awesome so now we have a get price function which is going to return the value of ethereum in terms of USD as a uint 256. now all we have to do is convert our message.value in terms of dollars using this get price function to

do that we're going to use our get conversion rate function this get conversion rate function is going to take a uint 256 ether amount as an input and we're going to convert this ether amount to its value in dollars this will be a public view function that will

return a uint 256. so first we're going to get the price of ethereum by doing uint 256 price equals our get price function that we just defined and then we're going to do uint 256 eth amount in USD equals eth price times eth

amount and then we actually have to divide by one e 18. the reason we divide by one e 18 is because both of these have 18 decimal places and if we multiply one one two three four five six seven eight one two three four five six seven eight

nine ten if we multiply these by each other where both of these are representing one we're actually going to get a number that's absolutely massive with 38 decimal places so we need to divide by 18 to produce it back down to what it should actually be an important

rule with working with math and solidity is you always want to multiply before you divide the reason for this is since again only whole numbers work in solidity if you were to do one divided by two you would end up getting 0. so you always

want to multiply first when we get to The Foundry section of this course testing all this math is going to be a lot easier if you're really struggling with some of the math bits right now I wouldn't let that slow you down because one of the things that AI is really good

at is doing a lot of these math conversions but in any case this ETH amount in USD is the number that we're looking for so we can say return ETH amount in USD and awesome now we have a get conversion rate function let's walk through this together just so that

you can understand what's actually going on let's say we want to see how much one ETH is worth well we're going to get the ETH price which let's say it's two thousand dollars but it's gonna be two thousand dollars with one two three four

five six seven eight one two three four five six seven eight nine ten eighteen zeros so each price might look something like this so that means we're going to do this 2000 with 18 decimal places times one ETH which is going to be one

one two three four five six seven eight one two three four five six seven eight eight nine ten we're gonna multiply those together and then since we're gonna get an additional 36 zeros then we just divide by one e 18 and that's how

we would get two thousand dollars equals one ETH it's gonna be two thousand of course with 18 decimal places one of the good reasons for using whole numbers in solidity is that it's not possible to have decimal places so we don't lose any

Precision, which is really good. So normally we should test this function before continuing, but since we're doing all this on a testnet and I promise you that I tested this all beforehand, we're just going to go ahead and keep going. So now that we've done all this work, if we

want to make sure that our users are sending at least five dollars, we can do get conversion rate of message.sender needs to be greater than minimum USD because get conversion rate takes in each amount as input, uses the pricing information to get the dollar value back, and we're going to check to see that the

dollar value of the message.value is greater than the minimum USD. However, since get conversion rate returns a value with 18 decimal places, we need to update our minimum USD to say five times ten raised to the 18th, or we could also do 5e18, or we could do five times one e

eighteen. I personally like this 5e18 syntax, but whatever works for you. I'm going to deploy this to a testnet just to demonstrate this working, but again you do not have to. So we're going to go ahead, delete our previous deployments,

injected web3, deploy, Metamask pops up. I'm going to go ahead and confirm. After a long delay, we'll get our deployed contract here. We have all this stuff. We can call get price and we can see the value of Ethereum right now is this with

18 decimal places, which is awesome. So it's around 1,896 dollars. So if I were to try to fund this with maybe 100 wei, I know for a fact that 100 wei isn't going to be enough. So if we call this fun

function we're actually going to get this gas estimation failed gas estimation error failed with the following message blah blah blah error execution reverted didn't send enough ETH which is actually exactly the error message that we have here it's exactly what our error message is here in our

require didn't send enough ETH so if we try to fund it we're going to get didn't send enough ETH and this gas estimation failed is a pretty common error that you'll see in Remix and Solidity and EVM in general it's kind of just the

boilerplate answer for something went wrong luckily we got the actual error message here so we could debug here's what's actually going on now you see here we could still send this transaction like I said we would actually spend gas to send a failed transaction and this isn't something

that we want to do so we're going to go ahead and cancel however if we did send maybe one one two three four five six seven eight one two three four five six seven eight nine so if we go to Google actually we can even see like an eighth two dollar

conversion rate so if we send 0.01 0.01 ETH right now is worth 18 it might be different depending on when you actually do it so we send 0.01 ETH in our Remix and we can do that by doing one one two

three four five six seven eight nine ten one two three four five six so that's one E16 and we hit fund now we actually see MetaMask pops up we don't get that gas estimation error we can go ahead and confirm and we can see

that our function is working as expected, which is great. It's reverting transactions that have less than five dollars and it's letting transactions go through that have at least five dollars. Awesome. If we select minimum USD, we can obviously see that here and we can see

we have 0.01 ethereum. Fantastic, and since we don't have a withdrawal function here, we actually can't pull it out, which is another reason why I say hey maybe don't test this right now, but great. We've confirmed our get conversion rate is working, so let's move on.

Lesson 4 Part 11 | More Solidity Math - Solidity & Foundry Full Course

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Lesson 4 Part 11 | More Solidity Math - Solidity & Foundry Full Course

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