# Data Types

Every value in Cairo is of a certain *data type*, which tells Cairo what kind of
data is being specified so it knows how to work with that data. This section covers two subsets of data types: scalars and compounds.

Keep in mind that Cairo is a *statically typed* language, which means that it
must know the types of all variables at compile time. The compiler can usually infer the desired type based on the value and its usage. In cases when many types are possible, we can use a cast method where we specify the desired output type.

`fn main() { let x: felt252 = 3; let y: u32 = x.try_into().unwrap(); }`

You’ll see different type annotations for other data types.

## Scalar Types

A *scalar* type represents a single value. Cairo has three primary scalar types:
felts, integers, and booleans. You may recognize
these from other programming languages. Let’s jump into how they work in Cairo.

### Felt Type

In Cairo, if you don't specify the type of a variable or argument, its type defaults to a field element, represented by the keyword `felt252`

. In the context of Cairo, when we say “a field element” we mean an integer in the range \( 0 \leq x < P \),
where \( P \) is a very large prime number currently equal to \( {2^{251}} + 17 \cdot {2^{192}} + 1 \). When adding, subtracting, or multiplying, if the result falls outside the specified range of the prime number, an overflow (or underflow) occurs, and an appropriate multiple of \( P \) is added or subtracted to bring the result back within the range (i.e., the result is computed \( \mod P \) ).

The most important difference between integers and field elements is division: Division of field elements (and therefore division in Cairo) is unlike regular CPUs division, where
integer division \( \frac{x}{y} \) is defined as \( \left\lfloor \frac{x}{y} \right\rfloor \)
where the integer part of the quotient is returned (so you get \( \frac{7}{3} = 2 \)) and it may or may not satisfy the equation \( \frac{x}{y} \cdot y == x \),
depending on the divisibility of `x`

by `y`

.

In Cairo, the result of \( \frac{x}{y} \) is defined to always satisfy the equation \( \frac{x}{y} \cdot y == x \). If y divides x as integers, you will get the expected result in Cairo (for example \( \frac{6}{2} \) will indeed result in `3`

).
But when y does not divide x, you may get a surprising result: for example, since \( 2 \cdot \frac{P + 1}{2} = P + 1 \equiv 1 \mod P \), the value of \( \frac{1}{2} \) in Cairo is \( \frac{P + 1}{2} \) (and not 0 or 0.5), as it satisfies the above equation.

### Integer Types

The felt252 type is a fundamental type that serves as the basis for creating all types in the core library.
However, it is highly recommended for programmers to use the integer types instead of the `felt252`

type whenever possible, as the `integer`

types come with added security features that provide extra protection against potential vulnerabilities in the code, such as overflow and underflow checks. By using these integer types, programmers can ensure that their programs are more secure and less susceptible to attacks or other security threats.
An `integer`

is a number without a fractional component. This type declaration indicates the number of bits the programmer can use to store the integer.
Table 3-1 shows the built-in integer types in Cairo. We can use any of these variants to declare the type of an integer value.

Length | Unsigned |
---|---|

8-bit | `u8` |

16-bit | `u16` |

32-bit | `u32` |

64-bit | `u64` |

128-bit | `u128` |

256-bit | `u256` |

32-bit | `usize` |

Each variant has an explicit size. Note that for now, the `usize`

type is just an alias for `u32`

; however, it might be useful when in the future Cairo can be compiled to MLIR.
As variables are unsigned, they can't contain a negative number. This code will cause the program to panic:

`fn sub_u8s(x: u8, y: u8) -> u8 { x - y } fn main() { sub_u8s(1, 3); }`

All integer types previously mentioned fit into a `felt252`

, except for `u256`

which needs 4 more bits to be stored. Under the hood, `u256`

is basically a struct with 2 fields: `u256 {low: u128, high: u128}`

.

Cairo also provides support for signed integers, starting with the prefix `i`

. These integers can represent both positive and negative values, with sizes ranging from `i8`

to `i128`

.
Each signed variant can store numbers from \( -({2^{n - 1}}) \) to \( {2^{n - 1}} - 1 \) inclusive, where `n`

is the number of bits that variant uses. So an i8 can store numbers from \( -({2^7}) \) to \( {2^7} - 1 \), which equals `-128`

to `127`

.

You can write integer literals in any of the forms shown in Table 3-2. Note
that number literals that can be multiple numeric types allow a type suffix,
such as `57_u8`

, to designate the type.
It is also possible to use a visual separator `_`

for number literals, in order to improve code readability.

Numeric literals | Example |
---|---|

Decimal | `98222` |

Hex | `0xff` |

Octal | `0o04321` |

Binary | `0b01` |

So how do you know which type of integer to use? Try to estimate the max value your int can have and choose the good size.
The primary situation in which you’d use `usize`

is when indexing some sort of collection.

### Numeric Operations

Cairo supports the basic mathematical operations you’d expect for all the integer
types: addition, subtraction, multiplication, division, and remainder. Integer
division truncates toward zero to the nearest integer. The following code shows
how you’d use each numeric operation in a `let`

statement:

`fn main() { // addition let sum = 5_u128 + 10_u128; // subtraction let difference = 95_u128 - 4_u128; // multiplication let product = 4_u128 * 30_u128; // division let quotient = 56_u128 / 32_u128; //result is 1 let quotient = 64_u128 / 32_u128; //result is 2 // remainder let remainder = 43_u128 % 5_u128; // result is 3 }`

Each expression in these statements uses a mathematical operator and evaluates to a single value, which is then bound to a variable.

