CODEWITHSUNDEEP
vectorjulia
DkRckr12
DkRckr12

Reputation: 183

Why is Julia not taking the correct data in the function?

I started learning how to program in Julia, and I'm making a pretty simple code, but it's not working as I wish, and I'm lost because I can't find where's the error.

Basically, I have a vector like this one: (1,0,0,1,1) and I made two functions that will change the entries of the vector.

The first function needs to change every entry of the vector for 1.

The second function needs to change every entry as follows: if the entry is 1, then change it for 0, and vice versa.

I have the next code:

function vectorMethodOne(vector1)
    for i = 1:length(vector1)
        if vector1[i] == 0
           vector1[i] = 1
        end
    end
    return vector1
end

function vectorMethodTwo(vector1)
    for i = 1:length(vector1)
        if vector1[i] == 0
           vector1[i] = 1
        elseif vector1[i] == 1
           vector1[i] = 0
        end
    end
    return vector1
end

The problem happens when I run the code like this:

vectorEx = rand(0:1, 5)
println("Original Vector:")
println(string(vectorEx))
println("Vector using method 1:")
vectorM1 = vectorMethodOne(vectorEx)
println(string(vectorM1))
println("Vector using method 2:")
vectorM2 = vectorMethodTwo(vectorEx)
println(string(vectorM2))

The output looks like this:

> Original Vector:
> [1,0,0,1,1]
> Vector using method 1:
> [1,1,1,1,1]
> Vector using method 2:
> [0,0,0,0,0]

But I want that the output looks like this:

> Original Vector:
> [1,0,0,1,1]
> Vector using method 1:
> [1,1,1,1,1]
> Vector using method 2:
> [0,1,1,0,0]

If I only run the vectorMethodTwo, it works like I want, like this:

vectorEx = rand(0:1, 5)
println("Original Vector:")
println(string(vectorEx))
println("Vector using method 2:")
vectorM2 = vectorMethodTwo(vectorEx)
println(string(vectorM2))

And the output looks like this:

> Original Vector:
> [1,0,0,1,1]
> Vector using method 2:
> [0,1,1,0,0]

But I want that every function run over the original vector (1,0,0,1,1) but the vectorMethodTwo is running over the modified vector (1,1,1,1,1) and I can't understand where's the error in my code.

Upvotes: 3

Views: 116

Answers (1)

Mateen Ulhaq
Mateen Ulhaq

Reputation: 27201

Let's look at your output:

> Original Vector:
> [1,0,0,1,1]
> Vector using method 1:
> [1,1,1,1,1]
> Vector using method 2:
> [0,0,0,0,0]

Odd. Method 2 looks like method 1 flipped. Let's check:

println(vectorMethodTwo([1,1,1,1,1]))

> [0,0,0,0,0]

Very suspicious! Why could this be happening? Please think about this before moving to the next section.

Your "functions" are mutating the vector. When you do this in vectorMethodOne,

vector1[i] = 1

, you are changing the contents of vector1 that was passed in. That vector1 refers to the same memory as vectorEx.

Do not write code that mutates your inputs (unless you name the function accordingly). Either create a copy of your vector before mutating it, or try a list comprehension:

function vectorMethodOne(vector1)
    return [x == 0 ? 1 : x for x in vector1]
end

function vectorMethodTwo(vector1)
    return [x == 0 ? 1 : x == 1 ? 0 : x for x in vector1]
end

These do not modify the contents of the input vector1 in any way.

Upvotes: 4

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