if else语句出现问题

我设计了一个简易计算器，但是if else语句出问题了，无论输入什么数字都进行加法运算，下附程序
#include<iostream>
#include<cstdlib>
   using namespace std;
    int main(){
        double a=0,b=0,c=0,d=0;
        cout<<"请选择所需要的功能。"<<endl;
        cout<<"1-----加"<<endl;
        cout<<"2-----减"<<endl;
        cout<<"3-----乘"<<endl;
        cout<<"4-----除"<<endl;
        cin>>c;
        if(c=1)
        {
            cout<<"请输入数字a:";
            cin>>a;
            cout<<"请输入数字b:";
            cin>>b;
            d=a+b;
            cout<<"计算结果是:";
            cout<<d;
         }else if(c=2)
         {
             cout<<"请输入数字a:";
            cin>>a;
            cout<<"请输入数字b:";
            cin>>b;
            d=a-b;
            cout<<"计算结果是:";
            cout<<d;
         }else if(c=3){
             cout<<"请输入数字a:";
            cin>>a;
            cout<<"请输入数字b:";
            cin>>b;
            d=a*b;
            cout<<"计算结果是:";
            cout<<d;
         }else if(c=4){
             cout<<"请输入数字a:";
            cin>>a;
            cout<<"请输入数字b:";
            cin>>b;
            d=a/b;
            cout<<"计算结果是:";
            cout<<d;
         }
         return 0;
         system("pause")
    }