Arrays in Julia programming language

Arrays

Julia, like most technical computing languages, provides a first-class array implementation. Most technical computing languages pay a lot of attention to their array implementation at the expense of other containers. Julia does not treat arrays in any special way. The array library is implemented almost completely in Julia itself, and derives its performance from the compiler, just like any other code written in Julia.
An array is a collection of objects stored in a multi-dimensional grid. In the most general case, an array may contain objects of type Any. For most computational purposes, arrays should contain objects of a more specific type, such as Float64 or Int32.
In general, unlike many other technical computing languages, Julia does not expect programs to be written in a vectorized style for performance. Julia’s compiler uses type inference and generates optimized code for scalar array indexing, allowing programs to be written in a style that is convenient and readable, without sacrificing performance, and using less memory at times.
In Julia, all arguments to functions are passed by reference. Some technical computing languages pass arrays by value, and this is convenient in many cases. In Julia, modifications made to input arrays within a function will be visible in the parent function. The entire Julia array library ensures that inputs are not modified by library functions. User code, if it needs to exhibit similar behaviour, should take care to create a copy of inputs that it may modify.

Basic Functions

1. ndims(A) — the number of dimensions of A
2. size(A,n) — the size of A in a particular dimension
3. size(A) — a tuple containing the dimensions of A
4. eltype(A) — the type of the elements contained in A
5. length(A) — the number of elements in A
6. nnz(A) — the number of nonzero values in A
7. stride(A,k) — the size of the stride along dimension k
8. strides(A) — a tuple of the linear index distances between adjacent elements in each dimension

Construction and Initialization

Many functions for constructing and initializing arrays are provided. In the following list of such functions, calls with a dims... argument can either take a single tuple of dimension sizes or a series of dimension sizes passed as a variable number of arguments.

1. Array(type, dims...) — an uninitialized dense array
2. cell(dims...) — an uninitialized cell array (heterogeneous array)
3. zeros(type, dims...) — an array of all zeros of specified type
4. ones(type, dims...) — an array of all ones of specified type
5. trues(dims...) — a Bool array with all values true
6. falses(dims...) — a Bool array with all values false
7. reshape(A, dims...) — an array with the same data as the given array, but with different dimensions.
8. copy(A) — copy A
9. similar(A, element_type, dims...) — an uninitialized array of the same type as the given array (dense, sparse, etc.), but with the specified element type and dimensions. The second and third arguments are both optional, defaulting to the element type and dimensions of A if omitted.
10. reinterpret(type, A) — an array with the same binary data as the given array, but with the specified element type.
11. rand(dims) — random array with Float64 uniformly distributed values in [0,1)
12. randf(dims) — random array with Float32 uniformly distributed values in [0,1)
13. randn(dims) — random array with Float64 normally distributed random values with a mean of 0 and standard deviation of 1
14. eye(n) — n-by-n identity matrix
15. eye(m, n) — m-by-n identity matrix
16. linspace(start, stop, n) — a vector of n linearly-spaced elements from start to stop.
17. fill!(A, x) — fill the array A with value x

The last function, fill!, is different in that it modifies an existing array instead of constructing a new one. As a convention, functions with this property have names ending with an exclamation point. These functions are sometimes called “mutating” functions, or “in-place” functions.

Comprehensions

Comprehensions provide a general and powerful way to construct arrays. Comprehension syntax is similar to set construction notation in mathematics:

A = [ F(x,y,...) for x=rx, y=ry, ... ]

The meaning of this form is that F(x,y,...) is evaluated with the variables x, y, etc. taking on each value in their given list of values. Values can be specified as any iterable object, but will commonly be ranges like 1:n or 2:(n-1), or explicit arrays of values like [1.2, 3.4, 5.7]. The result is an N-d dense array with dimensions that are the concatenation of the dimensions of the variable ranges rx, ry, etc. and each F(x,y,...) evaluation returns a scalar.
The following example computes a weighted average of the current element and its left and right neighbour along a 1-d grid.

julia> const x = rand(10)
[0.6017125321472665,0.55317268439850298,0.83375372173664064,0.20371170284589835,0.50800458572940888,0.52963052092498386,0.33042233578025493,0.49411133447814293,0.29570938193206264,0.81897111867503525]

julia> [ 0.5*x[i-1] + x[i] + 0.5*x[i+1] for i=2:length(x)-1 ]
[1.27090581134045655,1.21219591535884108,0.8745908565789231,0.87467569761484998,0.94884398167981576,0.84229326348181832,0.80717719333430171,0.95225060850865173]

NOTE: In the above example, x is declared as constant because type inference in Julia does not work as well on non-constant global variables.

