good good study, day day up!

vendor/gopl.io/ch4/append/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 88.
+
+// Append illustrates the behavior of the built-in append function.
+package main
+
+import "fmt"
+
+func appendslice(x []int, y ...int) []int {
+	var z []int
+	zlen := len(x) + len(y)
+	if zlen <= cap(x) {
+		// There is room to expand the slice.
+		z = x[:zlen]
+	} else {
+		// There is insufficient space.
+		// Grow by doubling, for amortized linear complexity.
+		zcap := zlen
+		if zcap < 2*len(x) {
+			zcap = 2 * len(x)
+		}
+		z = make([]int, zlen, zcap)
+		copy(z, x)
+	}
+	copy(z[len(x):], y)
+	return z
+}
+
+//!+append
+func appendInt(x []int, y int) []int {
+	var z []int
+	zlen := len(x) + 1
+	if zlen <= cap(x) {
+		// There is room to grow.  Extend the slice.
+		z = x[:zlen]
+	} else {
+		// There is insufficient space.  Allocate a new array.
+		// Grow by doubling, for amortized linear complexity.
+		zcap := zlen
+		if zcap < 2*len(x) {
+			zcap = 2 * len(x)
+		}
+		z = make([]int, zlen, zcap)
+		copy(z, x) // a built-in function; see text
+	}
+	z[len(x)] = y
+	return z
+}
+
+//!-append
+
+//!+growth
+func main() {
+	var x, y []int
+	for i := 0; i < 10; i++ {
+		y = appendInt(x, i)
+		fmt.Printf("%d  cap=%d\t%v\n", i, cap(y), y)
+		x = y
+	}
+}
+
+//!-growth
+
+/*
+//!+output
+0  cap=1   [0]
+1  cap=2   [0 1]
+2  cap=4   [0 1 2]
+3  cap=4   [0 1 2 3]
+4  cap=8   [0 1 2 3 4]
+5  cap=8   [0 1 2 3 4 5]
+6  cap=8   [0 1 2 3 4 5 6]
+7  cap=8   [0 1 2 3 4 5 6 7]
+8  cap=16  [0 1 2 3 4 5 6 7 8]
+9  cap=16  [0 1 2 3 4 5 6 7 8 9]
+//!-output
+*/