# Checking if two values are identical

**URL:** <https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301>\
**Category:** New to Julia\
**Created:** [June 30, 2020, 1:24pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301 "2020-06-30T13:24:59Z")\
**Posts on this page:** 18\
**Page:** 1

<div class="post-metadata">

**Author:** ![Nash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nash/32/14482_2.png) [@Nash](https://discourse.julialang.org/u/Nash)\
**Post date:** [June 30, 2020, 1:24pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/1 "2020-06-30T13:24:59Z")

</div>

I am attempting to check if two values are identical and then do stuff if they are not. My check involves:

```
stock[id,3] != flow[v,3]

```

Unfortunately, I can get the following

```
Any[0]

```

and

```
0

```

and Julia interprets that as being different.

What beginner’s trap have I stumbled into?

---

<div class="post-metadata">

**Author:** ![oheil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oheil/32/220745_2.png) [@oheil](https://discourse.julialang.org/u/oheil)\
**Post date:** [June 30, 2020, 1:33pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/2 "2020-06-30T13:33:45Z")

</div>

```julia
julia> typeof(Any[0])
Array{Any,1}

julia> typeof(0)
Int64

```

can’t be equal. Only the single element of your array can be equal to an Int64.  
There is no good solution, if you insist to compare the two different types.

Clearly

```julia
julia> Any[0][1]==0
true

julia> Any[0]==[0]
true

```

but this is trivial and seems not suitable for anything which call itself a program.

What do you try to do in general? (I expect this to be related to the other two questions about stock and flow) 😀

---

<div class="post-metadata">

**Author:** ![Nash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nash/32/14482_2.png) [@Nash](https://discourse.julialang.org/u/Nash)\
**Post date:** [June 30, 2020, 2:16pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/3 "2020-06-30T14:16:16Z")

</div>

The script that is causing problems is shown below. I start by getting information from an API and create a “flow” array and a “stock” array that is equal to flow. Then I gather more information from the API (I use a loop, from i to number, and use sleep(seconds) to call the API at set intervals). Armed with the new information, I then create a new flow, and look for differences between this new flow and stock to update stock where needed (i.e., where there are differences between the new flow and stock). This is done by looking in the first column of the arrays, where there is an identifier for a particular sports match. If the new flow contains an entirely new line not contained in stock (i.e., a new match is included in the new flow but not in stock), I add that line to stock.

That is the basic idea. But in checking to see differences, I get the trouble stated above. So, I wonder why the type of numbers in stock and flow can be different at all…

```julia
for i = 1:updates
    # ...
    # The API is called. Information is gathered and placed in arrays.
    # ...

    # An aggregated array (flow) is assembled after calling the API: 
    flow = [marketId price1]
    # If this is the first iteration of the update loop, flow is duplicated and named "stock":
    if i == 1
       global stock = flow
    end
    # After i > 1, stock is compared with the new flow to look for differences and update stock:
    for v = 1:length(flow[:,1])
        if isempty(findall(x->x==flow[v,1], stock[:,1])) == false
           id = findall(x->x==flow[v,1], stock[:,1])
           if stock[id,2] != flow[v,2]
              stock[id,2] .= flow[v,2]
           end
        end
        if isempty(findall(x->x==flow[v,1], stock[:,1])) == true
           stock = [permutedims(flow[v,:]);stock]
        end
    end

sleep(60)

end

```

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [June 30, 2020, 2:34pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/4 "2020-06-30T14:34:45Z")

</div>

> [@Nash](#):
>
> But in checking to see differences, I get the trouble stated above. S

I mean, you are checking if two different things (an array and an integer) are equal and Julia tells you no. There isn’t really much more to say… You probably have som error in your code so you compare the wrong things.

---

<div class="post-metadata">

**Author:** ![Nash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nash/32/14482_2.png) [@Nash](https://discourse.julialang.org/u/Nash)\
**Post date:** [June 30, 2020, 2:39pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/5 "2020-06-30T14:39:52Z")

</div>

Can you see if my code (in principle) should work if the types are the same?

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [June 30, 2020, 2:41pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/6 "2020-06-30T14:41:35Z")

</div>

It’s hard to understand what the code does. It would be best to reduce your code to the smallest possible example and then ask the question about that code.

---

<div class="post-metadata">

**Author:** ![oheil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oheil/32/220745_2.png) [@oheil](https://discourse.julialang.org/u/oheil)\
**Post date:** [June 30, 2020, 2:48pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/7 "2020-06-30T14:48:03Z")

</div>

My guess is that you mixup the dimensions of stock and flow.  
First you do

```julia
stock = flow

```

Later you do

```julia
stock = [permutedims(flow[v,:]);stock]

```

This looks fishy to me.

