security_keypad.txt

/*
 There is a security keypad at the entrance of a building. It has 9 numbers 1 - 9 in a 3x3 matrix format.
 1 2 3
 4 5 6
 7 8 9
 The security has decided to allow one digit error for a person but that digit should be horizontal or vertical. 
 Example: for 5 the user is allowed to enter 2, 4, 6, 8 or for 4 the user is allowed to enter 1, 5, 7. If the security code to enter is 1478 and if the user enters 1178 he should be allowed. Write a function to take security code from the user and print out if he should be allowed or not
*/

#include <iostream>
using namespace std;

int main(){
    string realPW = "1478";
    cout<<"the length of realPW is "<<realPW.length()<<endl;//has nothing to do with this problem, just to see the lengh of the string defined in this ways
    cout<<"Input your password:"<<endl;
    string guessPW;
    cin>>guessPW;
    cout<<"the length of guessPW is "<<guessPW.length()<<endl;//has nothing to do with this problem, just to see the lengh of the string defined in this ways
    /*Check the input.*/
    for (int i=0; i<guessPW.length(); ++i){
        if (guessPW[i]<'0' || guessPW[i]>'9'){
            cout<<"wrong input"<<endl;
            return -1;
        }
    }
    /*If the two strings have different length, then the input one must be incorrect.*/
    if (realPW.length()!=guessPW.length()){
        cout<<"The password is incorrect!"<<endl;
        return -1;
    }
    
    /*Note the starting digit in each string. The starting digits are its original digits.*/
    string allPossibleDigit[]={"14723","21358","32169","41756","52468","63459","71489","82579","93678"};
    
    int count=0;
    
    for (int i=0; i<realPW.length();++i){
        if (realPW[i]!=guessPW[i]){
            bool flag=0;
        for(int j=1; j<5; ++j){
            if(guessPW[i]==allPossibleDigit[(realPW[i]-'0')-1][j]){
                flag=1;
                break;
            }
        }
        if (flag==1) count++;
        else{
        cout<<"The password is incorrect!"<<endl;
            return 0;
        }
        }
    }
    if (count<=1) cout<<"correct!"<<endl;
    else cout<<"The password is incorrect!"<<endl;
    return 0;
}

REMARK:
1. We use string as input, which is easy and reasonable. It also can store char. So I discarded int arr[].
2. NOTE: at line42, you need write
 if(guessPW[i]==allPossibleDigit[(realPW[i]-'0')-1][j]){
INSTEAD OF
 if(guessPW[i]==allPossibleDigit[realPW[i]-1][j]){