smartcontractkit
addressToAmoundFunded[msg.sender] = msg.value; is changed into some typo in the video by was not shown through the demo
Lesson
Lesson 4
Could you please leave a link to the timestamp in the video where this error occurs? (You can right click a video and "copy video URL at current time")
No response
Operating System
Windows
Describe the bug
addressToAmoundFunded[msg.sender] = msg.value; this code was shown to be the one working and made sense but in the rest of the video I was able to spot the same code changed into addressToAmoundFunded[msg.sender] += msg.value; the later code is also working but I dont get it, its not shown in the video why that + is here, so I had to rewatch almost 30 min of the video like 2 times and still not understand why it is here and whether it should be here or not, but it works the same.
Ok so at this point in time the code is addressToAmoundFunded[msg.sender] = msg.value; and I right click the video link at current time for anyone to view: https://youtu.be/gyMwXuJrbJQ?t=15826 at this point https://youtu.be/gyMwXuJrbJQ?t=16464 the code is same and right after 15 seconds
https://youtu.be/gyMwXuJrbJQ?t=16474 at this point, the code is changed to addressToAmoundFunded[msg.sender] += msg.value;, that plus sign is confusing
At some point he commented on the need to account for this situation but did not provide the solution. I guess he fixed it in the repo code.
In C-like languages the += means 'add to whatever was already there'.
The first time an address sends us eth addressToAmoundFunded[msg.sender] += msg.value; will create a new entry in the mapping. I believe new entries are initialized to zero, and the +=will immediately add msg.value to it.
If the same address sends us some eth a second time, then the entry on the mapping already exists with some value and we add msg.value to what's already there.
the explanation make sense, but you are guessing i think that it was may be due to C-like language logic as you said you believe new entries are initialized to zero, may be we need a proper guidance on this one still???
"The concept of “undefined” or “null” values does not exist in Solidity, but newly declared variables always have a default value dependent on its type."
- https://docs.soliditylang.org/en/v0.8.14/types.html
"The default value for the uint or int types is 0."
- https://docs.soliditylang.org/en/v0.8.14/control-structures.html#default-value
"Mapping is a reference type as arrays and structs. Following is the syntax to declare a mapping type." "mapping(_KeyType => _ValueType)"
- https://www.tutorialspoint.com/solidity/solidity_mappings.htm
addressToAmoundFunded[msg.sender] += msg.value
With the provided information let's dissect the example above that there is some confusion about. addressToAmoundFunded is a mapping data type.
mapping(address => uint256) public addressToAmountFunded;
This means that addressToAmoundFunded has two data types that it references. The msg.sender the address part of addressToAmoundFunded and msg.value is the uint256 part. Since we know that uint256 has a default value of 0, it makes sense to shorten the code and just use addressToAmoundFunded[msg.sender] += msg.value. The += just means to add what's left of the sign what's to the right of it and assign the result of that to the variable.
addressToAmoundFunded[msg.sender] += msg.value is just a shorter way of writing addressToAmoundFunded[msg.sender] = msg.value + addressToAmoundFunded[msg.sender].
I think I went over everything. Let me know if you need anymore clarification about anything else.
