# Channels and Conditions: How to do notify channel is ready

**URL:** <https://discourse.julialang.org/t/channels-and-conditions-how-to-do-notify-channel-is-ready/36904>\
**Category:** New to Julia\
**Tags:** question, task\
**Created:** [April 2, 2020, 1:52pm UTC](https://discourse.julialang.org/t/channels-and-conditions-how-to-do-notify-channel-is-ready/36904 "2020-04-02T13:52:13Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![prototact](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/prototact/32/13847_2.png) [@prototact](https://discourse.julialang.org/u/prototact)\
**Post date:** [April 2, 2020, 1:52pm UTC](https://discourse.julialang.org/t/channels-and-conditions-how-to-do-notify-channel-is-ready/36904/1 "2020-04-02T13:52:14Z")

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Hi all,

I am doing an exercise with Julia and the Task related primitives.

It is a function that adds an array of integers from the left. There is also one symmetrical adding from the right.

In the main function i schedule both left and right, which should switch whenever the sum from one side exceeds the other, or the two meet before the sums become equal.

I use `notify` and wait on a condition to restart each task, basically doing

- if left is smaller, sum left, put, notify and wait, take and repeat
- if right etc.

The issue is that `put!` and `notify` seem to be executing independently of each other in sumleft.  
The print statement between them gives enough time for `put!` to finish, but without it whole program blocks. How does` notify` work? The main method also waits for the `put!` operation to finish and then resumes the loop (either left or right depending on which is larger) by taking from a channel.

```julia
channel1 = Channel{Tuple{Int,Int}}(3)
channel2 = Channel{Tuple{Int,Int}}(3)
array = [1,2,3,4,1,1,9]
cond1 = Condition()
cond2 = Condition()
cond3 = Condition()
cond4 = Condition()

function sumleft()
    accleft = 0
    for (i, elem) in enumerate(array)
        accleft += elem
        put!(channel1, (accleft, i))
        println("notifying left")
        notify(cond3)
        println("waiting for left...")
        wait(cond1)
    end
    accleft
end

function sumright()
    accright = 0
    for (j, elem) in enumerate(reverse(array))
        accright += elem
        put!(channel2, (accright, j))
        println("notifying right")
        notify(cond4)
        println("waiting for right...")
        wait(cond2)
    end
    accright
end

function main()
    a = Task(sumleft)
    b = Task(sumright)
    schedule(a)
    println("scheduled a")
    println("a status: ", istaskstarted(a))
    schedule(b)
    println("scheduled b")
    println("b status: ", istaskstarted(b))
    (accleft, accright) = 0, -1 
    (i ,j) = 0, 0
    while accleft != accright && i < length(array) - j
        println("notifying a and b")
        if accleft > accright
            println("take from channel right")
            notify(cond2)
            wait(cond4) 
            (accright, j) = take!(channel2)
        elseif accright > accleft
            println("take from channel left")
            notify(cond1)
            wait(cond3)
            (accleft, i) = take!(channel1)
        end
        println("left channel: ", accleft, ' ', i)
        println("right channel: ", accright, ' ', j)
    end
    [(accleft, i),(accright,j)]
end

```

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<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:** [April 2, 2020, 4:14pm UTC](https://discourse.julialang.org/t/channels-and-conditions-how-to-do-notify-channel-is-ready/36904/2 "2020-04-02T16:14:44Z")

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Please post code as text instead of a screenshot. Also, please post enough code so that people here can run it. [Please read: make it easier to help you](https://discourse.julialang.org/t/psa-make-it-easier-to-help-you/14757)