You should create some example data, which is in dimensions equal and for the entries similar to the data you receive from the API, i.e. an example for flow with more than 1 rows (or cols, its unclear what you receive).

You can post this example here or you may try again implementing your algorithm with the example.

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [June 30, 2020, 3:03pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/8 "2020-06-30T15:03:43Z")

</div>

To me it looks like you should be using a dictionary, not an array. It actually looks like you made a very inefficient implementation of `Dict`.

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [June 30, 2020, 3:13pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/9 "2020-06-30T15:13:04Z")

</div>

Something like this maybe

```julia
stock = Dict{String, Float64}() # or some appropriate types
for i in axes(flow, 1)
    key, value = flow[i, 1], flow[i, 2] 
    stock[key] = value
end

```

?

---

<div class="post-metadata">

**Author:** ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)\
**Post date:** [June 30, 2020, 3:14pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/10 "2020-06-30T15:14:04Z")

</div>

This is a very peculiar way of writing the loop, which makes it hard to see what’s going on, but it seems like you are basically trying to replace values in `stock` with values in `flow` if they are different now but were the same before?  
Some comments:

```julia
for v = 1:length(flow[:,1])

```

Here it would be idiomatic to use `size(flow, 2)`

```julia
if isempty(findall(x->x==flow[v,1], stock[:,1])) == false
    ...
end
if isempty(findall(x->x==flow[v,1], stock[:,1])) == true
    ...
end

```

Here you are unnecessarily performing the `findall` check twice, when it can only be either true or false. You could just use `if ... else ... end` to write:

```julia
if !isempty(findall(x->x==flow[v,1], stock[:,1]))
    ...
else
    ...
end

```

```julia
id = findall(x->x==flow[v,1], stock[:,1])

```

Here you are performing the check for a third time, which tells me you should have just saved the result the first time around.

I would probably write something like this (note this hasn’t been tested of course!)

```julia
for v = 1:size(flow, 2)

    matches = flow[v, 1] .== stock[:, 1]
    
    if sum(matches) > 0
        stock[matches, 2] .= flow[v, 2]
    else
       stock = [permutedims(flow[v,:]);stock]
    end
end

```

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [June 30, 2020, 3:39pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/11 "2020-06-30T15:39:58Z")

</div>

Again, don’t use arrays for this.

---

<div class="post-metadata">

**Author:** ![Nash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nash/32/14482_2.png) [@Nash](https://discourse.julialang.org/u/Nash)\
**Post date:** [June 30, 2020, 3:46pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/12 "2020-06-30T15:46:29Z")

</div>

I think I have found the main problem (there were others, as the advice in the thread revealed)

```
id = findall(x->x==flow[v,1], stock[:,1])

```

does not produce and integer, but an array.

Hence, when I compare

```
if stock[id,3] != flow[v,3]

```

I get an array in the first case, but an integer in the second, since v is an integer.

---

<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [June 30, 2020, 3:50pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/13 "2020-06-30T15:50:48Z")

</div>

You could use `findfirst` if you wanted to just grab the first match. That’ll return an integer.

`findall` returns an array because there might be more than one match.

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [June 30, 2020, 3:51pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/14 "2020-06-30T15:51:09Z")

</div>

You can use `findfirst` instead of `findall`.

But can you say why you have to use an array instead of a dictionary? I’m starting to feel invisible here.

---

<div class="post-metadata">

**Author:** ![Nash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nash/32/14482_2.png) [@Nash](https://discourse.julialang.org/u/Nash)\
**Post date:** [June 30, 2020, 3:52pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/15 "2020-06-30T15:52:41Z")

</div>

Sorry, DNF. It is my background in MATLAB that makes it easier for me to keep things looking like MATLAB. Not a great excuse, I know…

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [June 30, 2020, 3:54pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/16 "2020-06-30T15:54:49Z")

</div>

I am primarily a Matlab user too. Dictionaries are like Matlab’s `containers.Map`, if that helps.

---

<div class="post-metadata">

**Author:** ![Nash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nash/32/14482_2.png) [@Nash](https://discourse.julialang.org/u/Nash)\
**Post date:** [June 30, 2020, 3:58pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/17 "2020-06-30T15:58:38Z")

</div>

I must admit, I never used containers.Map before. But I will certainly examine Julia’s Dict to become familiar with it. Thank you for your advice.

---

<div class="post-metadata">

**Author:** ![lungben](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lungben/32/12314_2.png) [@lungben](https://discourse.julialang.org/u/lungben)\
**Post date:** [June 30, 2020, 6:48pm UTC](https://discourse.julialang.org/t/checking-if-two-values-are-identical/42301/18 "2020-06-30T18:48:34Z")

</div>

> [@Nash](#):
>
> `global stock = flow`

Using a global variable inside a for loop reduces performance due to type instability.  
You should put your code into a function and define stock inside the function, not global.