Indexing

The general syntax for indexing into an n-dimensional array A is:

X = A[I_1, I_2, ..., I_n]

where each I_k may be:

1. A scalar value
2. A Range of the form :, a:b, or a:b:c
3. An arbitrary integer vector, including the empty vector []

The result X has the dimensions (size(I_1), size(I_2), ..., size(I_n)), with location (i_1, i_2, ..., i_n) of X containing the value A[I_1[i_1], I_2[i_2], ..., I_n[i_n]].
Indexing syntax is equivalent to a call to ref:

X = ref(A, I_1, I_2, ..., I_n)

Example:

julia> x = reshape(1:16, 4, 4)
4x4 Int64 Array
1 5 9 13
2 6 10 14
3 7 11 15
4 8 12 16

julia> x[2:3, 2:end-1]
2x2 Int64 Array
6 10
7 11

Assignment

The general syntax for assigning values in an n-dimensional array A is:

A[I_1, I_2, ..., I_n] = X

where each I_k may be:

1. A scalar value
2. A Range of the form :, a:b, or a:b:c
3. An arbitrary integer vector, including the empty vector []

The size of X should be (size(I_1), size(I_2), ..., size(I_n)), and the value in location (i_1, i_2, ..., i_n) of A is overwritten with the value X[I_1[i_1], I_2[i_2], ..., I_n[i_n]].
Index assignment syntax is equivalent to a call to assign:

A = assign(A, X, I_1, I_2, ..., I_n)

Example:

julia> x = reshape(1:9, 3, 3)
3x3 Int64 Array
1 4 7
2 5 8
3 6 9

julia> x[1:2, 2:3] = -1
3x3 Int64 Array
1 -1 -1
2 -1 -1
3 6 9

Concatenation

Arrays can be concatenated along any dimension using the following syntax:

1. cat(dim, A...) — concatenate input n-d arrays along the dimension dim
2. vcat(A...) — Shorthand for cat(1, A...)
3. hcat(A...) — Shorthand for cat(2, A...)
4. hvcat(A...)

Concatenation operators may also be used for concatenating arrays:

1. [A B C...] — calls hcat
2. [A, B, C, ...] — calls vcat
3. [A B; C D; ...] — calls hvcat

Vectorized Operators and Functions

The following operators are supported for arrays. In case of binary operators, the dot version of the operator should be used when both inputs are non-scalar, and any version of the operator may be used if one of the inputs is a scalar.

1. Unary Arithmetic — -
2. Binary Arithmetic — +, -, *, .*, /, ./, \, .\, ^, .^, div, mod
3. Comparison — ==, !=, <, <=, >, >=
4. Unary Boolean or Bitwise — ~
5. Binary Boolean or Bitwise — &, |, $
6. Trigonometrical functions — sin, cos, tan, sinh, cosh, tanh, asin, acos, atan, atan2, sec, csc, cot, asec, acsc, acot, sech, csch, coth, asech, acsch, acoth, sinc, cosc, hypot
7. Logarithmic functions — log, log2, log10, log1p, logb, ilogb
8. Exponential functions — exp, expm1, exp2, ldexp
9. Rounding functions — ceil, floor, trunc, round, ipart, fpart
10. Other mathematical functions — min, max, abs, pow, sqrt, cbrt, erf, erfc, gamma, lgamma, real, conj, clamp

Implementation

The base array type in Julia is the abstract type AbstractArray{T,n}. It is parametrized by the number of dimensions n and the element type T. AbstractVector and AbstractMatrix are aliases for the 1-d and 2-d cases. Operations on AbstractArray objects are defined using higher level operators and functions, in a way that is independent of the underlying storage class. These operations are guaranteed to work correctly as a fallback for any specific array implementation.
The Array{T,n} type is a specific instance of AbstractArray where elements are stored in column-major order. Vector and Matrix are aliases for the 1-d and 2-d cases. Specific operations such as scalar indexing, assignment, and a few other basic storage-specific operations are all that have to be implemented for Array, so that the rest of the array library can be implemented in a generic manner for AbstractArray.
SubArray is a specialization of AbstractArray that performs indexing by reference rather than by copying. A SubArray is created with the sub function, which is called the same way as ref (with an array and a series of index arguments). The result of sub looks the same as the result of ref, except the data is left in place. sub stores the input index vectors in a SubArray object, which can later be used to index the original array indirectly.
StridedVector and StridedMatrix are convenient aliases defined to make it possible for Julia to call a wider range of BLAS and LAPACK functions by passing them either Array or SubArray objects, and thus saving inefficiencies from indexing and memory allocation.
The following example computes the QR decomposition of a small section of a larger array, without creating any temporaries, and by calling the appropriate LAPACK function with the right leading dimension size and stride parameters.

julia> a = rand(10,10);

julia> b = sub(a, 2:2:8,2:2:4)
4x2 SubArray of 10x10 Float64 Array
0.48291296659328276 0.31639301252254248
0.11191852765878418 0.80311033863988501
0.34377272170384798 0.12998312467801409
0.75207724893767547 0.48974544536835718

julia> (q,r,p) = qr(b);

julia> q
4x2 Float64 Array
-0.31610281030340204 0.38994108897230212
-0.80237370921615103 -0.5848318975546335
-0.12986390146593485 0.36571345172816944
-0.48929624071011685 0.61005841520202764

julia> r
2x2 Float64 Array
-1.00091806276211814 -0.65508286752651457
0.0 0.70738744643074303

julia> p
[2,1]