Expressions, Declarations and Statements

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Expressions

An expression is a sequence of applying both operators and function calls to operands (variables, values), which determines the calculations and actions done by the script. Expressions in Pine almost always produce a result (only annotation functions are an exception, such as study or fill. They produce side effects and will be covered later).

Here are some examples of simple expressions:

(high + low + close)/3
sma(high - low, 10) + sma(close, 20)

Variable Declarations

Variables in Pine are declared with the help of the special symbol = in the following way:

<identifier> = <expression>

In place of <identifier> will be the name of the declared variable. Examples of Variable Declarations:

src = close
len = 10
ma = sma(src, len) + high

Three variables were declared here: src, len and ma. Identifiers close and high are built-in variables. The identifier sma is a built-in function for calculating Simple Moving Average.

Variable Assignment

Mutable variable is such a variable which can be given a new value.

The operator := must be used to give a value to a variable. To use this operator, a special attribute must be used in the first line of a code: //@version=2. This attribute identifies the version of Pine Script. Mutable variables were introduced in version 2.

A variable must be declared before you can set a value for it (declaration of variables has been described above).

Type of a variable is identified on the declaration step. A variable can be given a value of expression only if both the expression and the variable belong to the same type, otherwise it will give you a compilation error.

Variable assignment example:

//@version=2
study("My Script")
price = close
if hl2 > price
    price := hl2
plot(price)

We also use an ‘if’ statement in this example.

Self Referencing Variables in version 2

Note: self referencing variables and forward referencing variables was removed in version 3.

The ability to reference the previous values of declared variables in expressions where they are declared (using the operator []) is a useful feature in Pine.These variables are called self referencing variables. For Example:

study("Fibonacci numbers")
fib = na(fib[1]) or na(fib[2]) ? 1 : fib[1] + fib[2]
plot(fib)

Note: For Version 3, this can be achieved using the syntax:

study("Fibonacci numbers v3")
fib = 0
fib := na(fib[1]) or na(fib[2]) ? 1 : fib[1] + fib[2]
plot(fib)

(See migration guide: https://www.tradingview.com/wiki/Pine_Version_3_Migration_Guide#Self-referenced_variables_are_removed) Expert tip: mod out the Fibonacci numbers by 1000 to generate a plot you can actually see:

study("Fibonacci numbers v3")
fib = 0
fib :=( na(fib[1]) or na(fib[2]) ? 1 : fib[1] + fib[2] ) % 1000
plot(fib)


The variable fib is a series of Fibonacci numbers : 1, 1, 2, 3, 5, 8, 13, 21, … Where the first two numbers are equal to 1 and 1 and each subsequent number is the sum of the last two. In the given example, the built-in function na is used and returns true if the value of its argument has still not been determined (is NaN). In the example produced below, the values fib[1] and fib[2] have not been determined on the first bar, while on the second bar fib[2] has not been determined. Finally, on the third bar both of them are defined and can be added. Fib.png

Footnote: Since the sequence of Fibonacci numbers grows rather fast, the variable ‘fib’ very quickly overflows. As such, the user should apply the given indicator on the monthly ‘M’ or yearly ‘Y’ resolution, otherwise the value ‘n/a’ will be on the chart instead of Fibonacci numbers.

Preventing NaN values, Functions na and nz

Self referencing variables allow for the accumulation of values during the indicator’s calculation on the bars. However there is one point to remember. For example, let's assume we want to count all the bars on the chart with the following script:

barNum = barNum[1] + 1

The self referencing variable ‘barNum’ refers to its own value on the previous bar, meaning, when the indicator will be calculated on every bar, the value barNum[1] will be equal to NaN. Therefore, on the first bar barNum[1] will not be defined (NaN). Adding 1 to NaN, NaN will still be the result. In total, the entire barNum series will be equal on every bar to NaN. On next bar, barNum = NaN + 1 = NaN and so on. In total, barNum will contain only NaN values.

In order to avoid similar problems, Pine has a built-in function nz. This function takes an argument and if it is equal to NaN then it returns 0, otherwise it returns the argument’s value. Afterwards, the problem with the bars’ calculation is solved in the following way:

barNum = nz(barNum[1]) + 1

There is an overloaded version of nz with two arguments which returns the second argument if the first is equal to NaN. Further information about ‘nz’ can be found here.

In addition, there is a simple function with one argument that returns a logical result called na. This function makes it possible to check if the argument is NaN or not. Check it out here.

