# 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.
```