<\/p>\n\n\n\n
Go programs are built from small pieces of reusable behavior. Two of the most important pieces are:<\/p>\n\n\n\n
At first they look almost identical.<\/p>\n\n\n\n
A function:<\/p>\n\n\n\n
func add(a int, b int) int {\n return a + b\n}\n<\/code><\/pre>\n\n\n\nA method:<\/p>\n\n\n\n
func (c Calculator) Add(a int, b int) int {\n return a + b\n}\n<\/code><\/pre>\n\n\n\nThe important difference is this part:<\/p>\n\n\n\n
(c Calculator)\n<\/code><\/pre>\n\n\n\nIt is called the receiver<\/strong>.<\/p>\n\n\n\nA method is therefore essentially a function associated with a receiver type. In Go’s specification, a method is defined as a function with a receiver.<\/p>\n\n\n\n
\n\n\n\n1. The Big Picture<\/h1>\n\n\n\n
Think about functions like this:<\/p>\n\n\n\n
Input\n |\n v\nFunction\n |\n v\nOutput\n<\/code><\/pre>\n\n\n\nExample:<\/p>\n\n\n\n
result := add(10, 20)\n<\/code><\/pre>\n\n\n\nThe function doesn’t need to belong to anything.<\/p>\n\n\n\n
Methods are different:<\/p>\n\n\n\n
Object \/ Value\n |\n v\n +--------+\n | Method |\n +--------+\n |\n v\n Result\n<\/code><\/pre>\n\n\n\nExample:<\/p>\n\n\n\n
account.Deposit(500)\n<\/code><\/pre>\n\n\n\nDeposit<\/code> represents behavior associated with an account<\/code>.<\/p>\n\n\n\nThe easiest mental model is:<\/p>\n\n\n\n
FUNCTION\n-------\nDo something.\n\nMETHOD\n------\nAsk a particular type\/value to do something.\n<\/code><\/pre>\n\n\n\n
\n\n\n\n2. Function vs Method in One Example<\/h1>\n\n\n\n
Consider calculating a rectangle’s area.<\/p>\n\n\n\n
Using a function<\/h2>\n\n\n\nfunc area(width float64, height float64) float64 {\n return width * height\n}\n<\/code><\/pre>\n\n\n\nCall it:<\/p>\n\n\n\n
result := area(10, 5)\n<\/code><\/pre>\n\n\n\nUsing a method<\/h2>\n\n\n\ntype Rectangle struct {\n Width float64\n Height float64\n}\n\nfunc (r Rectangle) Area() float64 {\n return r.Width * r.Height\n}\n<\/code><\/pre>\n\n\n\nCall it:<\/p>\n\n\n\n
rect := Rectangle{\n Width: 10,\n Height: 5,\n}\n\nresult := rect.Area()\n<\/code><\/pre>\n\n\n\nBoth approaches work.<\/p>\n\n\n\n
But they communicate different ideas.<\/p>\n\n\n\n
The function says:<\/p>\n\n\n\n
Calculate an area using these two numbers.\n<\/code><\/pre>\n\n\n\nThe method says:<\/p>\n\n\n\n
Rectangle, calculate your area.\n<\/code><\/pre>\n\n\n\nThat distinction becomes extremely useful as programs become larger.<\/p>\n\n\n\n
\n\n\n\nPART I \u2014 GO FUNCTIONS<\/h1>\n\n\n\n3. What Is a Function?<\/h1>\n\n\n\n
A function is a reusable block of code designed to perform a particular task.<\/p>\n\n\n\n
Basic syntax:<\/p>\n\n\n\n
func functionName(parameters) returnType {\n \/\/ statements\n}\n<\/code><\/pre>\n\n\n\nExample:<\/p>\n\n\n\n
func greet() {\n fmt.Println(\"Hello!\")\n}\n<\/code><\/pre>\n\n\n\nCall it:<\/p>\n\n\n\n
greet()\n<\/code><\/pre>\n\n\n\n
\n\n\n\n4. Your First Go Function<\/h1>\n\n\n\n
Complete program:<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\nfunc greet() {\n fmt.Println(\"Hello, Go!\")\n}\n\nfunc main() {\n greet()\n}\n<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
Hello, Go!\n<\/code><\/pre>\n\n\n\nLine-by-line explanation<\/h2>\n\n\n\nLine 1<\/h3>\n\n\n\npackage main\n<\/code><\/pre>\n\n\n\nEvery Go source file belongs to a package.<\/p>\n\n\n\n
