> For the complete documentation index, see [llms.txt](https://cs4998.cornellblockchain.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://cs4998.cornellblockchain.org/introduction/hello-world-pt.-2/units-and-global-variables.md).

# Units and Global Variables

Solidity provides a wide array of predefined units and variables that facilitate both logical and mathematical calculations. In this section we will go over some of the most commonly used ones.

### Units

Solidity provides two types of units: *ether units* and *time units*. 

#### Ether Units

Recall that the EVM denominates ether in terms of Wei (1 wei  = 10\*\*(-18) ether). Therefore, consider the following code that checks if an account has a balance of 1 ether:

```solidity
if (account.balance == 10**18) {}
```

Having to specify an exact integer, especially for integers as large as `10**18`, can lead to both bugs and legibility issues. Rather than specifying an exact value, we can utilize units.

```solidity
if (account.balance == 1 ether) {}
```

In the code above, we are checking if the account has exactly 1 ether. Under the hood, we are still checking if the account has a balance of `10**18`, but it is obvious that using units makes our code easier to read and less prone to bugs. Likewise, we can use units in state/local variables as well:

```solidity
uint num = 1 ether;
```

Solidity provides us with the following ether units:

* `wei`
* `gwei`
* `ether`

#### Time Units

The EVM (and computers in general) denominate time in terms of seconds. Dealing with seconds might be a headache for some and for this reason, Solidity provides us with *time units*. As an example, consider the following code:

```solidity
function redeem() public {
        if (lastTimeCalled + 3600 < currentTime) {}
}
```

In the code above, we have a function which performs some logic if it has been more than a hour since the last time it was called; we check this by adding `3600` to `lastTimeCalled`. Again, denominating seconds explicitly introduces both legibility issues and the potential for bugs. Instead, we can use time units:

```solidity
function redeem() public {
        if (lastTimeCalled + 1 hours < currentTime) {}
}
```

Listed below are the time units that Solidity provides:

* `seconds`
* `minutes`
* `hours`
* `days`
* `weeks`

Again, note that all these units are denominated in terms of seconds.

### Global Variables

As the name might suggest, *global variables* are values that are available at a global level. Listed below are some of the most commonly-used global variables:

* `block.gaslimit`: the gas limit of the current block
* `block.number`: the number of the current block
* `block.timestamp`: the time at which the current block was published
* `msg.data`: the data attached with the current message
* `msg.sender`: the sender of the current message
* `msg.value`: the amount of ether (denominated in wei) that was sent with the current message
* `tx.gasprice`: the price per gas paid for the current transaction
* `tx.origin`: the account that sent the current transaction

#### Transactions Vs Messages

Although transactions and messages might seem synonymous, there is a big difference between the two terms. Consider the following transaction:

> I call contract A. Halfway through the execution of contract A, contract A calls contract B.

Transaction here refers to the entire execution context. In this scenario, `tx.origin` is my address (`tx.origin` is always the address of an EOA). Messages, meanwhile, refer to a specific execution environment. In the scenario above, there are two messages:

* Message 1: consists of me as the sender and contract A as the recipient.
* Message 2: consists of contract A as the sender and contract B as the recipient.

An easy way to think about messages is that a new message is created whenever the from/to values of the execution environment changes.&#x20;

### Resources

Solidity Docs: <https://docs.soliditylang.org/en/v0.8.17/units-and-global-variables.html>