Solidity Basics

[thecil]

@bdoherty

In relation to Error Handling - require()… Is there any cost in gas to do any of the checks in require? Because if it fails it returns the gas, so could that be used as a way to spam the network? If for example you kept calling contracts with params that will fail at the require stage…

Hi @bdoherty, hope you are well.

Now the require() function is used to verify that the arguments passed to the function are valid before the execution of themself, it does not have any gas cost, functions cost gas when they are about to being processed by a miner, the gas is for the miner basically.

Take an example on:
Lets say i have a function that must verify my age before it can proceed to the next step of his logic.
So if my age is above the assigned by my require it will not execute the function logic.

function payMe(uint _age) public{
 //verify that _age is inside range
require(_age < 50, "You'r age is above the range, can't continue" );
//if _age is valid, continue execution
doSomething();
} 

If you have any more questions, please let us know so we can help you!

Carlos Z.

[bdoherty]

Hi Carlos,

Im great! Thanks for the feedback. So its not costing gas yet because this happens before it gets to a miner. So in your example, where and who is doing the processing to check the require statements? Is it happening on the EVM? If so would there be a cost in relation to resources to check the requires etc? So if you were to repeatedly call some function with lots of requires would things start to fall over at any point?

Hope I make sense!
Brian

[thecil]

@bdoherty

Is it happening on the EVM?

Yes, the EVM have access to the smart contract, any call on a function will be handle by the EVM.

@bdoherty

If so would there be a cost in relation to resources to check the requires etc?

There is basically no use of big resources to check the arguments that must be passed to a function.

@bdoherty

So if you were to repeatedly call some function with lots of requires would things start to fall over at any point?

Is a simple calculation for the network, computers are very precise in terms of calculations so is almost impossible that calling multiple times a function with different arguments will fall, maths are very precise and even more because the EVM goes on binary code which is the most tiny and exact representation of what the computer must calculate.

Basically, the EVM verify the arguments, since this a very simple operation, no big resources are used, also it does not matter how many times a function is called if this one has been programmed decently to verify the inserted data before it’s execution (thats why require is important, to verify data before execution).

If you have any more questions, please let us know so we can help you!

Carlos Z.

[seer]

@filip
In the video series about storage you mention 3 locations:

• Storage
• Memory
• Calldata

However, when I do the quiz afterwards, I only found 2 of these (Storage + Memory, Calldata was not in the list) which turned out to be wrong. In my breakdown, it shows “Stack” as third option. You never mentioned this in your video’s.

Is this the old name for Calldata perhaps? Or are we missing something here?

[steve777]

hi everybody i am in solidity basics and when I click on deploy it give me this

sender doesn’t have enough funds to send tx. The upfront cost is: 3000000

does anybody can help me with that please , thanks!!!

[thecil]

Hi @steve777, hope you are well.

Could you please share a screenshot? If you are using remix, i would like to verify your compiler config, maybe also the code can help us solve your issue

If you have any more questions, please let us know so we can help you!

Carlos Z.

[jon_m]

Hey @seer,

It seems that your post about the data location quiz has been missed. Sorry about that!

There was actually a problem with this quiz. It hadn’t been updated from the old course which only discussed Memory, Storage and the Stack. It’s now been updated to coincide with the changes in the new course, and what you’ve been taught in the videos. So you may want to just retake the quiz, and you should find it makes sense now

Apologies for the confusion and inconvenience!

[novosel]

@keithra1948

Hey Philip,

Enjoying the Solidity course so far just had one question. You showed that there are 2 ways to write out the createPerson function:

The short way

(eg. people.push(Person(people.length, name, age, height));

the long way

(eg. 
Person memory newPerson
       newPerson.id = people.length;
       newPerson.name = name; 
        etc
       people.push(newPerson);

Is there a reason that Person is used for the 1st method and newPerson for the 2nd way?

Thanks,

Hey guys, they say that there’s no such thing as a stupid question, so here goes. Mine is a follow-on to keitra1948’s post above – but it relates to mapping.

In the video on Structs, Filip first demonstrates his “clearest” way (i.e., the function createPerson . . .) to code adding a new person to the people array, and then he demonstrates the alternative (i.e., “Person memory newPerson . . .”). See: @10:05 of the Structs video.

The distinction between the two methods as well as the subsequently explained reasons why we might choose to use one versus the other seems obvious enough but I must be missing something because what I don’t understand is why, Filip, you didn’t code the presentation in the Mappings video using the function createPerson . . . people.push(Person(people.length, name, age, height)) method?

In the Mappings video, you removed the “people.push(Person(people.length, name, age, height))” altogether, without explanation, and proceeded to use the less “clean” newPerson method to complete your Mappings illustration.

So, my question is, how do I code the mapping using the functionCreatePerson . . . people.push(Person(people.length, name, age, height))? I’ve tried to work this out on my own but I’m just spinning wheels.

Best regards!

P.S. I see that you’ve also removed the “people.push(Person(people . . .” expression from your GitHub structs.sol file. Is there a reason for that as well?

[jon_m]

Great question @novosel,

Have a look at this post. That should answer your question, but please come back and let us know if anything is still unclear or if you have any further questions

[novosel]

Ah! Jon_m, THANK YOU!!

