Assignment, Move, Force-Assignment, and Swapping Operators

Assignment operator (=)

The binary assignment operator = can be used to assign a new value to a variable. It is only allowed in a statement and is not allowed in expressions:

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var a = 1

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a = 2

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// `a` is `2`

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var b = 3

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var c = 4

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// Invalid: The assignment operation cannot be used in an expression.

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a = b = c

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// Instead, the intended assignment must be written in multiple statements.

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b = c

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a = b

Assignments to constants are invalid.

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let a = 1

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// Invalid: Assignments are only for variables, not constants.

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a = 2

The left-hand side of the assignment operand must be an identifier. For arrays and dictionaries, this identifier can be followed by one or more index or access expressions.

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// Declare an array of integers.

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let numbers = [1, 2]

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// Change the first element of the array.

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numbers[0] = 3

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// `numbers` is `[3, 2]`

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// Declare an array of arrays of integers.

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let arrays = [[1, 2], [3, 4]]

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// Change the first element in the second array

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arrays[1][0] = 5

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// `arrays` is `[[1, 2], [5, 4]]`

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let dictionaries = {

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true: {1: 2},

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false: {3: 4}

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}

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dictionaries[false][3] = 0

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// `dictionaries` is `{

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// true: {1: 2},

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// false: {3: 0}

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//}`

Move operator (<-)

The move operator (<-) is unique to Cadence and is used to move resource types from one location to another. It works similar to the assignment operator (=) you're used to from most programming languages, except that the data in the location on the right side of the statement is destroyed by the operation:

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// Declare a resource named `SomeResource`, with a variable integer field.

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access(all)

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resource SomeResource {

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access(all)

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var value: Int

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init(value: Int) {

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self.value = value

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}

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}

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// Declare a constant with value of resource type `SomeResource`.

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let a: @SomeResource <- create SomeResource(value: 5)

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// *Move* the resource value to a new constant.

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let b <- a

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// Invalid Line Below: Cannot use constant `a` anymore as the resource that it

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// referred to was moved to constant `b`.

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a.value // Error: a no longer exists

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// Constant `b` owns the resource.

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b.value // equals 5

Force-assignment operator (<-!)

The force-assignment operator (<-!) assigns a resource-typed value to an optional-typed variable if the variable is nil. If the variable being assigned to is non-nil, the execution of the program aborts.

The force-assignment operator is only used for resource types.

Swapping operator (<->)

The binary swap operator <-> can be used to exchange the values of two variables. It is only allowed in a statement and is not allowed in expressions:

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var a = 1

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var b = 2

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a <-> b

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// `a` is `2`

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// `b` is `1`

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var c = 3

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// Invalid: The swap operation cannot be used in an expression.

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a <-> b <-> c

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// Instead, the intended swap must be written in multiple statements.

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b <-> c

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a <-> b

Both sides of the swap operation must be variable, assignment to constants is invalid.

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var a = 1

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let b = 2

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// Invalid: Swapping is only possible for variables, not constants.

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a <-> b

Both sides of the swap operation must be an identifier, followed by one or more index or access expressions.