# Confusion with arrays and functions

**URL:** <https://discourse.julialang.org/t/confusion-with-arrays-and-functions/39371>\
**Category:** New to Julia\
**Tags:** arrays\
**Created:** [May 12, 2020, 9:58pm UTC](https://discourse.julialang.org/t/confusion-with-arrays-and-functions/39371 "2020-05-12T21:58:14Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![SambitMishra98](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sambitmishra98/32/14263_2.png) [@SambitMishra98](https://discourse.julialang.org/u/SambitMishra98)\
**Post date:** [May 12, 2020, 9:58pm UTC](https://discourse.julialang.org/t/confusion-with-arrays-and-functions/39371/1 "2020-05-12T21:58:14Z")

</div>

I was using a function inside a loop to modify a 2-Dimensional array variable (say `DATA`) in my code when I realised that the variable changes even when I have not given `DATA` as an argument or tried to return it to the main body of the program. Instead of the code throwing an error (that `DATA` is not defined in the function), I got the output I wanted from the code. Consequently, I tried out more possibilities and got even more confused. Each of the possibilities I tried where I expected some error to pop up, I did not get any error and instead got the expected answer.

Why did the compiler not give an error when I did not mention all the arguments?  
How does the compiler know what to return at the end of the function?  
How do the returned values get stored in the intended location?  
What is a good programming practice when modifying an array using a function?

**[CODE]**

```julia
Data1 = 1000 * ones(10);
Data2 = 2000 * ones(10);
Data3 = 3000 * ones(10);
Data4 = 4000 * ones(10);
Data5 = 5000 * ones(10);
Data6 = 6000 * ones(10);
Data7 = 7000 * ones(10);
Data8 = 8000 * ones(10);

function ChangeData1(i) Data1[i] = Data1[i] + i; end;
function ChangeData2(i) Data2[i] = i; end;
function ChangeData3(Data3,i) Data3[i] = Data3[i] + i; end;
function ChangeData4(Data4,i) Data4[i] = i; end;
function ChangeData5(Data5,i) Data5[i] = Data5[i] + i; return Data5 end;
function ChangeData6(Data6,i) Data6[i] = i; return Data6 end;
function ChangeData7(i) Data7[i] = Data7[i] + i; return Data7 end;
function ChangeData8(i) Data8[i] = i; return Data8 end;

for i in 1:10
    ChangeData1( i);
    ChangeData2( i);
    ChangeData3(Data3,i);
    ChangeData4(Data4,i);
    ChangeData5(Data5,i);
    ChangeData6(Data6,i);    
    ChangeData7( i);
    ChangeData8( i);    
end

println(Data1')
println(Data2')
println(Data3')
println(Data4')
println(Data5')
println(Data6')
println(Data7')
println(Data8')

```

**[OUTPUT]**

```julia
[1001.0 1002.0 1003.0 1004.0 1005.0 1006.0 1007.0 1008.0 1009.0 1010.0]
[1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0]
[3001.0 3002.0 3003.0 3004.0 3005.0 3006.0 3007.0 3008.0 3009.0 3010.0]
[1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0]
[5001.0 5002.0 5003.0 5004.0 5005.0 5006.0 5007.0 5008.0 5009.0 5010.0]
[1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0]
[7001.0 7002.0 7003.0 7004.0 7005.0 7006.0 7007.0 7008.0 7009.0 7010.0]
[1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0]

```

**[EDIT]**  
I realised that if I use variable names in function different from those used in main code, some functions throw errors. So the following are more observations:

**[CODE]**

```julia
Data1 = 1000 * ones(10);
Data2 = 2000 * ones(10);
Data3 = 3000 * ones(10);
Data4 = 4000 * ones(10);
Data5 = 5000 * ones(10);
Data6 = 6000 * ones(10);

function ChangeData1(Data,j) Data[j] = Data[j] + j; return Data end;
function ChangeData2(Data,j) Data[j] = j; return Data end;
function ChangeData3(Data,j) Data[j] = Data[j] + j; return Data end;
function ChangeData4(Data,j) Data[j] = j; return Data end;
function ChangeData5(Data,j) Data[j] = Data[j] + j; end;
function ChangeData6(Data,j) Data[j] = j; end;

for i in 1:10
    Data1 = ChangeData3(Data1,i);
    Data2 = ChangeData4(Data2,i);
    ChangeData3(Data3,i);
    ChangeData4(Data4,i);
    ChangeData5(Data5,i);
    ChangeData6(Data6,i);    
end

println(Data1')
println(Data2')
println(Data3')
println(Data4')
println(Data5')
println(Data6')

```

**[OUTPUT]**

```julia
[1001.0 1002.0 1003.0 1004.0 1005.0 1006.0 1007.0 1008.0 1009.0 1010.0]
[1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0]
[3001.0 3002.0 3003.0 3004.0 3005.0 3006.0 3007.0 3008.0 3009.0 3010.0]
[1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0]
[5001.0 5002.0 5003.0 5004.0 5005.0 5006.0 5007.0 5008.0 5009.0 5010.0]
[1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0]

```

---

<div class="post-metadata">

**Author:** ![raminammour](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/raminammour/32/13572_2.png) [@raminammour](https://discourse.julialang.org/u/raminammour)\
**Post date:** [May 12, 2020, 10:21pm UTC](https://discourse.julialang.org/t/confusion-with-arrays-and-functions/39371/2 "2020-05-12T22:21:33Z")

</div>

Hello,

You may find this page helpful: [Scope of Variables · The Julia Language](https://docs.julialang.org/en/v1/manual/variables-and-scoping/).

The compiler does not error because the variables (data1-8) are in the global scope (`Main.Data1`,…) even when you don’t pass them as argument.

The return value of a function is the return value of the last expression by default. Unless you have an explicit `return ...`.

While `ChangeData1(i)` and `ChangeData3(Data1 #not 3,i)` will mutate `Data1` in the same way. When `Data1` is an argument, the j[ulia compiler produces more efficient code](https://docs.julialang.org/en/v1/manual/performance-tips/). So a “better practice” is passing in an argument. However, it is not _needed_, and may not matter for non performance critical code.

Hope this helps.

---

<div class="post-metadata">

**Author:** ![SambitMishra98](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sambitmishra98/32/14263_2.png) [@SambitMishra98](https://discourse.julialang.org/u/SambitMishra98)\
**Post date:** [May 13, 2020, 6:13pm UTC](https://discourse.julialang.org/t/confusion-with-arrays-and-functions/39371/3 "2020-05-13T18:13:12Z")

</div>

Thanks a lot for clarifying it! The links were helpful.