How to add an element to the start of a Pine Script array?

How to add an element to the start of a Pine Script array?

TL;DR
Use array.unshift() to prepend elements, maintain array size, and manage rolling windows in Pine Script.

At a Glance

Difficulty	Beginner
Time to implement	5-10 min
Category	Arrays

Quick Actions

• Open in AI Editor
• Browse PineScript Examples
• Connect to Live Trading

Quick Start

//@version=5
indicator("Array prepend demo", overlay=false)

arr = array.new_float()
array.unshift(arr, close)  // add latest close to front

plot(array.size(arr) > 0 ? arr[0] : na, "Newest value")

Tip. `array.unshift()` increases the array size by one. If you keep a fixed-length buffer, remove the last element afterwards.

Why It Matters

Sometimes you want the newest values at index 0 for easy access (e.g., to maintain a rolling list or feed custom logic). Prepending lets you treat arr[0] as “most recent” with minimal bookkeeping.

What You’ll Learn

• Creating arrays and adding elements to the front
• Maintaining a fixed-size rolling buffer
• Displaying array contents for debugging
• Combining unshift() with other array operations

Quick Reference

Call	Purpose
array.new_*()	Creates array storage for integers, floats, etc.
array.unshift(id, value)	Inserts value at index 0.
array.pop(id)	Removes the last element (useful to keep size constant).
array.get(id, index)	Reads values at a specific index.
array.size(id)	Returns the number of elements currently stored.

Implementation Blueprint

1. Create or reuse the array
   Decide which data type you need—float, int, bool, etc.—and initialise once with var.

   var arr = array.new_float()

2. Prepend the new value
   Call array.unshift() each time you want to add to the front.

   array.unshift(arr, close)

3. Optionally trim the array
   If you only need the N most recent elements, pop the last one when size exceeds your target.

   if array.size(arr) > 10
       array.pop(arr)

Example Playbook

//@version=5
indicator("Rolling window with unshift", overlay=false)

length = input.int(5, "Window size", minval=1)
var float[] values = array.new_float()

array.unshift(values, close)
if array.size(values) > length
    array.pop(values)

rollingAvg = array.size(values) > 0
    ? array.sum(values) / array.size(values)
    : na

plot(close, "Close", color=color.gray)
plot(rollingAvg, "Rolling avg (front-based)", color=color.orange)

Why this works.

• Newest values always reside at index 0, simplifying custom calculations.
• Popping the array after exceeding `length` keeps the window fixed-size.
• The example demonstrates how to derive statistics from the front-based buffer.

Pro Tips & Pitfalls

• Avoid repeatedly creating arrays inside the main code block; use var to initialise once.
• Accessing array elements is zero-based—arr[0] is the newest after unshift().
• For performance, try to keep array sizes small if you frequently call unshift() and pop().
• If you only need historical values occasionally, consider using built-in series indexing (value[bar_index]) instead of arrays.

Troubleshooting & FAQ

Why am I seeing runtime error when accessing arr[0]?

Ensure the array is not empty (array.size(arr) > 0) before reading index 0.

Do I need to clear the array each bar?

No. Arrays persist across bars. Use array.clear(arr) only when you want to reset the buffer manually.

Key Takeaways

• array.unshift() is the go-to method for prepending values.
• Combine it with array.pop() to manage rolling windows of data.
• Always guard against empty arrays when reading indices.
• Arrays give you flexible storage beyond built-in series referencing.

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Keep Going

• Open AI Editor to apply the pattern with guided assistance
• Browse PineScript Examples for ready-to-run templates
• Connect to Live Trading when you are ready to automate

Adapted from tradingcode.net, optimised for Algo Trade Analytics users.