main<\/code> is special because it is used for executable programs.<\/p>\n\n\n\n
\n\n\n\nLine 2<\/h3>\n\n\n\nimport \"fmt\"\n<\/code><\/pre>\n\n\n\nImports Go’s fmt<\/code> package.<\/p>\n\n\n\nWe need it because we use:<\/p>\n\n\n\n
fmt.Println()\n<\/code><\/pre>\n\n\n\n
\n\n\n\nLine 3<\/h3>\n\n\n\nfunc greet() {\n<\/code><\/pre>\n\n\n\nfunc<\/code> tells Go that we’re declaring a function.<\/p>\n\n\n\ngreet<\/code> is the function’s name.<\/p>\n\n\n\nThe empty parentheses:<\/p>\n\n\n\n
()\n<\/code><\/pre>\n\n\n\nmean the function receives no parameters.<\/p>\n\n\n\n
{<\/code> begins the function body.<\/p>\n\n\n\n
\n\n\n\nLine 4<\/h3>\n\n\n\nfmt.Println(\"Hello, Go!\")\n<\/code><\/pre>\n\n\n\nPrints text.<\/p>\n\n\n\n
\n\n\n\nLine 5<\/h3>\n\n\n\n}\n<\/code><\/pre>\n\n\n\nEnds the greet<\/code> function.<\/p>\n\n\n\n
\n\n\n\nLine 6<\/h3>\n\n\n\nfunc main() {\n<\/code><\/pre>\n\n\n\nEvery executable Go program starts execution from the main<\/code> function in package main<\/code>.<\/p>\n\n\n\n
\n\n\n\nLine 7<\/h3>\n\n\n\ngreet()\n<\/code><\/pre>\n\n\n\nCalls our function.<\/p>\n\n\n\n
\n\n\n\n5. Function Parameters<\/h1>\n\n\n\n
Functions become much more useful when we give them input.<\/p>\n\n\n\n
Example:<\/p>\n\n\n\n
func greet(name string) {\n fmt.Println(\"Hello,\", name)\n}\n<\/code><\/pre>\n\n\n\nCall it:<\/p>\n\n\n\n
greet(\"Alice\")\n<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
Hello, Alice\n<\/code><\/pre>\n\n\n\nUnderstanding the parameter<\/h2>\n\n\n\nname string\n<\/code><\/pre>\n\n\n\nmeans:<\/p>\n\n\n\n
parameter name : name\nparameter type : string\n<\/code><\/pre>\n\n\n\nThe value:<\/p>\n\n\n\n
\"Alice\"\n<\/code><\/pre>\n\n\n\nis called an argument<\/strong> when we call the function.<\/p>\n\n\n\nTechnically:<\/p>\n\n\n\n
Parameter = variable declared by function\n\nArgument = actual value supplied when calling it\n<\/code><\/pre>\n\n\n\n
\n\n\n\n6. Multiple Parameters<\/h1>\n\n\n\nfunc add(a int, b int) {\n fmt.Println(a + b)\n}\n<\/code><\/pre>\n\n\n\nCall:<\/p>\n\n\n\n
add(10, 20)\n<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
30\n<\/code><\/pre>\n\n\n\nGo also allows parameters of the same type to be shortened:<\/p>\n\n\n\n
func add(a, b int) {\n fmt.Println(a + b)\n}\n<\/code><\/pre>\n\n\n\nBoth mean exactly the same thing.<\/p>\n\n\n\n
\n\n\n\n7. Returning a Value<\/h1>\n\n\n\n
Instead of printing the result inside a function, it is usually more reusable to return it.<\/p>\n\n\n\n
func add(a, b int) int {\n return a + b\n}\n<\/code><\/pre>\n\n\n\nCall it:<\/p>\n\n\n\n
result := add(10, 20)\n\nfmt.Println(result)\n<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
30\n<\/code><\/pre>\n\n\n\nLine-by-line<\/h2>\n\n\n\nfunc add(a, b int) int {\n<\/code><\/pre>\n\n\n\nThe first int<\/code> describes the parameters:<\/p>\n\n\n\na, b int\n<\/code><\/pre>\n\n\n\nThe final int<\/code> describes the returned value:<\/p>\n\n\n\nfunc add(a, b int) int\n ^\n return type\n<\/code><\/pre>\n\n\n\nThen:<\/p>\n\n\n\n
return a + b\n<\/code><\/pre>\n\n\n\ncalculates the value and gives it back to the caller.<\/p>\n\n\n\n
\n\n\n\n8. Why Returning Is Usually Better Than Printing<\/h1>\n\n\n\n
Compare these functions.<\/p>\n\n\n\n
