Compiler principle: Implement advanced Expression evaluator (beginner)

#define _crt_secure_no_warnings#include <stdio.h> #include <stdlib.h> #include <string.h>// Processing space void Delspace (char *str) {char *p1 = Str;char *P2 = Str;while ((*p1 = * (p2++)) = = ' + ') {if (*p1! = ') {p1++;}}} int isnum (char ch)//Determines whether the number {int is = 0;if (ch >= ' 0 ' && ch <= ' 9 ') {is = 1;} return is;} Get the string inside the parentheses char *mkh (char *str, int *pindex)//pair The parentheses to remove the contents of the parentheses {char *pstr = null;int num = 0;//record How many parentheses the logarithmic int leftindex = *pin   Dex;do {switch (* (str+ (*pindex))) {case ' (': Num++;break;case ') ': if (0==num) {(*pindex) + +;  Pstr = malloc (sizeof (char) * (*pindex-leftindex));//Allocate memory size strncpy_s (PSTR, *pindex-leftindex, str + leftindex, *pindex   -leftindex-1);           return pstr; } else {num--;//handles closing parenthesis}break;}} while (* (str+ (*pindex) + +)! = ') ';} Double Getnum (char *str, int *pindex) {Double value = 0.0;int index = *pindex;while (* (str+index) = = ' (') {char *psubstr = NUL L;*pindex = ++INDEX;PSUBSTR = MKH (str, piNdex);d ouble getValue (char *str); value = GetValue (PSUBSTR); free (psubstr);//release memory Psubstr = = Null;return value;}  while (Isnum (* (str + index)))//str[index]{value = value * + (Str[index]-' 0 ');//character to integer index++;//move forward}if (* (str + index) == '.') {Double Xiaoshu = 1.0;while (isnum (* (str + ++index))//loop to non-digit after decimal point {Xiaoshu/= 10;//decimal Value + = xiaoshu* (* (str + index)-' 0 ');}} *pindex = Index;return value;} Double Getfirtnum (char *str, int *pindex) {Double value = 0.0;value = Getnum (str, pindex);//Gets a data while (1) {if (* (str + (*p index) = = ' * ') {(*pindex) ++;value *= getnum (str, pindex);//Calculate Multiplication}else if (* (str + (*pindex)) = = '/') {(*pindex) ++;value/= g Etnum (str, pindex);//calculate multiplication}else{break;}} return value;}  Double GetValue (char *str) {Double value = 0.0;int index = 0;value = Getfirtnum (str, &index);//Gets First data while (1) {char ch = * (str + index);//Remove character index++;//loop traversal switch (CH) {case ' + ': return value;case ' + ': value + = Getfirtnum (str, &index); BR Eak;case '-': Value-= Getfirtnum (str, &index); Break;default: Break;}}} void Main () {printf ("Enter the expression to evaluate:"), char str[1024] = {0};scanf ("%[^\n]", str);d elspace (str);p rintf ("\ n evaluates expression value =%f\n", GetValue (str)); system ("Pause");}

Test results:

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Compiler principle: Implement advanced Expression evaluator (beginner)