How Slice Internals Work

Go slices are dynamic views of underlying arrays where len is the current element count and cap is the total allocated array size. The append function adds elements to the slice, automatically allocating a new, larger underlying array and copying data if the current capacity is exceeded.

s := make([]int, 0, 5) // len=0, cap=5
s = append(s, 1)       // len=1, cap=5
s = append(s, 2, 3)    // len=3, cap=5
s = append(s, 4, 5, 6) // len=6, cap=10 (new array allocated)

Think of a slice as a window looking at a row of boxes (the array). The length is how many boxes you are currently using, while the capacity is the total number of boxes available in that row. When you fill up your current row, the system automatically builds a bigger row, moves your boxes over, and gives you a new window to keep adding more.