Why Does \"panic: runtime error: index out of range\" Occur When a Go Slice Has a Non-Null Length?

Panic: Runtime Error: Index Out of Range When Array Length Is Not Null

The provided code snippet attempts to iterate through a string to separate words containing vowels. However, it encounters a "panic: runtime error: index out of range" error, despite the array length not being null.

To understand this error, it's essential to delve into the underlying slice data structure.

Understanding Slices

A slice in Go is a descriptor of an array segment. It comprises three components: a pointer to the actual array, the segment's length, and its capacity (the maximum length the segment can accommodate).

When creating a slice using make([]string, 0, 4), an empty underlying array is allocated, with an initial length of 0 and a capacity of 4.

Making a Distinction

result := make([]string, 0, 4) differs from result := make([]string, 4) in the following way:

• make([]string, 0, 4): Initializes an empty array, meaning accessing result[0] will cause a panic.
• make([]string, 4): Initializes an array with 4 elements, allowing access to result[0] through result[3].

Understanding append

The append function adds elements to a slice's end. If the slice has enough capacity, it reslices the destination to accommodate the new elements. Otherwise, a new underlying array is allocated.

Applying to the Code

In the provided code, the line result := make([]string, 0, 4) creates an empty slice. Attempting to access result[0] within the loop will result in a panic.

To rectify this issue, use append to populate the slice, like in the following code snippet:

<code class="go">for i := 0; i < len(strs); i++ {
    if strings.ContainsAny(strs[i], "eaiu") {
        result = append(result, strs[i])
    }
}</code>

Alternatively, you can simplify the code using a range-based loop:

<code class="go">for _, s := range strs {
    if strings.ContainsAny(s, "eaiu") {
        result = append(result, s)
    }
}</code>

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