assignment 3-1 calculate.cpp

this code is code c++. have many lesson

#include<iostream>
#include<string>
using namespace std;

int main()
{
  string a,m1,m2;
  cout << "Equation : " ;
  getline(cin,a);
  
  int n=0,s1,s2;
  if(a.find("+")!= -1){
  n = a.find("+");
  m1 = a.substr(0,n);
  m2 = a.substr(n+1);
  s1 = m1.length();
  s2 = m2.length();
 
  float b1=0,b2=0,output;
        
        if(s1 == 3){
                   b1 = (int)(m1.at(0)-'0')*100;
                   b1 += (int)(m1.at(1)-'0')*10;
                   b1 += (int)(m1.at(2)-'0');}
        else if(s1 == 2){
                   b1 = (int)(m1.at(0)-'0')*10; 
                   b1 += (int)(m1.at(1)-'0'); }         
        else if(s1 == 1){
                   b1 = (int)(m1.at(0)-'0');}
        if(s2 == 3){
                   b2 = (int)(m2.at(0)-'0')*100; 
                   b2 += (int)(m2.at(1)-'0')*10; 
                   b2 += (int)(m2.at(2)-'0');}
        else if(s2 == 2){
                   b2 = (int)(m2.at(0)-'0')*10;
                   b2 += (int)(m2.at(1)-'0'); }        
        else if(s2 == 1){
                   b2 = (int)(m2.at(0)-'0');}
        output = b1+b2;
        cout << "Answer : " << output << endl;
        }
        
if(a.find("-")!= -1){
  n = a.find("-");
  m1 = a.substr(0,n);
  m2 = a.substr(n+1);
  s1 = m1.length();
  s2 = m2.length();
 
  float b1=0,b2=0,output;
        
        if(s1 == 3){
                   b1 = (int)(m1.at(0)-'0')*100;
                   b1 += (int)(m1.at(1)-'0')*10;
                   b1 += (int)(m1.at(2)-'0');}
        else if(s1 == 2){
                   b1 = (int)(m1.at(0)-'0')*10; 
                   b1 += (int)(m1.at(1)-'0'); }         
        else if(s1 == 1){
                   b1 = (int)(m1.at(0)-'0');}
        if(s2 == 3){
                   b2 = (int)(m2.at(0)-'0')*100; 
                   b2 += (int)(m2.at(1)-'0')*10; 
                   b2 += (int)(m2.at(2)-'0');}
        else if(s2 == 2){
                   b2 = (int)(m2.at(0)-'0')*10;
                   b2 += (int)(m2.at(1)-'0'); }        
        else if(s2 == 1){
                   b2 = (int)(m2.at(0)-'0');}
        output = b1-b2;
        cout << "Answer : " << output << endl;
}

if(a.find("*")!= -1){
  n = a.find("*");
  m1 = a.substr(0,n);
  m2 = a.substr(n+1);
  s1 = m1.length();
  s2 = m2.length();
 
  float b1=0,b2=0,output;
        
        if(s1 == 3){
                   b1 = (int)(m1.at(0)-'0')*100;
                   b1 += (int)(m1.at(1)-'0')*10;
                   b1 += (int)(m1.at(2)-'0');}
        else if(s1 == 2){
                   b1 = (int)(m1.at(0)-'0')*10; 
                   b1 += (int)(m1.at(1)-'0'); }         
        else if(s1 == 1){
                   b1 = (int)(m1.at(0)-'0');}
        if(s2 == 3){
                   b2 = (int)(m2.at(0)-'0')*100; 
                   b2 += (int)(m2.at(1)-'0')*10; 
                   b2 += (int)(m2.at(2)-'0');}
        else if(s2 == 2){
                   b2 = (int)(m2.at(0)-'0')*10;
                   b2 += (int)(m2.at(1)-'0'); }        
        else if(s2 == 1){
                   b2 = (int)(m2.at(0)-'0');}
        output = b1*b2;
        cout << "Answer : " << output << endl;
}

if(a.find("/")!= -1){
  n = a.find("/");
  m1 = a.substr(0,n);
  m2 = a.substr(n+1);
  s1 = m1.length();
  s2 = m2.length();
 
  float b1=0,b2=0,output;
        
        if(s1 == 3){
                   b1 = (int)(m1.at(0)-'0')*100;
                   b1 += (int)(m1.at(1)-'0')*10;
                   b1 += (int)(m1.at(2)-'0');}
        else if(s1 == 2){
                   b1 = (int)(m1.at(0)-'0')*10; 
                   b1 += (int)(m1.at(1)-'0'); }         
        else if(s1 == 1){
                   b1 = (int)(m1.at(0)-'0');}
        if(s2 == 3){
                   b2 = (int)(m2.at(0)-'0')*100; 
                   b2 += (int)(m2.at(1)-'0')*10; 
                   b2 += (int)(m2.at(2)-'0');}
        else if(s2 == 2){
                   b2 = (int)(m2.at(0)-'0')*10;
                   b2 += (int)(m2.at(1)-'0'); }        
        else if(s2 == 1){
                   b2 = (int)(m2.at(0)-'0');}
        output = b1/b2;
        cout << "Answer : " << output << endl;
}
  
   
   
   
system("PAUSE");
return 0;    
}