Solidity Tutorial: all about Array

In Solidity, an array is an ordered list of items that is indexed numerically, starting at 0.

Array types

In this section, we provide two types of categorisation for arrays:

• Fixed-size vs dynamic size arrays
• One-dimensional vs Multi-Dimensional arrays.

Fixed-size vs Dynamic-Size array

The Solidity documentation make the distinction between fixed-size and dynamic size arrays. In the examples below, T is the element type and k is the array length / size. If k = 5 , the array can hold a maximum of 5 values.

• Fixed Size: T[k]

// array that can hold 5 unsigned integers maximum
uint[5] my_array;

• Dynamic Size: T[]

// array containing an undefined number of strings
string[] my_array;

One Dimensional vs Multi-Dimensional arrays

One Dimensional and Multi-Dimensional arrays can be both of Fixed-Size or Dynamic-Size. The One Dimensional arrays are essentially the examples cited above. Here are the definitions

• T[k] : One Dimensional, Fixed-size
• T[] : One Dimensional, Dynamic-size

Multi-dimensional arrays are essentially nested arrays (An array that contain other arrays). These however, come in three forms in Solidity. We use two-dimensional array in this first example…

• T[k][k] : Two-Dimensional, Fixed-size
• T[][] : Two-Dimensional, Dynamic-size
• T[][k] or T[k][] : Two-Dimensional, Mixed-size

… but you will see that multi-dimensional arrays can have any level of nesting ! Here are some examples.

• T[2][2][2] : Three-Dimensional, Fixed-Size (all k are the same)
• T[2][8][4][12] : Four-Dimensional, Fixed-Sizes ( k‘s are of different values)
• T[][][][][] : Five-Dimensional, Dynamic-Size
• T[][3][2][][9][] : Six-Dimensional, Mixed-Size