Why is an array multiplied by [0] equal to the first element?

Question

I have the following code:

print ([12,2][0])

The result is 12

print ([12,2][1])

The result is 2. Why?

Question update

I can't understand:

target_f = self.model.predict(state)
target_f[0][action] = target

https://github.com/keon/deep-q-learning/blob/master/dqn.py#L51

Why does target_f have an index [0] then another index [action]? What is this doing?

In my C programming experience, it should be a 2D array index. Is it the same here?

You're not doing any multiplication here. Your just making a list `[12,2]` and indexing it with `[0]` or `[1]`. — Ignatius, Jul 18 '19 at 07:22
@ThierryLathuille I don't think this question is about slice notation, is it? — Ignatius, Jul 18 '19 at 07:23
@Taegyung The question about slice notation is broader, simple indexing is also explained in the answers. — Thierry Lathuille, Jul 18 '19 at 07:25
Because that’s how arrays and lists work. 0 is the first position, 1 is the second, and so forth. — Claire, Jul 18 '19 at 07:25

Relandom · Accepted Answer · 2019-07-18T08:52:08.010

It is because [12, 2] is a list, and next notation: [0] or [1] is indexing.

You can test it if you try to print: print([12, 2][2]) you should get index out of range error.

EDIT: to answer your second question:

It is hard to say. target_f = self.model.predict(state) - it is some kind of structure and I can't find information about this structure in the link you put above.

But we can consider some similar structure. Let's say you have:

target_f = [{'action': 'value1', 'action2': 'valuex'}, {'action': 'value2', 'another_key': 'another_value'}]

In your code target_f[0][action] = target:

[0] is index in list. It stands for fist element of list: {'action': 'value1', 'action2': 'valuex'}

[action] is key in the dictionary. It stands for value1.

Since you are typing: target_f[0][action] = target thats mean you want to update your value1 by new value target. Your new stucture will looks like that:

target_f = [{'action': target, 'action2': 'valuex'}, {'action': 'value2', 'another_key': 'another_value'}]

can you also explain what does it mean in 'target_f[0][action] = target'? — Mark, Jul 18 '19 at 08:33
It depends on the kind of structure, but I assume that target_f is a list of dictionaries. Hope explanation above will help you understand it ;) — Relandom, Jul 18 '19 at 08:53

Why is an array multiplied by [0] equal to the first element?

1 Answers1