Version A<\/h2>\n\n\n\nfunc add(a, b int) {\n fmt.Println(a + b)\n}\n<\/code><\/pre>\n\n\n\nVersion B<\/h2>\n\n\n\nfunc add(a, b int) int {\n return a + b\n}\n<\/code><\/pre>\n\n\n\nVersion B is much more reusable.<\/p>\n\n\n\n
You can:<\/p>\n\n\n\n
total := add(10, 20)\n<\/code><\/pre>\n\n\n\nor:<\/p>\n\n\n\n
fmt.Println(add(10, 20))\n<\/code><\/pre>\n\n\n\nor:<\/p>\n\n\n\n
finalPrice := add(10, 20) * 2\n<\/code><\/pre>\n\n\n\nor:<\/p>\n\n\n\n
if add(10, 20) > 25 {\n fmt.Println(\"Large result\")\n}\n<\/code><\/pre>\n\n\n\nA useful design principle is:<\/p>\n\n\n\n
\nFunctions that calculate values should generally return those values rather than deciding how they are displayed.<\/p>\n<\/blockquote>\n\n\n\n
\n\n\n\n9. Multiple Return Values<\/h1>\n\n\n\n
Go functions can return multiple values.<\/p>\n\n\n\n
This feature is used extensively in idiomatic Go.<\/p>\n\n\n\n
Example:<\/p>\n\n\n\n
func divide(a, b float64) (float64, bool) {\n if b == 0 {\n return 0, false\n }\n\n return a \/ b, true\n}\n<\/code><\/pre>\n\n\n\nCall:<\/p>\n\n\n\n
result, ok := divide(10, 2)\n\nfmt.Println(result)\nfmt.Println(ok)\n<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
5\ntrue\n<\/code><\/pre>\n\n\n\n
\n\n\n\n10. Multiple Returns and Errors<\/h1>\n\n\n\n
One of the most important Go patterns is:<\/p>\n\n\n\n
value, err := function()\n<\/code><\/pre>\n\n\n\nExample:<\/p>\n\n\n\n
package main\n\nimport (\n \"errors\"\n \"fmt\"\n)\n\nfunc divide(a, b float64) (float64, error) {\n if b == 0 {\n return 0, errors.New(\"cannot divide by zero\")\n }\n\n return a \/ b, nil\n}\n\nfunc main() {\n result, err := divide(10, 2)\n\n if err != nil {\n fmt.Println(\"Error:\", err)\n return\n }\n\n fmt.Println(\"Result:\", result)\n}\n<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
Result: 5\n<\/code><\/pre>\n\n\n\n
\n\n\n\n11. Understanding the Error Pattern<\/h1>\n\n\n\n
Look carefully at:<\/p>\n\n\n\n
func divide(a, b float64) (float64, error)\n<\/code><\/pre>\n\n\n\nThe function returns two things:<\/p>\n\n\n\n
\nfloat64<\/code><\/li>\n\n\n\nerror<\/code><\/li>\n<\/ol>\n\n\n\nOn failure:<\/p>\n\n\n\n
return 0, errors.New(\"cannot divide by zero\")\n<\/code><\/pre>\n\n\n\nOn success:<\/p>\n\n\n\n
return a \/ b, nil\n<\/code><\/pre>\n\n\n\nnil<\/code> means there is no error.<\/p>\n\n\n\nThe caller checks:<\/p>\n\n\n\n
if err != nil {\n<\/code><\/pre>\n\n\n\nThis pattern appears everywhere in Go.<\/p>\n\n\n\n
Examples include:<\/p>\n\n\n\n
file, err := os.Open(...)\n<\/code><\/pre>\n\n\n\ndata, err := os.ReadFile(...)\n<\/code><\/pre>\n\n\n\nvalue, err := strconv.Atoi(...)\n<\/code><\/pre>\n\n\n\n
\n\n\n\n12. Ignoring a Return Value<\/h1>\n\n\n\n
Suppose:<\/p>\n\n\n\n
value, err := divide(10, 2)\n<\/code><\/pre>\n\n\n\nbut you don’t need value<\/code>.<\/p>\n\n\n\nUse the blank identifier:<\/p>\n\n\n\n
_, err := divide(10, 2)\n<\/code><\/pre>\n\n\n\nSimilarly:<\/p>\n\n\n\n
value, _ := divide(10, 2)\n<\/code><\/pre>\n\n\n\nis syntactically possible, although ignoring errors without a reason is usually a bad practice.<\/p>\n\n\n\n
\n\n\n\n13. Named Return Values<\/h1>\n\n\n\n
Go allows returned values to have names.<\/p>\n\n\n\n
Example:<\/p>\n\n\n\n
