Slices

package main

import "fmt"

func main() {
    // 1) Create
    nums := []int{1, 2, 3, 4, 5}        // literal
    var empty []int                     // nil slice — len=0, cap=0
    pre := make([]int, 5)               // [0,0,0,0,0]
    cap10 := make([]int, 0, 10)         // empty, capacity 10

    fmt.Println(len(nums), cap(nums))

    // 2) Append — re-assign the return value
    nums = append(nums, 6, 7, 8)
    fmt.Println(nums, len(nums), cap(nums))

    // 3) Slice expression — VIEW into the same backing array
    sub := nums[1:4]                    // [2, 3, 4]
    sub[0] = 99                         // mutates nums too!
    fmt.Println(nums)

    // 4) Three-index slice — control capacity, prevent aliasing surprise
    safe := nums[1:4:4]                 // len=3, cap=3 — append allocates new array
    safe = append(safe, 100)
    fmt.Println(nums, safe)             // nums unchanged

    // 5) Copy — independent slice
    dst := make([]int, len(nums))
    copy(dst, nums)
    dst[0] = -1
    fmt.Println(nums, dst)

    // 6) Delete an index (no built-in — use append)
    nums = append(nums[:2], nums[3:]...)

    // 7) Insert at index
    nums = append(nums[:2], append([]int{77}, nums[2:]...)...)

    // 8) Pre-allocate when you know the size — avoids reallocations
    out := make([]int, 0, len(input))
    for _, x := range input {
        if x > 0 { out = append(out, x) }
    }

    // 9) Range — i is index, v is COPY of the element
    for i, v := range nums {
        v *= 2                          // doesn't modify nums[i]
        nums[i] = v                     // this does
    }

    // 10) nil vs empty slice
    var n []int
    e := []int{}
    fmt.Println(n == nil, e == nil)     // true, false
    fmt.Println(len(n), len(e))         // 0, 0   — both safe to range over
}

nums := []int{1, 2, 3}
nums = append(nums, 4)
fmt.Println(nums, len(nums), cap(nums))