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The GCS is a very complicated system to calculate the exact location of a point on the planet. There many coordinate systems used on Earth, but I will discuss the two main ones: GCS and UTM. Think about the planet as an orange being scored for “easy peeling” – you make straight strips that are closer at the top and bottom but wider apart at the circumference. The reason that we need to think about coordinate systems as wedges and not plain grid system is the fact that earth is a ball, and a square grid won’t work due to the curvature. Coordinate SystemsĬoordinate systems are a way to “cut” the planet to wedges – both horizontal ones an d vertical ones – in order to give each location that grid “address” that I mentioned above. Also this is as complicated as it will get here in this series, to try to stick it out – it will get easier after this, I promise! In the next few paragraphs I will try to explain it, but if you feel that you lose me, just stop, have a break and come back to it. There is actually a whole scientific field called geodesy just on coordinate systems, so it can be a huge subject. Now this subject can be really, really complicated. Grid maps are inherently very simple and they usually use a cartesian system (X,Y system) that gives a point an X address and a Y address, resulting in an X,Y point.Ī grid system is only good if it can be used to describe that address universally, and for that we have coordinate systems. The purpose of the grid system is to give each point in the map an identifier, an address, by which we can refer to it by. If you look far enough back in the history of Go repository, you can find examples of maps created with the new operator.If this is the first time you are joining the Map Reading series, you should start from the first post.Ī grid system on a map is usually square and is represented by drawn lines on the map creating those squares.
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There is no magic save the rewriting of map syntax by the compiler into calls to functions in runtime/hmap.go. Maps have the same pointer semantics as any other pointer value in a Go program. As you saw above, a map is just a pointer to a runtime.hmap structure. Maps, like channels, but unlike slices, are just pointers to runtime types. We moved away from that when we realized that no one ever wrote `map` without writing `*map`.Īrguably renaming the type from * mapint to mapint, while confusing because the type does not look like a pointer, was less confusing than a pointer shaped value which cannot be dereferenced. In the very early days what we call maps now were written as pointers, so you wrote *mapint. Shouldn’t it return a *mapint? Ian Taylor answered this recently in a golang-nuts thread 1. It’s a good question that if maps are pointer values, why does the expression make(mapint) return a value with the type mapint. } If maps are pointers, shouldn’t they be *mapvalue? package mainįmt.Println(unsafe.Sizeof(m), unsafe.Sizeof(p)) // 8 8 (linux/amd64) We cannot see this from normal Go code, but we can confirm that a map value is the same size as a uintptr–one machine word. If bucket != nil, bucket can be used as the first bucket.įunc makemap(t *maptype, hint int64, h *hmap, bucket unsafe.Pointer) *hmapĪs you see, the type of the value returned from runtime.makemap is a pointer to a runtime.hmap structure. If h != nil, the map can be created directly in h. can be created on the stack, h and/or bucket may be non-nil. If the compiler has determined that the map or the first bucket
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The compiler replaces it with a call to runtime.makemap, which has the signature // makemap implements a Go map creation make(mapv, hint) When you write the statement m := make(mapint) If you’re not satisfied with this explanation, read on. This leaves the question, if maps are not references variables, what are they?Ī map value is a pointer to a runtime.hmap structure. In my previous post I showed that Go maps are not reference variables, and are not passed by reference.