EVALUATION D'UNE EXPRESSION PARENTHÉSÉE

#include<stdio.h>
#include<string.h>
           
   void tostring(int n,char* s){
           
      char r[12];
      sprintf(r,"%d",n);
      strcpy(s,r);
   }
           
   int operation(char* s){
           
      int a,b,i,j;
      char s1[11],s2[11];
      s1[0]=s[0];
   
   
      for(i=1;i<(int)strlen(s)&&s[i]!='+'&&s[i]!='-'&&s[i]!='*'&&s[i]!='/'&&s[i]!='%';i++)
         s1[i]=s[i];
      s1[i]='\0';
   
      sscanf(s,"%d",&a);
   
      if(i==(int)strlen(s))
         return a;
   
   
   
      for(j=i+1;j<(int)strlen(s);j++)
         s2[j-i-1]=s[j];
   
      s2[strlen(s)-i-1]='\0';
      sscanf(s2,"%d",&b);
   
   
      switch(s[i]){
         case '+':
            return a+b;
         case '-':
            return a-b;
         
         case '*':
            return a*b;
         
         case '/':
            return a/b;
         
         case '%':
            return a%b;
         default:
            return 0;
      }
   }
           
   int calculer(char *s){
           
      int i=-1,j=0,k;
      char r[2000],tampon[2000];
      while(s[j]!=')'&&j<(int)strlen(s))
      {   
         if(s[j]=='(')
            i=j;
         j++;
      }
      if(i==-1)
         return operation(s);
      for(k=0;k<i;k++)
         r[k]=s[k];
      r[k]='\0';
   
      for(k=i+1;k<j;k++)
         tampon[k-i-1]=s[k];
      tampon[k]='\0';
   
      tostring(calculer(tampon),tampon);
      strcat(r,tampon);
   
   
      for(k=j+1;k<(int)strlen(s);k++)
         tampon[k-j-1]=s[k];
      tampon[strlen(s)-j-1]='\0';
   
      strcat(r,tampon);
      printf("= %s\n",r);
      return calculer(r); 
   }


           
   int main(){
           
      char s[2000];
   while(1){
      scanf("%s",s);
      printf("votre expression = %d\n",calculer(s));
      }

      return 0;
   }