Why does my Go code throw a \'panic: runtime error: index out of range\' error even when the array length isn\'t null?

Go "panic: runtime error: index out of range" When Array Length Isn't Null

Understanding Array Length and Capacity

Arrays in Go store a fixed number of values and have a specified length. The length of an array determines the number of elements it contains. However, Go slices, which are an abstraction of arrays, introduce another concept: capacity. Capacity represents the maximum number of elements a slice can hold.

When creating a slice using make([]T, len, cap), where T is the element type, you're specifying the length and capacity. If the capacity is greater than the length, the slice can expand to accommodate additional elements without reallocating memory. If the capacity is zero, it uses the length as the capacity.

Resolving the Error

In the provided code snippet, the make function is used to create a []string slice with a length of 0 and a capacity of 4:

<code class="go">result := make([]string, 0, 4)</code>

This means that the slice can initially hold 0 strings and can expand to hold up to 4 without reallocating memory.

The error "panic: runtime error: index out of range" occurs because accessing an element at an index beyond the slice's length is invalid. In the original code snippet, the loop tries to access elements starting from result[0], but there are no elements at that index since the slice is empty.

Adding Elements to the Slice

To resolve the error, we need to append elements to the slice using the append function. The append function returns a new slice with the added elements:

<code class="go">result = append(result, "foo")</code>

Each time we append an element, the slice's length increases, and if the capacity is reached, a new underlying array is allocated with a larger capacity. In this case, the first append increases the length to 1, and since the capacity is 4, no reallocation is necessary.

Simplified Code Example

Here's a simplified example that uses append to add elements to the slice without encountering the error:

<code class="go">package main

import "fmt"

func main() {
    result := make([]string, 0, 4)

    // Append elements to the slice
    result = append(result, "foo")
    result = append(result, "bar")

    // Access the elements using valid indices
    fmt.Println(result[0]) // Output: foo
    fmt.Println(result[1]) // Output: bar
}</code>

In this example, the make function creates a slice with an initial length of 0 and a capacity of 4. Then, we use append to add two elements to the slice. Finally, we can access the elements using valid indices without encountering the "index out of range" error.

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