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Gepser Hoil
Asked: 2020-03-24 20:32:29 +0800 CST 2020-03-24 20:32:29 +0800 CST

"index out of range" in Go

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Driving arraysin Go I realize that there are two ways to declare them.

form 1

When I declare the array with the fixed size

var a [5]int

a[0] = 1

The above does not give any problem.

form 2

When I declare the array without the size.

var b []int

The above does not give any problem, but when I try to assign a value to it, like so:

b[0] = 1

I get this error:

panic: runtime error: index out of range
goroutine 1 [running]:
panic(0x5fb80, 0xc82000a0a0)
    /usr/local/go/src/runtime/panic.go:464 +0x3e6
main.main()
    /Users/my/path/main.go:8 +0x38
exit status 2

Why do I get that error?

You'll notice that the error message is a bit confusing since it talks about goroutines, which I'm not using yet (I don't even know what they are, I just know they exist).

go

1 Answers

  1. Best Answer

    Actually there are not two ways to declare an array in go, there is only one way, and it is the first of the two that the example describes:

    var a [5]int

    In go, the length of the array must always be part of its type, this is because this data structure will never change its size at runtime, therefore the syntax to declare an array is:

    var name [length]T

    intwhere length must be a non-negative type constant and T is the data type that it will contain. Obviously, once the array of 5 elements of type is created int, initializing one of these fields is simple, the way you did it:

    a[0] = 1

    Now in the second way you describe you are not declaring an array, because the length is missing, but a data structure called slice . You can think of it as a structure that has a pointer to an array and a length. One of several ways to declare the slice is how you do it in form two:

    var b []int

    In the statement above, you are declaring a named sliceb that will point to an array of type int. Compared to the syntax when declaring an array, if the value of is not specified length, go assumes that you are creating a slice . Now, according to your example, you just declared the variable bof type int, but now you must initialize it, otherwise the value of this variable will be nil(a data type very similar to that nullin conventional programming languages).

    To initialize the value of a slice , one possible way is to use the make(). This receives as a parameter the type of data that it wants to build, its initial length and an optional parameter that indicates the maximum value to which it can expand the capacity of the slice , as you will notice, this structure does allow you to expand, or decrease, dynamically the length of the array. Therefore, your form 2 is missing the statement:

    //Construyo un slice de longitud inicial 5 y longitud máxima 10.
b = make([]int, 5, 10)

    And now yes, you can assign values ​​to the slice

    b[0] = 1

    The error displayed when executing the code is somewhat obscure, because in reality there will always be an array behind a slice , whether you create it with the function make()or build it from an array you have already created (such as the array ain your code). In any case, according to its form 2, the initial length of the slice is nil, and when referencing object 0 of this slice, it will generate index out of rangean error and not an exception-like error like NullPointerExceptionin Java, because it actually niljust references the value of variables that are not initialized.

    On the relationship with goroutine

    A goroutine is a function that is executed concurrently with the main thread. If you have programmed in Java you can think of them as similar to the concept of Threads ., if not, think about this: your main program (the main execution thread) is your main, or the one you execute when you run the program that contains the code you need. This program can "delegate" tasks to run concurrently (at the same time) as the main program. In go, you accomplish this through a goroutine. By your post you indicate that you have no experience in this topic, so I invite you to search more about this beautiful feature of go. Regarding your question, when the program finds that it is trying to access an array element that does not exist, it throws an 'index out of range' error. In go, this error generates a call to the panic function, which aborts the execution of the program.

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