I come from a Python/Ruby/JavaScript background. I understand how pointers work, however, I'm not completely sure how to leverage them in the following situation.
Let's pretend we have a fictitious web API that searches some image database and returns a JSON describing what's displayed in each image that was found:
[
{
"url": "https://c8.staticflickr.com/4/3707/11603200203_87810ddb43_o.jpg",
"description": "Ocean islands",
"tags": [
{"name":"ocean", "rank":1},
{"name":"water", "rank":2},
{"name":"blue", "rank":3},
{"name":"forest", "rank":4}
]
},
...
{
"url": "https://c3.staticflickr.com/1/48/164626048_edeca27ed7_o.jpg",
"description": "Bridge over river",
"tags": [
{"name":"bridge", "rank":1},
{"name":"river", "rank":2},
{"name":"water", "rank":3},
{"name":"forest", "rank":4}
]
}
]
My goal is to create a data structure in Go that will map each tag to a list of image URLs that would look like this:
{
"ocean": [
"https://c8.staticflickr.com/4/3707/11603200203_87810ddb43_o.jpg"
],
"water": [
"https://c8.staticflickr.com/4/3707/11603200203_87810ddb43_o.jpg",
"https://c3.staticflickr.com/1/48/164626048_edeca27ed7_o.jpg"
],
"blue": [
"https://c8.staticflickr.com/4/3707/11603200203_87810ddb43_o.jpg"
],
"forest":[
"https://c8.staticflickr.com/4/3707/11603200203_87810ddb43_o.jpg",
"https://c3.staticflickr.com/1/48/164626048_edeca27ed7_o.jpg"
],
"bridge": [
"https://c3.staticflickr.com/1/48/164626048_edeca27ed7_o.jpg"
],
"river":[
"https://c3.staticflickr.com/1/48/164626048_edeca27ed7_o.jpg"
]
}
As you can see, each image URL can belong to multiple tags at the same time. If I have thousands of images and even more tags, this data structure can grow very large if image URL strings are copied by value for each tag. This is where I want to leverage pointers.
I can represent the JSON API response by two structs in Go, func searchImages()
mimics the fake API:
package main
import "fmt"
type Image struct {
URL string
Description string
Tags []*Tag
}
type Tag struct {
Name string
Rank int
}
// this function mimics json.NewDecoder(resp.Body).Decode(&parsedJSON)
func searchImages() []*Image {
parsedJSON := []*Image{
&Image {
URL: "https://c8.staticflickr.com/4/3707/11603200203_87810ddb43_o.jpg",
Description: "Ocean islands",
Tags: []*Tag{
&Tag{"ocean", 1},
&Tag{"water", 2},
&Tag{"blue", 3},
&Tag{"forest", 4},
},
},
&Image {
URL: "https://c3.staticflickr.com/1/48/164626048_edeca27ed7_o.jpg",
Description: "Bridge over river",
Tags: []*Tag{
&Tag{"bridge", 1},
&Tag{"river", 2},
&Tag{"water", 3},
&Tag{"forest", 4},
},
},
}
return parsedJSON
}
Now the less optimal mapping function that results in a very large in-memory data structure can look like this:
func main() {
result := searchImages()
tagToUrlMap := make(map[string][]string)
for _, image := range result {
for _, tag := range image.Tags {
// fmt.Println(image.URL, tag.Name)
tagToUrlMap[tag.Name] = append(tagToUrlMap[tag.Name], image.URL)
}
}
fmt.Println(tagToUrlMap)
}
I can modify it to use pointers to the Image
struct URL
field instead of copying it by value:
// Version 1
tagToUrlMap := make(map[string][]*string)
for _, image := range result {
for _, tag := range image.Tags {
// fmt.Println(image.URL, tag.Name)
tagToUrlMap[tag.Name] = append(tagToUrlMap[tag.Name], &image.URL)
}
}
It works and my first question is what happens to the result
data structure after I build the mapping in this way? Will the Image
URL
string fields be left in memory somehow and the rest of the result
will be garbage collected? Or will the result
data structure stay in memory until the end of the program because something points to its members?
Another way to do this would be to copy the URL to an intermediate variable and use a pointer to it instead:
// Version 2
tagToUrlMap := make(map[string][]*string)
for _, image := range result {
imageUrl = image.URL
for _, tag := range image.Tags {
// fmt.Println(image.URL, tag.Name)
tagToUrlMap[tag.Name] = append(tagToUrlMap[tag.Name], &imageUrl)
}
}
Is this better? Will the result
data structure be garbage collected correctly?
Or perhaps I should use a pointer to string in the Image
struct instead?
type Image struct {
URL *string
Description string
Tags []*Tag
}
Is there a better way to do this? I would also appreciate any resources on Go that describe various uses of pointers in depth. Thanks!
https://play.golang.org/p/VcKWUYLIpH7
UPDATE: I'm worried about optimal memory consumption and not generating unwanted garbage the most. My goal is to use the minimal amount of memory possible.