"Zhejiang University patest" 02-3. To find the value of a prefix expression

The arithmetic expression has the form of prefix notation, infix notation and suffix notation. The prefix expression means that the two-tuple operator precedes two operands, such as the prefix expression for 2+3* (7-4) +8/4: + + 2 * 3-7 4/8 4. Please design the program to calculate the result value of the prefix expression.

Input Format Description:

Enter a prefix expression of no more than 30 characters in a row, containing only the + 、-、 *, \, and operands, separated by a space between the different objects (operands, operation symbols).

Output Format Description:

The output prefix expression evaluates to 1 digits after the decimal point, or error message "error".

Sample input and output:

Serial number	Input	Output
1	+ + 2 * 3-7 4/8 4	13.0
2	/-25 + *-2 3 4/8 4	12.5
3	/5 + *-2 3 4/8 2	ERROR
4	+10.23	10.2

"Solution"

The evaluation of the prefix expression, the algorithm is not difficult, I think the more difficult place is the processing of strings and some of the details of the place, compared to spend time.

The algorithm is as follows:

"For the evaluation of a prefix expression, the first is to scan the expression from right to left, starting with the first character on the right, if the current character is a number, until the end of the number string is recorded, if the operator, the right is the nearest two" number string "as the corresponding operation, as a new" number string " and recorded. The value of the last operation is the value of the expression, which is always scanned to the leftmost end of the expression. For example, the prefix expression "-1 + 2 3" Evaluation, scan to 3 o'clock, record the number string, scan to 2 o'clock, record the number string, when the scan to +, will + right to do two adjacent numbers of binary operators, recorded as 2+3, the result is 5, record the new number string, and continue to scan to the left, scan to 1 o'clock, Record this number string, scan to-when, move-right to the next two-digit string operator, recorded as 1-5, the result is-4, so the value of the expression is-4. "--from Baidu encyclopedia entry" prefix expression "

According to this algorithm to write out on the stack, time complexity O (n).

The AC code is as follows:

1#include <stdio.h>2#include <string.h>3#include <stdlib.h>4#include <math.h>5 6 #defineException ();7printf"ERROR");8Exit0);9 Ten intContain (Charch) One { A     if(ch=='+'|| ch=='-'|| ch=='*'|| ch=='/')return 1; -     return 0; - } the  - floatCalcfloatAfloatBCharch) - { -     floatans; +     Switch(CH) -     { +          Case '+': AAns = a +b; at              Break; -          Case '-': -Ans = ab; -              Break; -          Case '*': -Ans = A *b; in              Break; -          Case '/': to             if(b = =0) {exception ();} +Ans = A/b; -              Break; the     } *     returnans; $ }Panax Notoginseng  - floatAddbit (floatAnsintBooCharChintPower) the { +     if(!boo)return(ans*Ten+ch-'0'); A     Else  return(Ans+ (float) (ch-'0')/pow (Ten, Power)); the } +  - floatStr2flo (Char*ch) $ { $     floatans; -     intPower =0; -     intBoo =0, positive =1; the      -     if(*ch = ='-') Wuyi     { thech++; -Positive =0; Wu     } -     Else if(*ch = ='+') ch++; About      $Ans = *ch-'0'; -ch++; -      -      while(*ch! =' '&& *ch! =' /'&& *ch! ='\ n') A     { +         if(*ch>='0'&& *ch<='9') the         { -             if(Boo) power++; $Ans = addbit (ans, Boo, *ch, power); the         }  the         Else if(*ch=='.') the         { theBoo =1; -         } inch++; the     } the      About     if(positive)returnAnsElse return-ans; the } the  the intMain () + { -     Charstr[ *]; the     intn, I, tail =0;Bayi     floatans[ *]; the     intBoo =0; the      - gets (str); -      then = strlen (str)-1; the      the      while(N >=0) the     { -         if(contain (Str[n])) the         { the             if(Tail <2) {exception ();} the             Else 94             { theans[tail-2] = Calc (ans[tail-1], ans[tail-2], str[n]); thetail--;  then = n-2;98             } About         }  -         Else if(Str[n]! =' ')101         {102              while(str[n]!=' '&& n>=0) n--;103ans[tail++] = Str2flo (str+n+1);104n--; the         }106         Elsen--;107     }108     109     if(Tail = =1) the     {111printf"%.1f", ans[0]); the     } 113     Else{exception ();} the      the     return 0; the}

"Zhejiang University patest" 02-3. To find the value of a prefix expression