func calculate(a, b int) (sum int, product int) {\n sum = a + b\n product = a * b\n\n return\n}\n<\/code><\/pre>\n\n\n\nCall:<\/p>\n\n\n\n
s, p := calculate(5, 3)\n<\/code><\/pre>\n\n\n\nResult:<\/p>\n\n\n\n
s = 8\np = 15\n<\/code><\/pre>\n\n\n\nBecause the results were declared as:<\/p>\n\n\n\n
(sum int, product int)\n<\/code><\/pre>\n\n\n\nthe function can use:<\/p>\n\n\n\n
return\n<\/code><\/pre>\n\n\n\nwithout explicitly listing them.<\/p>\n\n\n\n
This is called a naked return<\/strong>.<\/p>\n\n\n\nRecommendation<\/h3>\n\n\n\n
Named returns are useful when they make a function’s meaning clearer.<\/p>\n\n\n\n
For short functions:<\/p>\n\n\n\n
func dimensions() (width int, height int)\n<\/code><\/pre>\n\n\n\ncan be helpful.<\/p>\n\n\n\n
But avoid large functions full of naked returns because they become difficult to understand.<\/p>\n\n\n\n
\n\n\n\n14. Functions Are Passed Values<\/h1>\n\n\n\n
An important Go concept is:<\/p>\n\n\n\n
\nFunction arguments are passed by value.<\/p>\n<\/blockquote>\n\n\n\n
Consider:<\/p>\n\n\n\n
func changeNumber(n int) {\n n = 100\n}\n<\/code><\/pre>\n\n\n\nThen:<\/p>\n\n\n\n
number := 10\n\nchangeNumber(number)\n\nfmt.Println(number)\n<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
10\n<\/code><\/pre>\n\n\n\nWhy?<\/p>\n\n\n\n
The function receives its own copy of number<\/code>.<\/p>\n\n\n\nConceptually:<\/p>\n\n\n\n
main:\n\nnumber = 10\n\n copy\n |\n v\n\nchangeNumber:\n\nn = 10\nn = 100\n<\/code><\/pre>\n\n\n\nThe original number<\/code> remains 10<\/code>.<\/p>\n\n\n\n
\n\n\n\n15. Functions with Pointer Parameters<\/h1>\n\n\n\n
If you want a function to modify the caller’s value, you can pass a pointer.<\/p>\n\n\n\n
func changeNumber(n *int) {\n *n = 100\n}\n<\/code><\/pre>\n\n\n\nCall:<\/p>\n\n\n\n
number := 10\n\nchangeNumber(&number)\n\nfmt.Println(number)\n<\/code><\/pre>\n\n\n\nOutput:<\/p>\n\n\n\n
100\n<\/code><\/pre>\n\n\n\nImportant symbols<\/h2>\n\n\n\n&number\n<\/code><\/pre>\n\n\n\nmeans:<\/p>\n\n\n\n
\nGive me the address of number<\/code>.<\/p>\n<\/blockquote>\n\n\n\nAnd:<\/p>\n\n\n\n
*n\n<\/code><\/pre>\n\n\n\nmeans:<\/p>\n\n\n\n
\nAccess the value stored at the address contained in n<\/code>.<\/p>\n<\/blockquote>\n\n\n\nThis same concept becomes extremely important when we study pointer receiver methods.<\/p>\n\n\n\n
\n\n\n\n16. Variadic Functions<\/h1>\n\n\n\n
Sometimes you don’t know how many arguments a function will receive.<\/p>\n\n\n\n
Use:<\/p>\n\n\n\n
...\n<\/code><\/pre>\n\n\n\nExample:<\/p>\n\n\n\n
func sum(numbers ...int) int {\n total := 0\n\n for _, number := range numbers {\n total += number\n }\n\n return total\n}\n<\/code><\/pre>\n\n\n\nCalls:<\/p>\n\n\n\n
fmt.Println(sum())\nfmt.Println(sum(10))\nfmt.Println(sum(10, 20))\nfmt.Println(sum(10, 20, 30, 40))\n<\/code><\/pre>\n\n\n\nOutputs:<\/p>\n\n\n\n
0\n10\n30\n100\n<\/code><\/pre>\n\n\n\nThe specification defines a function whose final parameter uses ...<\/code> as a variadic function<\/strong>.<\/p>\n\n\n\nInside the function:<\/p>\n\n\n\n
numbers\n<\/code><\/pre>\n\n\n\nbehaves like a slice:<\/p>\n\n\n\n
[]int\n<\/code><\/pre>\n\n\n\n
\n\n\n\n17. Passing a Slice to a Variadic Function<\/h1>\n\n\n\n
Suppose:<\/p>\n\n\n\n
values := []int{10, 20, 30}\n<\/code><\/pre>\n\n\n\n