Lesson 4. Added a + on the mapping implementation on the fund() function

[davidinci7]

I noticed that on the FundMe.sol contract, on the fund() function. A + sign is all of the sudden added on the mapping addressToAmountFunded, after Patrick explains about for loop. Is there a difference in execution on the first code addressToAmountFunded[msg.sender] = msg.value; to addressToAmountFunded[msg.sender] += msg.value;.

[krakxn]

Yeah, there is a difference. I suppose visually is easier to understand so I will take a simple example:

let a, b;
a = 4;
b = 6;

// The following assigns the value of b to a 
a = b; // a = 6 now

// Whereas, the following is equivalent to a = a + b
a += b; // a = 6 + 4, so a = 10

So, in your case addressToAmountFunded[msg.sender] = msg.value; will be wrong.

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

// The above is equivalent to the following
addressToAmountFunded[msg.sender] = addressToAmountFunded[msg.sender] + msg.value;

To sum it up, the += notation is a shorthand just is the case with anonymous functions in JavaScript.

[alymurtazamemon]

@davidinci7 Above @krakxn well explained the mathematics behind adding +. I just want to add the purpose of using += instead of =.

We want everyone to fund our contract along with it we want to trace that which address gives how much amount.

So if we use;

addressToAmountFunded[msg.sender] = msg.value; 

Every time an address fund the contract its latest amount will be saved in the mapping. Ex. if the first time an address funds 1 ETH and the next time it funds 2 ETH. The mapping will tell us that the address has only funded 2 ETH because that was the last funded amount but that is not true because the same address has funded a total of 3 ETH. That's what we are doing by using the + operator.

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

This will remember the latest as well as past all funds and will tell that the address has funded a total of 3 ETH.

[krakxn]

Clarifying the above,

As,

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

addressToAmountFunded[msg.sender] on the right hand side of the = searches for the amount funded by msg.sender (i.e. the "key" of the dictionary here, its address). So, we need to add this to update that key's value.

[davidinci7]

It's crystal clear now, thanks to you both!

[krakxn]

All the best!