# Constant

There are only 2 data forms that can be represented by constants directly: scalar and vector. Other data forms including pair, matrix, set, dictionary, and table must be returned from a function call. Functiondef and handle are also constants.

```
$ 3;
// an integer constant
$ 3.9;
// a double constant
$ 1 2 3;
// an integer vector constant
$ (1, 2, 3)
// a constant tuple
$ `IBM`YAHOO`MS`GOOG
// a string vector constant
$ true;
// a boolean constant
$ NULL;
// special constant NULL
```

There are 2 built-in constants: pi and e. Built-in constants cannot be redefined.

```
$ pi*2;
6.283185
$ log e;
1
$ e=1;
Syntax Error: [line #1] Please use '==' rather than '=' as equal operator in non-sql expression.
```

**Integer constants**

Integer constants are constant data elements that have no fractional parts or exponents.

1. data forms: SCALAR, PAIR, VECTOR, MATRIX, SET, DICT, TABLE.

```
$ x=set(1 2 3)
$ if(form(x) == SET){y=10};
$ y;
10
$ form x;
4
```

2. data types: VOID, BOOL, CHAR, SHORT, INT, LONG, DATE, MONTH, TIME, MINUTE, SECOND, DATETIME, TIMESTAMP, NANOTIME, NANOTIMESTAMP, FLOAT, DOUBLE, SYMBOL, STRING, UUID, FUNCTIONDEF, HANDLE, CODE, DATASOURCE, RESOURCE, ANY, ANY DICTIONARY, DATEHOUR, IPADDR, INT128, BLOB, COMPLEX, POINT, DURATION.

```
$ x=(`ABC, 123);
$ type x;
24
$ (type x) == ANY;
1
```

3. graph types: LINE, PIE, COLUMN, BAR, AREA, HISTOGRAM, SCATTER.

```
$ plot(1..5 as value, `IBM`MSFT`GOOG`XOM`C, `rank, BAR)
```

4. location of the internal cursor in the seek function: HEAD, CURRENT, TAIL. The following example returns the length of a file.

```
$ def fileLength(f): file(f).seek(0, TAIL)
$ fileLength("test.txt");
14
```

5. types of objects to be undefined in the undef command: VAR (variable), SHARED (shared variable) or DEF (function definition).

```
$ x=1 2 3;
$ undef(`x, VAR);
```

We can declare constant variables with: const variableName = value.

Constant variables are not mutable. After we declare a constant variable, we can’t bind another object to the constant variable, nor can we modify the value of the bound object.

```
$ const a = 1 2 3;
$ a=4 5 6;
Syntax Error: [line #1] Constant variable [a] can't be modified.
$ const a = 10;
$ a+=1;
Syntax Error: [line #1] Constant variable [a] can't be modified.
$ const x = 1..10;
$ x[5]=0;
Syntax Error: [line #1] Constant variable [x] can't be modified.
```