if (pwdTextField && [pwdTextField length] > 6 ){

  for(NSString* existUserName in userNamesArr){
     if(existUserName isEqualToString:txtUserName.text){ //txtUserName is your UItextField
         UIAlertView *alert=[[UIAlertView alloc]initWithTitle:@"" message:@"User name already exists" delegate:self cancelButtonTitle:@"Try with different user name" otherButtonTitles:nil];

        [alert show];
        [alert release];
        return;
    }else{
         // ** Save New User to your database **
    }
}else{
     UIAlertView *alert=[[UIAlertView alloc]initWithTitle:@"" message:@"Password should be atleast 6 characters" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];

    [alert show];
    [alert release];
}

}

if (textField.text.length > 6){//Do something}