# Why are two functions behave differently?

**URL:** <https://discourse.julialang.org/t/why-are-two-functions-behave-differently/93848>\
**Category:** Performance\
**Tags:** question\
**Created:** [February 1, 2023, 4:07am UTC](https://discourse.julialang.org/t/why-are-two-functions-behave-differently/93848 "2023-02-01T04:07:01Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![hungpham3112](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hungpham3112/32/34315_2.png) [@hungpham3112](https://discourse.julialang.org/u/hungpham3112)\
**Post date:** [February 1, 2023, 4:07am UTC](https://discourse.julialang.org/t/why-are-two-functions-behave-differently/93848/1 "2023-02-01T04:07:01Z")

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I’m doing some exercises in the [ThinkJulia](https://benlauwens.github.io/ThinkJulia.jl/latest/book.html#_exercises_12) book.  
Specifically, I try to solve the exercise 10-10:

> Write a function that reads the file `words.txt` and builds an array with one element per word. Write two versions of this function, one using `push!` and the other using the idiom `t = [t..., x]`. Which one takes longer to run? Why?

Here is my code:

```julia
function version1(filename::String)
    array = []
    open(filename) do file
        for line in readlines(file)
            push!(array, line)
        end
    end
    array
end

function version2(filename::String)
    array = []
    open(filename) do file
        for line in readlines(file)
            array = [array..., line]
        end
    end
    array
end

@time println(version1(joinpath("..", "words.txt")))
@time println(version2(joinpath("..", "words.txt")))

```

When running two function, I observed that `version1` prints the array after running function while `version2` hangs for a while and then print the array.

```julia
version 1:
3.709137 seconds (4.47 M allocations: 115.453 MiB, 0.39% gc time, 2.21% compilation time)
version 2:
77.762709 seconds (5.06 M allocations: 96.644 GiB, 1.11% gc time, 0.14% compilation time)

```

What actually runs under 2 function? Why did we see the big gap between them? Thank you.

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<div class="post-metadata">

**Author:** ![mrufsvold](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mrufsvold/32/31600_2.png) [@mrufsvold](https://discourse.julialang.org/u/mrufsvold)\
**Post date:** [February 1, 2023, 5:37am UTC](https://discourse.julialang.org/t/why-are-two-functions-behave-differently/93848/2 "2023-02-01T05:37:20Z")

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Your first function allocates an array and then asks for that array to be given one more element of space over and over.

Your second function allocates a whole new array each time that is one element bigger than the last and then copies all the old words into the new array, adding one new word to the end.

Allocation and copying data are both very expensive operations, so the second one takes a lot longer!

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**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:** [February 1, 2023, 5:53am UTC](https://discourse.julialang.org/t/why-are-two-functions-behave-differently/93848/3 "2023-02-01T05:53:30Z")

</div>

A few more things:

1. Splatting of dynamically sized containers is very slow.
2. The used vector is untyped: `array = []`. This compounds the problem. Use a typed container: `array = String[]`.
3. The function does not solve the exercise correctly. It should return a vector with one _word_ per element, not one _line_ per element.