          a[top]=tmp*ans;
          ans=0;
        }
        if (flag==4){double tmp=a[top];
          a[top]=tmp/ans;
          ans=0;
        }
      } 
    }
    for (int i=1; i<=top; i++)
      sum+=a[i];
    printf("%.2f\n", sum);
  }
  return 0;
}

后缀表达式

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
using namespace std;

double a[1005];
int top;
char str[105];

int is_op(char ch){
  if (ch=='+' || ch=='-' || ch=='*' || ch=='/'){
    return 1;
  }else{
    return 0;
  }
}
int main(){
  while (cin.getline(str, 105)){
    memset(a, 0, sizeof(a));
    top=0;
    int len=strlen(str);
    int tsum=0;
    for (int i=0; i<len; i++){
      if (str[i]>='0' && str[i]<='9'){
        tsum=tsum*10+str[i]-'0';
      }else if (str[i]==' ' && tsum!=0){
        a[++top]=tsum;
        tsum=0; 
      }else if (is_op(str[i])){
        double y=a[top];  top--;
        double x=a[top];  top--;
        double z;
        if (str[i]=='+'){
          z=x+y;
        }else if (str[i]=='-'){
          z=x-y;
        }else if (str[i]=='*'){
          z=x*y;
        }else if (str[i]=='/'){
          z=x/y;
        }
        a[++top]=z;
      }
    }
    printf("%.1f\n", a[top]);
  }
  return 0;
}