# array\_difference

Returns elements that exist in the first array but not in the second array. Similar to set subtraction (A - B).

```liquid
{% assign all_skus = "SKU1,SKU2,SKU3,SKU4,SKU5" | split: "," %}
{% assign processed_skus = "SKU2,SKU4" | split: "," %}

{% assign remaining = all_skus | array_difference: processed_skus %}
```

Output:

```
SKU1, SKU3, SKU5
```

#### Syntax

```liquid
{{ arrayA | array_difference: arrayB }}
```

| Parameter | Description                  |
| --------- | ---------------------------- |
| `arrayA`  | Source array                 |
| `arrayB`  | Array of elements to exclude |

#### Visual Representation

```
Array A: [1, 2, 3, 4, 5]
Array B: [3, 4, 5, 6, 7]

Result:  [1, 2]  (elements in A that are NOT in B)
```

#### Examples

**Find products not yet synced:**

```liquid
{% assign all_product_ids = shopify_products | map: "id" %}
{% assign synced_product_ids = sync_log | map: "product_id" %}

{% assign unsynced_ids = all_product_ids | array_difference: synced_product_ids %}

{% log "Products not yet synced: " | append: unsynced_ids.size %}

{% for product_id in unsynced_ids %}
  {% comment %} Sync this product {% endcomment %}
{% endfor %}
```

**Find removed tags:**

```liquid
{% assign old_tags = "sale,featured,new,bestseller" | split: "," %}
{% assign new_tags = "featured,bestseller" | split: "," %}

{% assign removed_tags = old_tags | array_difference: new_tags %}
{% log "Tags removed: " | append: removed_tags | join: ", " %}
```

Output:

```
Tags removed: sale, new
```

**Identify discontinued SKUs:**

```liquid
{% assign current_catalog_skus = current_products | map: "sku" %}
{% assign new_catalog_skus = import_data | map: "sku" %}

{% assign discontinued = current_catalog_skus | array_difference: new_catalog_skus %}

{% if discontinued.size > 0 %}
  {% log "Discontinued SKUs:" %}
  {% log discontinued %}

  {% for sku in discontinued %}
    {% comment %} Archive or mark as discontinued {% endcomment %}
  {% endfor %}
{% endif %}
```

**Filter out excluded customers:**

```liquid
{% assign all_customer_ids = customers | map: "id" %}
{% assign vip_customer_ids = vip_list | map: "customer_id" %}

{% assign non_vip_ids = all_customer_ids | array_difference: vip_customer_ids %}

{% log "Regular customers: " | append: non_vip_ids.size %}
{% log "VIP customers: " | append: vip_customer_ids.size %}
```

**Find orders missing from fulfillment report:**

```liquid
{% assign all_order_names = orders | map: "name" %}
{% assign fulfilled_order_names = fulfillment_report | map: "order_name" %}

{% assign unfulfilled = all_order_names | array_difference: fulfilled_order_names %}

{% if unfulfilled.size > 0 %}
  {% log "Orders not in fulfillment report:" %}
  {% for order_name in unfulfilled %}
    {% log order_name %}
  {% endfor %}
{% endif %}
```

#### Notes

* Both inputs must be arrays; throws error otherwise
* Preserves order of elements from the first array
* Duplicate elements in arrayA are preserved if they don't exist in arrayB
* Returns empty array if all elements of arrayA exist in arrayB
* Comparison uses strict equality (===)
* See also: `array_intersection`, `array_symmetric_difference`