Using .update to update values

Question

If we have a hash and we want to update one of the values or keys we can easily use the .update method like the following

def foo(params)
    params+=1
end

hsh = {:a=>1,:b=>2,:c=>3}

hsh.update(hsh) do |key,oldVal|
    foo(oldVal)
end

p hsh

However suppose a new hash, and I want to copy its properties into another hash and add one to the value. How would I exactly do this? Here is my attempt

def foo(params)
    params+=1
end
new_hsh = {:a=>1,:b=>2,:c=>3}
hsh = {}
hsh.update(new_hsh) do |key,value,newVal|
    newVal = foo(value)
end

p hsh

Which outputs {:a=>1,:b=>2,:c=>3}. So it is copying new_hsh, but not updating the value.

According to docs

Adds the contents of other_hash to hsh. If no block is specified, entries with duplicate keys are overwritten with the values from other_hash, otherwise the value of each duplicate key is determined by calling the block with the key, its value in hsh and its value in other_hash.

So it is copying like the docs say.. but how can I update it?

Thanks

EDIT

tried

def foo(params)
    params+=1
end
new_hsh = {:a=>1,:b=>2,:c=>3}
hsh = {}
hsh.update(new_hsh) do |key,value,newVal|
    foo(newVal)
end

p hsh

In your first snippet, you're not updating _one_ of values. You're updating __all__ of them. — Sergio Tulentsev, Aug 29 '16 at 14:55
@SergioTulentsev, do you know why my edited snippet does not work? — bill, Aug 29 '16 at 17:10
of course. Because `update` doesn't find any duplicate keys (the receiver hash is empty) and therefore does not call the block. — Sergio Tulentsev, Aug 29 '16 at 17:22
I see. Is `update` not useful for transferring one things from one hash to another then? I know there are other ways but I am curious — bill, Aug 29 '16 at 17:26
Why, `.update` is quite usable for this. It does it very well. It's just that you require something on top of it. :) — Sergio Tulentsev, Aug 29 '16 at 17:52
I see. so its possible and I just haven't thought of a solution with `.update` yet right? XD — bill, Aug 29 '16 at 17:59
In the docs it says `update(other_hash) → hsh`, so when do you want to use `update` to manipulate another hash based on one? — bill, Aug 29 '16 at 18:55
You only use `.update` to copy other hash as is. It is not suited for any additional pre-processing. — Sergio Tulentsev, Aug 29 '16 at 18:58
lol oh ok. You would think the method for that would be like `.copy` or something, which probably exists. — bill, Aug 29 '16 at 18:59

Robert · Answer 1 · 2016-08-29T14:57:57.600

1

Dont pass in 3 parameters. Just pass in 2 like this. Iterate over each key, value pair of your new_hsh and set the key equal to the new value after being passed to the foo method and push into your hsh.

def foo(params)
params+=1
end
new_hsh = {:a=>1,:b=>2,:c=>3}
hsh = {}
new_hsh.each do |key,value|
    hsh[key] = foo(value)
end

p hsh

edited Aug 29 '16 at 14:57

answered Aug 29 '16 at 14:52

Robert

86
1
5

Great, this does indeed work, how do i implement this using `.update` though? – bill Aug 29 '16 at 15:13
Try this question http://stackoverflow.com/questions/5215713/ruby-what-is-the-easiest-method-to-update-hash-values – Velocibadgery Aug 29 '16 at 16:53
My working example is the same as the suggested ones in that question. I just wanted to know if I can do this with `update`, with regards to a new hash. Is there any way to do so? – bill Aug 29 '16 at 17:09

Robert · Accepted Answer · 2016-08-31T16:29:58.587

1

Here is another answer. This works but also modifies the new_hsh. But you need to set the hsh = new_hsh before calling update on hsh.

def foo(params)
params+=1
end

new_hsh = {:a=>1,:b=>2,:c=>3}
hsh = new_hsh
hsh.update(hsh) do |key,value|
    hsh[key] = foo(value)
end

p hsh

edited Aug 31 '16 at 16:29

answered Aug 29 '16 at 15:26

Robert

86
1
5

yeah thats what I said at the top. – Robert Aug 29 '16 at 19:59
This updates `new_hsh` because `hashes` are objects, and its being changed by reference from the line `hsh=new_hsh` right? – Muntasir Alam Aug 30 '16 at 12:53

Using .update to update values

2 Answers2