Loops & Iteration

{% for item in items %}
  - {{ item }}
{% endfor %}

Render the body for each item, with item bound to the loop var.

Loop variables (loop namespace in Jinja2):

• loop.index: 1-based.
• loop.index0: 0-based.
• loop.first, loop.last: booleans.
• loop.length: total items.
• loop.cycle('a','b'): cycle through values.

{% for user in users %}
  {{ loop.index }}. {{ user.name }}
{% endfor %}

Renders:

1. Alice
2. Bob
3. Carol

Else clause for empty:

{% for item in items %}
  {{ item }}
{% else %}
  No items.
{% endfor %}

Renders else branch only if iterable was empty.

Nested loops:

• Inner loop's loop variable shadows outer.
• Access outer via loop.outer.index (Jinja2 syntax varies).

Iterables:

• Lists, tuples.
• Dicts: iterate keys; or (key, value) via for k, v in d.items().
• Strings: iterate chars.
• Custom: any iterable.

Iteration over expensive sequences: lazy generators don't always work. Some engines materialize.

Performance: loops in templates over large datasets are slow. Pre-compute in code.

Break/continue: not in standard Jinja2. Some engines have. Conditional emit instead:

{% for i in items %}
  {% if i > 10 %}{{ i }}{% endif %}
{% endfor %}