2.3. Data types

Field elements

A field element is a value in the domain of Z/pZ, where p is the prime number set by default to

p = 21888242871839275222246405745257275088548364400416034343698204186575808495617

As such, field elements are operated in arithmetic modulo p. The circom language is parametric to this number, and it can be changed without affecting the rest of the language (using GLOBAL_FIELD_P). To understand why we need to operate over Z/pZ and when it makes sense to change p, take a look at the Background section.

Field element is the default type for all signals and basic variables.


There is no explicit boolean type. In circom we consider:

  • TRUE : Any field element different from 0.

  • FALSE : Only the element 0.


Arrays can hold a finite number of elements (known at compilation time) of the same type (signal, var or the same type of components or arrays again).

  • The elements are numbered from zero on, and can be accessed using the corresponding index of their position.

  • Array access is made using square brackets.

  • Declaration of an array of a given type is made by adding [] aside of the variable identifier and including the size between the brackets (which should be defined using constant values and/or numeric parameters of templates).

  • The notation m[i,j] for arrays of arrays (matrices) is not allowed. The access and the declaration should be consistent with their type and hence we access and declare with m[i][j], since m[i] is an array.

Below there are few examples of declarations with and without initialization:

var x[3] = [2,8,4];
var z[n+1]; // where n is a parameter of a template
var dbl[16][2] = base;
var y[5] = someFunction(n);

The following case would result in a compilation error, since the size of the array is not explicitly given:

var z = [2, 8, 4];

The type of signals need to be declared as they cannot be assigned globally as an array. They are assigned by positions.