In arrays, see if last four elements of 1st array match last 4 elements of 2nd array?

Question

How is it possible to see if the last four elements of an array match the last four elements of the second array?

For instance, I'm writing a password program. First, the user is asked their birth date. Then they are asked to input a password.

for the birth date array, the user enters '05/14/1984' for the password array, the user enters 'coke_1984'

I'm trying to see if the last four of birth date are the same as the last four of the password, if all four match, then add 1 to score.

I'm completely stuck! The code I have now has an "invalid conversion from 'char' to 'const char*'"

for(i = len; i <= len; i--)
{
    if(strcmp(birthdate, password[i]) == 0)
    {
        dateCMP = true;
    }else{dateCMP = false;}
}
if(dateCMP = true)
{
    score += 1;
    cout << "Your date of birth should not be the last four characters";
}

Answer 1

Make your life easier and take advantage of the already provided STL facilities like std::string.

Below is a function that takes as entries 2 strings and returns true if their last 4 characters are equal and false otherwise:

bool
compare_last_4_characters(std::string const &str1, std::string const &str2) {
  std::size_t sz1 = str1.size();
  std::size_t sz2 = str2.size();
  if(sz1 > 3 && sz2 > 3) {
    return str1.substr(sz1 - 4, sz1) == str2.substr(sz2 - 4, sz2);
  }
  return false;
}

LIVE DEMO

If you can't use std::string below is a version that works without them:

bool
compare_last_4_characters(char const *str1, char const *str2) {
  int sz1 = strlen(str1) - 4;
  int sz2 = strlen(str2) - 4;  
  if(sz1 >= 0 && sz2 >= 0) return !strcmp(str1 + sz1, str2 + sz2);
  return false;
}

LIVE DEMO

Answer 2

#include <iostream>
#include <utility>
#include <algorithm>

//size_t literal: see http://stackoverflow.com/questions/22346369/initialize-integer-literal-to-stdsize-t
constexpr std::size_t operator "" _z(unsigned long long n)
{
    return n;
}

bool lastFourEquals(const char* str1, size_t strlen1, const char* str2, size_t strlen2)
{
    //variant 1: do not allow shorter strings
    //if(strlen1 < 4 || strlen2 < 4) return false;

    //Variant 2: maximum of last for equals (e.g. "123"=="0123")
    size_t maxLen =std::min(std::max(strlen1, strlen2), 4_z);

    for (int i = 1; i <= maxLen; i++)
    {
        size_t pos1 = strlen1 - i;
        size_t pos2 = strlen2 - i;
        if (str1[pos1] != str2[pos2]) return false; //Found difference
    }

    return true;
}

int main()
{
    const char* test1 = "05/14/1984";
    const char* test2 = "coke_1984";

    bool eq = lastFourEquals(test1, strlen(test1), test2, strlen(test2));

    std::cout << (eq ? "true" : "false") << std::endl;
}

Answer 3

I figured it out by using another array. I just made an array called compare to store values of password if password[i] == birthdate[i]... then used an if to strcmp(compare, birthdate)...

for(i = len-4; i < len; i++)
{
    if(birthdate[i] == password[i])
    {
        compare[i] = password[i];
    }
}
if(strcmp(compare, birthdate))
{
    score += 1;
}

Thanks for your attempts to help!

Answer 4

I think this works! I finally got it. It's a pain in the ass not being able to use the string library. Please let me know! You guys have been so much help. Don't worry about my other arguments in the function, I'm not using those quite yet.

//this function runs to see if the first letter of last name matched
//first letter of password, if the birthdate

int checkStrength(char password[SIZE], char lastName[SIZE], char 
birthdate[SIZE], char carMake[SIZE], int favNum)
{

//array and variables for use through the checks
char compareDate[SIZE];
int weakness = 0;
int len = strlen(password) - 4;
int i;
int c = 0;

//this for loop compares arrays, if comparison is found, adds element to
//compareDate and then uses an if statement to strcmp compare and birthdate
//if compare and birthdate are then the same, add weakness
for(i = 0; i < len; i++)
{
    if(birthdate[c] == password[len])
    {
        compareDate[c] = password[i];
        c += 1;
    }
}
if(strcmp(compareDate, birthdate) == 1)
{
    weakness += 1;
    cout << "Your birth year should not be the last four character of your 
    password.";
    cout << compareDate;
}

//this checks to see if the car make array is used in order through password,
//if it is, add weakness


return weakness;

}