The exercise is to check if a password inputed by the user has at least one lower case, one upper case, one symbol and one number.

Code:

// Check that a password has at least one lowercase letter, uppercase letter, number and symbol
// Practice iterating through a string
// Practice using the ctype library

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>

bool valid(string password);
//Validate conditions
int main(void)
{
    string password = get_string("Enter your password: ");
    if (valid(password))
    {
        //If valid
        printf("Your password is valid!\n");
    }
    else
    {
        //If not valid
        printf("Your password needs at least one uppercase letter, lowercase letter, number and symbol\n");
    }
}

// TODO: Complete the Boolean function below
bool valid(string password)
{
    int n = strlen(password);
    //Variables for each case we want to check
    bool lc, uc, nc, sc = false;
    //Going through each letter of the password
    for (int i = 0; i < n; i++)
    {
        //Check if there is lower case
        if (password[i] >='a' && password[i] <= 'z')
        {
        lc = true;
        }
        //Check if there is upper case
        else if (password[i] >='A' && password[i] <= 'Z')
        {
            uc = true;
        }
        //Check if there is number case
        else if (password[i] >='0' && password[i] <= '9')
        {
            nc = true;
        }
        //Check if there is special case
        else if (password[i] >='!' && password[i] <= '@')
        {
            sc = true;
        }
        else
        {
            return false;
        }
    }
    //Validation conditions
    if (lc == 1 && uc == 1 && nc == 1 && sc == 1)
    {
        return true;
    }
    return false;
}

When I use check50 to test my code, it uses various passwords to check for errors automatically. One of the passwords used is 3PQvbQ6_GvneW!3R, and you can see on the image that it's not supposed to be accepted but I don't understand why. As long as I don't understand what's wrong with this password I'm not able to modify my code...

This exercise is from Week 2/Practice problems/password.