// here is my code
    #include <iostream>
    #include <ctype.h>
    #include <string.h>
    #include <fstream>
    #include <cstring>
    #define MAX 15
    using namespace std;
    char fourdigOfnum(char *s1,char *s2) {
        int i=0;
        while(i<4)
        {
            s1[i]=s2[i];
            ++i;
        }
        s1[i]='\0';
    }
    int main() {
        char num_dig[4],simCard[MAX];
        char mobile_num[MAX];
        ofstream file;
        file.open("mobile.txt",ios::out);
        cout<<"Enter a Mobile-Number : "; cin>>mobile_num;
        if(mobile_num[0]==0||strlen(mobile_num)==11)
        {
            //num_dig=fourdigOfnum(str_num);
            fourdigOfnum(num_dig,mobile_num);
            if(num_dig[strlen(num_dig-1)]==0)
            {
                strncpy(simCard,"Jazz",sizeof(simCard));
            }
            else if(num_dig[strlen(num_dig-1)]==1)
            {
                strncpy(simCard,"Zong",sizeof(simCard));
            }
            else if(num_dig[strlen(num_dig-1)]==2)
            {
                strncpy(simCard,"Warid",sizeof(simCard));
            }
            else if(num_dig[strlen(num_dig-1)]==3)
            {
                strncpy(simCard,"Ufone",sizeof(simCard));
            }
            else if(num_dig[strlen(num_dig-1)]==4)
            {
                strncpy(simCard,"Telenor",sizeof(simCard));
            }
        } else {
            cout<<"You entered a Invalid number "<<endl;
        } 
        if(file.fail())
        {
            cout<<"Sorry File is failed to open "<<endl;
        } else {
            file<<"Your Mobile-Number is "<<mobile_num<<endl;
            file<<"This is a "<<simCard<<" number "<<endl;
        }
        file.close();
        system("pause");
        return 0;
    }