trigger Account on Account (after insert, after update) {
    List<Contact> contactsForUpsert = New List<Contact>();

    // We want to avoid SOQL inside the for loop, because it might be a reason of troubles
    // and in order to follow best practices, thus we queried all needed information about Contacts
    // which are related with Accounts which are under insert/update
    // You have to think in 200 record's context when working with triggers
    // -->
    Map<Id, Contact> contactsByAccountId = new Map<Id, Contact>();
    List<Contact> contacts = [SELECT Id,
                                     AccountId,
                                     mailingstreet,
                                     mailingcity,
                                     mailingstate,
                                     mailingpostalcode
                            FROM Contact WHERE AccountId IN: Trigger.newMap.keySet()];
    for (Contact cont: contacts) {
        contactsByAccountId.put(cont.accountId, cont);
    }
    // <-- finished collecting information about Contacts. All contacts were placed in Map<AccountId, Contact>

    for (Account account: Trigger.new) {
        // old records are available only in update call
        Account old;
        if (Trigger.isUpdate) {
            old = Trigger.oldMap.get(account.Id);
        }
        // we want to perfoem the update of contact address fields in two cases:
        //  1. we insert new account, but there is a question: is that right business case for inserting Contact record here?
        //  2. we updated ShippingAddress in an exisitng account, I do believe that we do not want to update Contact record 
        //  each time when other fields were changed
        Boolean isShippingAddressChangedOrNew = Trigger.isInsert ? true :
                                                (   account.ShippingStreet != old.ShippingStreet
                                                 || account.ShippingCity != old.ShippingCity
                                                 || account.ShippingState != old.ShippingState
                                                 || account.ShippingPostalCode != old.ShippingPostalCode
                                                 || account.ShippingCountry != old.ShippingCountry) ? true : false;
        // check that field was changed
        if (isShippingAddressChangedOrNew) {
            // get existing Contact if exist
            Contact relatedContact = contactsByAccountId.get(account.Id);
            if (relatedContact == null) { // for insert call we create new Contact record
                relatedContact           = new Contact();
                relatedContact.AccountID = account.Id;
            }
            relatedContact.mailingstreet     = account.Shippingstreet;
            relatedContact.mailingcity       = account.Shippingcity;
            relatedContact.mailingstate      = account.Shippingstate;
            relatedContact.mailingpostalcode = account.shippingpostalcode;
            contactsForUpsert.add(relatedContact);
        }

    }

    try {
        upsert contactsForUpsert;
    } catch(Dmlexception e) {
        System.debug(LoggingLevel.ERROR, 'Contact insert from createopp.trigger has failed with message:' + e.getMessage());
    }
}

trigger Account on Account (after insert, after update) {
    List<Contact> contactsForUpsert = New List<Contact>();

    // We want to avoid SOQL inside the for loop, because it might be a reason of troubles
    // and in order to follow best practices, thus we queried all needed information about Contacts
    // which are related with Accounts which are under insert/update
    // You have to think in 200 record's context when working with triggers
    // -->
    Map<Id, Contact> contactsByAccountId = new Map<Id, Contact>();
    List<Contact> contacts = [SELECT Id,
                                     AccountId,
                                     mailingstreet,
                                     mailingcity,
                                     mailingstate,
                                     mailingpostalcode
                            FROM Contact WHERE AccountId IN: Trigger.newMap.keySet()];
    for (Contact cont: contacts) {
        contactsByAccountId.put(cont.accountId, cont);
    }
    // <-- finished collecting information about Contacts. All contacts were placed in Map<AccountId, Contact>

    for (Account account: Trigger.new) {
        // old records are available only in update call
        Account old;
        if (Trigger.isUpdate) {
            old = Trigger.oldMap.get(account.Id);
        }
        // we want to perfoem the update of contact address fields in two cases:
        //  1. we insert new account, but there is a question: is that right business case for inserting Contact record here?
        //  2. we updated ShippingAddress in an exisitng account, I do believe that we do not want to update Contact record 
        //  each time when other fields were changed
        Boolean isShippingAddressChangedOrNew = Trigger.isInsert ? true :
                                                (   account.ShippingStreet != old.ShippingStreet
                                                 || account.ShippingCity != old.ShippingCity
                                                 || account.ShippingState != old.ShippingState
                                                 || account.ShippingPostalCode != old.ShippingPostalCode
                                                 || account.ShippingCountry != old.ShippingCountry) ? true : false;
        // check that field was changed
        if (isShippingAddressChangedOrNew) {
            // get existing Contact if exist
            Contact relatedContact = contactsByAccountId.get(account.Id);
            if (relatedContact == null) { // for insert call we create new Contact record
                relatedContact           = new Contact();
                relatedContact.AccountID = account.Id;
                relatedContact.LastName = 'test';
            }
            relatedContact.mailingstreet     = account.Shippingstreet;
            relatedContact.mailingcity       = account.Shippingcity;
            relatedContact.mailingstate      = account.Shippingstate;
            relatedContact.mailingpostalcode = account.shippingpostalcode;
            contactsForUpsert.add(relatedContact);
        }

    }

    try {
        upsert contactsForUpsert;
    } catch(Dmlexception e) {
        System.debug(LoggingLevel.ERROR, 'Contact insert from createopp.trigger has failed with message:' + e.getMessage());
    }
}

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