**Constants** are just like variables except that their value cannot be altered. A constant contains a fixed value that cannot be altered by the program during its execution. Constants can be of any data type defined in the programming language.

# Declaring constants
Constants are declared using the `let` keyword. We can also provide a *type* annotation while declaring a constant to define the kind of values the constant can store.


import Swift

let firstConstant = 53
print(firstConstant)

// Declaring constant by also providing type annotation
let anotherConstant:Float = 3.14159
print(anotherConstant)

# Modifying constants
There is no possible way to alter the value of a constant once we have assigned it a value; doing so would result in an error. The following code shows that if we try to assign a new value to a constant, our program will output an error.

import Swift

let firstConstant = 53
print(firstConstant)

// Modifying the value of the constant
firstConstant = 23
print(firstConstant)

What are constants in Swift?

Declaring constants

Modifying constants