Why am I getting \"panic: runtime error: index out of range\" even though my Go slice has a non-zero capacity?

Troubleshooting "panic: runtime error: index out of range" When Array Length Is Not Null

In Go, when working with arrays or slices, it's crucial to understand the concept of length and capacity. In the given code snippet, the issue stems from initializing the result array using make([]string, 0, 4).

Understanding the Role of make([]string, 0, 4)

make([]string, 0, 4) creates a slice of type []string with an initial length of 0 and a capacity of 4. Capacity defines the maximum number of elements that can be stored in the slice without reallocating memory.

Distinguishing Between Length and Capacity

Length indicates the number of elements currently present in the slice, while capacity represents the maximum size it can hold. If the slice already holds enough elements, adding more items within the capacity will not cause a reallocation. However, exceeding the capacity will trigger a reallocation, which involves creating a new underlying array and copying the existing elements.

In your code, make([]string, 0, 4) initializes an empty slice with a capacity of 4. Attempting to access elements beyond the current length (index 0) will result in a runtime error because there are no elements at that index.

Resolving the Issue

The error can be resolved by either initializing the slice with a sufficient initial length or using append to dynamically grow the slice as needed.

Option 1: Initialize with a Sufficient Length

If you know the approximate number of elements that will be stored in the slice, you can initialize it with an appropriate length. For example, result := make([]string, 4) initializes a slice with a length and capacity of 4.

Option 2: Use Append

Append appends elements to the end of a slice. If the slice has sufficient capacity, it is resliced to accommodate the new elements. If not, a new underlying array is allocated. By using append repeatedly, you can gradually grow the slice as needed without worrying about exceeding the capacity.

Simplified Code Using Append

Here's a simplified version of your code using append:

<code class="go">package main

import (
    "fmt"
    "strings"
)

func main() {
    strs := strings.Fields("Political srt")
    var result []string
    for _, s := range strs {
        if strings.ContainsAny(s, "eaiu") {
            result = append(result, s)
        }
    }
    fmt.Println(result)
}</code>

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