Appendix B contains a list of all operators that Cairo provides.

### The Boolean Type

As in most other programming languages, a Boolean type in Cairo has two possible
values: `true`

and `false`

. Booleans are one `felt252`

in size. The Boolean type in
Cairo is specified using `bool`

. For example:

`fn main() { let t = true; let f: bool = false; // with explicit type annotation }`

When declaring a `bool`

variable, it is mandatory to use either `true`

or `false`

literals as value. Hence, it is not allowed to use integer literals (i.e. `0`

instead of false) for `bool`

declarations.

The main way to use Boolean values is through conditionals, such as an `if`

expression. We’ll cover how `if`

expressions work in Cairo in the Control Flow section.

### String Types

Cairo doesn't have a native type for strings but provides two ways to handle them: short strings using simple quotes and ByteArray using double quotes.

#### Short strings

A short string is an ASCII string where each character is encoded on one byte (see the ASCII table). For example:

`'a'`

is equivalent to`0x61`

`'b'`

is equivalent to`0x62`

`'c'`

is equivalent to`0x63`

`0x616263`

is equivalent to`'abc'`

.

Cairo uses the `felt252`

for short strings. As the `felt252`

is on 251 bits, a short string is limited to 31 characters (31 * 8 = 248 bits, which is the maximum multiple of 8 that fits in 251 bits).

You can choose to represent your short string with an hexadecimal value like `0x616263`

or by directly writing the string using simple quotes like `'abc'`

, which is more convenient.

Here are some examples of declaring short strings in Cairo:

`fn main() { let my_first_char = 'C'; let my_first_char_in_hex = 0x43; let my_first_string = 'Hello world'; let my_first_string_in_hex = 0x48656C6C6F20776F726C64; let long_string: ByteArray = "this is a string which has more than 31 characters"; }`

#### Byte Array Strings

With the `ByteArray`

struct added in Cairo 2.4.0, you are not limited to 31 characters anymore. These `ByteArray`

strings are written in double quotes like in the following example:

`fn main() { let my_first_char = 'C'; let my_first_char_in_hex = 0x43; let my_first_string = 'Hello world'; let my_first_string_in_hex = 0x48656C6C6F20776F726C64; let long_string: ByteArray = "this is a string which has more than 31 characters"; }`

## Type Casting

In Cairo, you can convert scalar types from one type to another by using the `try_into`

and `into`

methods provided by the `TryInto`

and `Into`

traits from the core library.

The `try_into`

method allows for safe type casting when the target type might not fit the source value. Keep in mind that `try_into`

returns an `Option<T>`

type, which you'll need to unwrap to access the new value.

On the other hand, the `into`

method can be used for type casting when success is guaranteed, such as when the source type is smaller than the destination type.

To perform the conversion, call `var.into()`

or `var.try_into()`

on the source value to cast it to another type. The new variable's type must be explicitly defined, as demonstrated in the example below.

`fn main() { let my_felt252 = 10; // Since a felt252 might not fit in a u8, we need to unwrap the Option<T> type let my_u8: u8 = my_felt252.try_into().unwrap(); let my_u16: u16 = my_u8.into(); let my_u32: u32 = my_u16.into(); let my_u64: u64 = my_u32.into(); let my_u128: u128 = my_u64.into(); // As a felt252 is smaller than a u256, we can use the into() method let my_u256: u256 = my_felt252.into(); let my_usize: usize = my_felt252.try_into().unwrap(); let my_other_felt252: felt252 = my_u8.into(); let my_third_felt252: felt252 = my_u16.into(); }`

## The Tuple Type

A *tuple* is a general way of grouping together a number of values with a
variety of types into one compound type. Tuples have a fixed length: once
declared, they cannot grow or shrink in size.

We create a tuple by writing a comma-separated list of values inside parentheses. Each position in the tuple has a type, and the types of the different values in the tuple don’t have to be the same. We’ve added optional type annotations in this example:

`fn main() { let tup: (u32, u64, bool) = (10, 20, true); }`

The variable `tup`

binds to the entire tuple because a tuple is considered a
single compound element. To get the individual values out of a tuple, we can
use pattern matching to destructure a tuple value, like this:

`fn main() { let tup = (500, 6, true); let (x, y, z) = tup; if y == 6 { println!("y is 6!"); } }`

This program first creates a tuple and binds it to the variable `tup`

. It then
uses a pattern with `let`

to take `tup`

and turn it into three separate
variables, `x`

, `y`

, and `z`

. This is called *destructuring* because it breaks
the single tuple into three parts. Finally, the program prints `y is 6!`

as the value of
`y`

is `6`

.

We can also declare the tuple with value and types, and destructure it at the same time. For example:

`fn main() { let (x, y): (felt252, felt252) = (2, 3); }`

## The Unit Type ()

A *unit type* is a type which has only one value `()`

.
It is represented by a tuple with no elements.
Its size is always zero, and it is guaranteed to not exist in the compiled code.

You might be wondering why you would even need a unit type? In Cairo, everything is an expression, and an expression that returns nothing actually returns `()`

implicitly.