The difference between na and nz: na returns a Boolean value (True / False), and is therefore useful in constructing logical expressions (if na(x), ...). nz is a "filler", as it fills NaN values of a series with zeros (in the case of nz(x)) or with a user-specified value (in the case of nz(x, y)). Note: the double-argument version nz(x, y) is equivalent to the logical construction na(x) ? y : x. (This is a ternary operation, which can be read: "if na(x) then y else x.")

Simple Moving Average without applying the Function ‘sma’

While using self referencing variables, it’s possible to write the equivalent of the built-in function sma which calculates the Simple Moving Average.

study("Custom Simple MA", overlay=true)
src = close
len = 9
sum = nz(sum[1]) - nz(src[len]) + src
plot(sum/len)

The variable ‘sum’ is a moving sum with one window that has a length ‘len’. On each bar the variable ‘sum’ is equal to its previous value, then the leftmost value in a moving window is subtracted from ‘sum’ and a new value, which entered the moving window (the rightmost), is added. This is the algorithm optimized for vector languages, see Moving Average for a detailed basic algorithm description.

Further, before the graph is rendered, the ‘sum’ is divided by the window size ‘len’ and the indicator is displayed on the chart as the Simple Moving Average.

Self referencing variables can also be used in functions written by the user. This will be discussed later.

‘if’ statement

If statement defines what block of statements must be executed when conditions of the expression are satisfied.

To have access to and use the if statement, one should specify the version of Pine Script language in the very first line of code: //@version=2

General code form:

var_declarationX = if condition
    var_decl_then0
    var_decl_then1
    
    var_decl_thenN
    return_expression_then
else
    var_decl_else0
    var_decl_else1
    
    var_decl_elseN
    return_expression_else

where:

  • var_declarationX — this variable gets the value of the if statement
  • condition — if the condition is true, the logic from the block then (var_decl_then0, var_decl_then1, etc) is used, if the condition is false, the logic from the block ‘else’ (var_decl_else0, var_decl_else1, etc) is used.
  • return_expression_then, return_expression_else — the last expression from the block then or from the block else will return the final value of the statement. If declaration of the variable is in the end, its value will be the result.

The type of returning value of the if statement depends on return_expression_then and return_expression_else type (their types must match: it is not possible to return an integer value from then, while you have a string value in else block).

Example:

// This code compiles
x = if close > open
    close
else
    open
// This code doesn’t compile
x = if close > open
    close
else
    "open"

It is possible to omit the else block. In this case if the condition is false, an “empty” value (na, or false, or “”) will be assigned to the var_declarationX variable.

Example:

x = if close > open
    close
// If current close > current open, then x = close.
// Otherwise the x = na.

The blocks “then” and “else” are shifted by 4 spaces. If statements can include each other, +4 spaces:

x = if close > open
    b = if close > close[1]
        close
    else
        close[1]
    b
else
    open

It is possible to ignore the resulting value of an if statement (“var_declarationX=“ can be omited). It may be useful if you need the side effect of the expression, for example in strategy trading:

if (crossover(source, lower))
    strategy.entry("BBandLE", strategy.long, stop=lower,                    
                   oca_name="BollingerBands",
                   oca_type=strategy.oca.cancel, comment="BBandLE")
else
    strategy.cancel(id="BBandLE")

‘for’ statement

for statement allows to execute a number of instructions repeatedly. To use for statements, a special attribute must be used in the first line of a code: //@version=2. This attribute identifies the version of Pine Script. for statements were introduced in version 2.

General code form:

var_declarationX = for counter = from_num to to_num [by step_num]
    var_decl0
    var_decl1
    
    continue
    
    break
    
    var_declN
    return_expression

where:

  • counter - a variable, loop counter.
  • from_num - start value of the counter.
  • to_num - end value of the counter. When the counter becomes greater than to_num (or less than to_num in case from_num > to_num) the loop is broken.
  • step_num - loop step. Can be omitted (in the case loop step = 1). If from_num is greater than to_num loop step will change direction automatically, no need to specify negative numbers.
  • var_decl0, … var_declN, return_expression - body of the loop. It must be shifted by 4 spaces or 1 tab.
  • return_expression - returning value. When a loop is finished or broken, the returning value is given to the var_declarationX.
  • continue - a keyword. Can be used only in loops. It switches the loop to next iteration.
  • break - a keyword. Can be used only in loops. It breaks the loop.

Loop example:

//@version=2
study("My sma")
my_sma(price, length) =>
    sum = price
    for i = 1 to length-1
        sum := sum + price[i]
    sum / length
plot(my_sma(close,14))

Variable ‘sum’ is a mutable variable and a new value can be given to it by an operator := in body of the loop. Also note that we recommend to use a built-in function sma for Moving Average as it calculates faster.


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