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Diffstat (limited to 'doc/go_tutorial.txt')
| -rw-r--r-- | doc/go_tutorial.txt | 77 |
1 files changed, 44 insertions, 33 deletions
diff --git a/doc/go_tutorial.txt b/doc/go_tutorial.txt index 6ab6094c0..9c08bd278 100644 --- a/doc/go_tutorial.txt +++ b/doc/go_tutorial.txt @@ -227,14 +227,15 @@ In Go, since arrays are values, it's meaningful (and useful) to talk about pointers to arrays. The size of the array is part of its type; however, one can declare -a <i>slice</i> variable, to which one can assign a pointer to -any array -with the same element type or—much more commonly—a <i>slice -expression</i> of the form "a[low : high]", representing -the subarray indexed by "low" through "high-1". -Slices look a lot like arrays but have +a <i>slice</i> variable to hold a reference to any array, of any size, +with the same element type. +A <i>slice +expression</i> has the form "a[low : high]", representing +the internal array indexed from "low" through "high-1"; the resulting +slice is indexed from "0" through "high-low-1". +In short, slices look a lot like arrays but with no explicit size ("[]" vs. "[10]") and they reference a segment of -an underlying, often anonymous, regular array. Multiple slices +an underlying, usually anonymous, regular array. Multiple slices can share data if they represent pieces of the same array; multiple arrays can never share data. @@ -243,39 +244,43 @@ regular arrays; they're more flexible, have reference semantics, and are efficient. What they lack is the precise control of storage layout of a regular array; if you want to have a hundred elements of an array stored within your structure, you should use a regular -array. +array. To create one, use a compound value <i>constructor</i>—an +expression formed +from a type followed by a brace-bounded expression like this: + + [3]int{1,2,3} + +In this case the constructor builds an array of 3 "ints". When passing an array to a function, you almost always want to declare the formal parameter to be a slice. When you call -the function, take the address of the array and Go will -create (efficiently) a slice reference and pass that. +the function, slice the array to create +(efficiently) a slice reference and pass that. +By default, the lower and upper bounds of a slice match the +ends of the existing object, so the concise notation "[:]" +will slice the whole array. Using slices one can write this function (from "sum.go"): --PROG progs/sum.go /sum/ /^}/ -and invoke it like this: - ---PROG progs/sum.go /1,2,3/ - Note how the return type ("int") is defined for "sum()" by stating it after the parameter list. -The expression "[3]int{1,2,3}"—a type followed by a -brace-bounded -expression—is a constructor for a value, in this case an array -of 3 "ints". -Putting an "&" -in front gives us the address of a unique instance of the value. We pass the -pointer to "sum()" by (implicitly) promoting it to a slice. + +To call the function, we slice the array. This intricate call (we'll show +a simpler way in a moment) constructs +an array and slices it: + + s := sum([3]int{1,2,3}[:]) If you are creating a regular array but want the compiler to count the elements for you, use "..." as the array size: - s := sum(&[...]int{1,2,3}) + s := sum([...]int{1,2,3}[:]) -In practice, though, unless you're meticulous about storage layout within a -data structure, a slice itself—using empty brackets and no -"&"—is all you need: +That's fussier than necessary, though. +In practice, unless you're meticulous about storage layout within a +data structure, a slice itself—using empty brackets with no size—is all you need: s := sum([]int{1,2,3}) @@ -415,7 +420,7 @@ object. We could write n.name = name return n -but for simple structures like "File" it's easier to return the address of a nonce +but for simple structures like "File" it's easier to return the address of a composite literal, as is done here on line 21. We can use the factory to construct some familiar, exported variables of type "*File": @@ -471,15 +476,21 @@ We can now use our new package: --PROG progs/helloworld3.go /package/ END -The ''"./"'' in the import of ''"./file"'' tells the compiler to use our own package rather than +The ''"./"'' in the import of ''"./file"'' tells the compiler +to use our own package rather than something from the directory of installed packages. +(Also, ''"file.go"'' must be compiled before we can import the +package.) -Finally we can run the program: +Now we can compile and run the program: - % helloworld3 + $ 6g file.go # compile file package + $ 6g helloworld3.go # compile main package + $ 6l -o helloworld3 helloworld3.6 # link - no need to mention "file" + $ helloworld3 hello, world can't open file; err=No such file or directory - % + $ Rotting cats ---- @@ -549,11 +560,11 @@ even though under the covers it holds a pointer to a "struct". Here it is in action: <pre> - % echo abcdefghijklmnopqrstuvwxyz | ./cat + $ echo abcdefghijklmnopqrstuvwxyz | ./cat abcdefghijklmnopqrstuvwxyz - % echo abcdefghijklmnopqrstuvwxyz | ./cat --rot13 + $ echo abcdefghijklmnopqrstuvwxyz | ./cat --rot13 nopqrstuvwxyzabcdefghijklm - % + $ </pre> Fans of dependency injection may take cheer from how easily interfaces |