Proper way to MAP

[Mark-Four]

During the course and instructions on mapping, I found that there were two ways in which Patrick did this:

First Way:

nameToFavoriteNumber[_name] = _favoriteNumber;

Second Way:

addressToAmount[msg.sender] += msg.value;

My question is, What is the correct way of mapping a value? Do we need to add the '+=' sign or just the '=' sign should do?

[krakxn]

I will keep this as simple as possible for beginners:

The first way - think of the mapping nameToFavoriteNumber as a kind of dictionary; with name as key, which points to the data (here, favouriteNumber) which it contains. And you can access that data by doing nameToFavoriteNumber[key] (which is essentially calling the key which points to our data).
Now to be precise, since we are using the assignment operator = here, it assigns _favoriteNumber (or data) to the key _name (or our key)

Example (visual representation for interpretation, not precise):
nameToFavoriteNumber["Alice"] = 7
nameToFavoriteNumber = {"Alice": 7} // Alice points to 7 (Alice ----> 7) inside our mapping

The second way - We are using += here (as, sum = sum + a can be written as sum+=a, a shorthand)
So, we are essentially doing: Address = Amount + msg.value

To be completely technical, it is essentially a basic hash map

BTW, inapt category: Since this is a Q&A question, it is best advised to mark it as such, so people with doubts can reference to the marked answer in the future. Hope this helps and all the best with the course!

[Mark-Four]

Thank you @krakxn for an easy to understand reply. I'm diving a little deeper into this, to understand what a basic hash map is.

Sorry about posting in the wrong category. I'm new to github; please may I know how to mark it as Q&A?

[krakxn]

No need to apologize, rather feel free to ask questions when you are a beginner (being pragmatic and collaborative helps in this field), and you can do it by finding the category heading: there should be a gear icon next to the selected category (ie, General), via which you can select the appropriate category (which, in this case, is Q&A) and mark whichever response satisfies your problem as the answer, for others to refer in the future. Cheers!

[alymurtazamemon]

@Mark-Four Both ways are correct and both have their own use case.

nameToFavoriteNumber[_name] = _favoriteNumber;

This is only assigning a favorite number.

addressToAmount[msg.sender] += msg.value;

On the other hand, this is assigning the value as well as incrementing. This is equivalent to this:

addressToAmount[msg.sender] = addressToAmount[msg.sender] + msg.value;

Now you can see both ways are the same.

[Mark-Four]

Thank you for answering @ali-thegilfoyle!

May I know why we would like to increment in the case of 'addressToAmount' and not in the case of 'nameToFavoriteNumber'?

As I understand, and @krakxn replied above, the key points to some data. Therefore, why simply assign in the first case and assign+increment in the second?

[alymurtazamemon]

@Mark-Four It all depends on the logic/use case.

1. The first use case is from a simple storage contract, where we are allowing the user to store his favorite number, user can update his favorite number but every time it will be a single value. So here we want to replace with a new value not want to add to it.

2. The second use case is from a fund me a contract, where we are allowing the user to fund a contract, and with mapping, we want to know how much a particular user funded the contract. That is why we want to add new funds to previous funds every time user funds the contract.

Hope it clears your query.

[krakxn]

Thank you for answering @ali-thegilfoyle!

May I know why we would like to increment in the case of 'addressToAmount' and not in the case of 'nameToFavoriteNumber'?

As I understand, and @krakxn replied above, the key points to some data. Therefore, why simply assign in the first case and assign+increment in the second?

As @ali-thegilfoyle elaborated; also, one note though as to why we increment in the second case, it is because (in addressToAmountFunded) a single key (the specific address) then gets to store how much it has funded (amount). In laymans terms, tracks the sum of amount for that specific address. (if we had used the assignment operator it will just replace the amount and not store the total amount for that address) And the fact that in the first case we just had to let the key (name) point to the favoriteNumber (no sum needed obviously, we just replace what favouriteNumber the name has).

[Mark-Four]

Thank you @ali-thegilfoyle and @krakxn! Your answers helped clarify my query perfectly! Thank you for all your help once again.