Variables

This primer demonstrates variable declaration syntax.

Variable Declarations

Variables are declared with the var keyword. Variable declarations must have a type, initializer, or both. Here, we declare myVariable1 to be an int. Since there was no initializer, it is initialized to the default value for the int type: 0.

var myVariable1: int;
writeln("myVariable1 = ", myVariable1);

Here we declare a variable without a type. Since 1.618 is a real value, the variable is inferred to be of type real. Note that we print out the type by casting it to a string.

var myVariable2 = 1.618;
writeln("myVariable2 = ", myVariable2, " (type = ",
        myVariable2.type:string, ")");

Variables can also be declared using type aliases. Here we declare a type alias myType to be uint(16) and declare myVariable3 to be of type myType.

type myType = uint(16);
var myVariable3: myType = 3;
writeln("myType = ", myType:string);
writeln("myVariable3 = ", myVariable3, " (type = ",
        myVariable3.type:string, ")");

Constants

Instead of var, const and param can be used to declare runtime constants and compile-time constants respectively. A const must be initialized in place, but can have its value generated at runtime. A param must be known at compile time.

const myConst: real = sqrt(myVariable2);
param myParam = 3.14;
writeln("myConst = ", myConst, ", myParam = ", myParam);

Configs

At module scope, all three variable kinds can be qualified by the config keyword. This allows the initial value to be overridden on the command line. A config var or config const may be overridden when the program is executed; a config param may be overridden when the program is compiled. Similarly, type aliases maybe be qualified by the config keyword. The comment following each declaration shows how the value can be modified.

config var cfgVar = "hello";         // ./variables --cfgVar="world"
config const cfgConst: bool = false; // ./variables --cfgConst=true
config param cfgParam = 4;           // chpl variables.chpl -s cfgParam=1
config type cfgType = complex;       // chpl variables.chpl -s cfgType=imag
writeln("cfgVar = ", cfgVar,
        ", cfgConst = ", cfgConst,
        ", cfgParam = ", cfgParam,
        ", cfgType = ", cfgType:string);

Type and Value Propagation

Variable types and values propagate to the left. Here, a is a floating point number initialized to 1.0, b and c are integers initialized to 0, d and e are floating point numbers initialized to 1.0, and f is a floating point number initialized to 0.0.

var a = 1.0, b, c: int, d, e = 1.0, f: real;
writeln("a = ", a, ", b = ", b, ", c = ", c,
        ", d = ", d, ", e = ", e, ", f = ", f);