Do you happen to know why Filip abandoned using this method?

If it’s a no-harm, no-foul way to use exclusively the expressions that don’t write to the state, then I’ll simply focus on that route. I’m a beginner so I’ll have to think this through.

Cheers!

[jon_m]

@novosel

P.S. I see that you’ve also removed the “people.push(Person(people . . .” expression from your GitHub structs.sol file. Is there a reason for that as well?

And just a follow-up on your other question…
Both methods are valid, and I’m only hazzarding a guess when I say that the first method may have been removed to avoid the potential confusion that you yourself have highlighed.
If you have only just started this older Solidity course, you may wish to restart with the new updated course instead. It’s literally just been released about a couple of weeks ago and has been updated for the newer versions of Solidity and the examples and assignmnets are based on v0.7.5 (instead of 0.5.12 in the course you are following).
We’ve kept the old course available for students who already started it, so by all means continue with it, and finish that one first, if you prefer. But please bear in mind that Solidity has been evolving and so there are some important changes that you will eventually need to be aware of.

[jon_m]

@novosel

Do you happen to know why Filip abandoned using this method?

Just answered that one… our communication paths have crossed

@novosel

If it’s a no-harm, no-foul way to use exclusively the expressions that don’t write to the state, then I’ll simply focus on that route.

No, you can use either method. The more long-winded method, using more steps and including the temporary “middle-man” local variable can be better when multiple values and/or more complex data structures are involved, simply because the code is much clearer, easier to understand, and probably safer to manipulate and build on. It can also be easier for beginners to understand because it’s broken down into smaller logical steps rather than these being less visible with the one-giant-leap approach. Personally, I like the conciseness achieved with the 1st approach, so it’s also a matter of style and personal preference. Obviously each particular use case will be different, and there may be other case-specific reasons which favour either one method or the other.

Does that go most of the way to clearing up your uncertainties?

[novosel]

That all makes perfect sense. Thx jon_m.

[novosel]

@jon_m

/* Alternative version of createPerson function, not shown in the Mappings video
   (based on Alternative 1 in the previous video, when using an array) */
function createPerson(string memory name, uint age, uint height) public {
   address creator = msg.sender;
   people[creator] = Person(name, age, height);
}

Maybe I jumped the gun. Can someone syntax-check me on the above?

I’m getting a TypeError message using that code:

"TypeError: Type address is not implicitly convertible to expected type uint256. people[creator] = Person(name, age, height); ^------^ "

[jon_m]

Hi @novosel,

You have probably coded either your struct or mapping slightly differently to the ones I based my solution on. You need to adapt the structure in the solution I’ve given you to your own contract. Here is my full contract which contains the code you’ve tried to use. This compiles successfully in Remix, and should help you to identify why it’s throwing an error when you try to implement it in your contract. If this doesn’t help to identify the issue, then post the full contract that you are getting the error with, and I should be able to work out the problem from that.

pragma solidity 0.5.12;

contract MappingExample {
    
    struct Person {
        string name;
        uint age;
        uint height;
        
    }
    
    mapping(address => Person) private people;
    
    function createPerson(string memory name, uint age, uint height) public {
        address creator = msg.sender;
        people[creator] = Person(name, age, height);
        /*
        Person memory newPerson;
        newPerson.name = name;
        newPerson.age = age;
        newPerson.height = height;
        people[creator] = newPerson;
        */
    }
    
    function getPerson() public view returns(string memory name, uint age, uint height) {
        address creator = msg.sender;
        return (people[creator].name, people[creator].age, people[creator].height);
    }
    
}

[matottjm]

Just a couple of questions I had regarding the special variables…

1. Does interacting with the msg.sender and/or msg.value considered interacting something outside the function and therefore not a “pure” function?
2. If floating point variables aren’t supported how is the msg.value represented? I was under the impression you can send fractional ethereum in a transaction.

Thanks,
Jeremy

[Javier_Flores]

Well this is my second time around, I dont understand the UPDATE process, but im re-taking the updated course from scratch. I know theres a few things that are updated. i dont mind really, but I guess this time it should stick, lol.

There should be a smart contract bootcamp at the academy.

[Javier_Flores]

Ok, im doing this course over again because its 0.7.5 now, and its Updated. So now I am having this issue please take a look at the screenshot, if you can help with this , Its well appreciated. Thank you.

I dont know why I am receiving error from compiler, im following along step by step with Filip. Who state that variable can be outside of function. As you can see I keep getting error, why is this?

``pragma solidity 0.7.5;

// this is a complete contract able to run IDE

string message = "Hello World";

// contract statement

contract HelloWorldNew{
// all variables and functions

function hello() public returns(string memory){
    return "Hello World";
}

}`
type or paste code here

:thinking:

[zai]

Hey @Javier_Flores. I guess the concern is that you declared the string message outside the contract. Variable can be outside the function but should still be inside the contract header.
I’m not really sure if my answer is right but my code is just fine putting the string message inside the contract.
I really hope this helps!

[Javier_Flores]

Oh man I see it now, geez, thank so much!

JF