Converting a Pointer to an Array or List

Some functions in Global Mapper that give a collection of information may return a pointer to the start of an array rather than the array itself. This is because the Python implementation of the Global Mapper SDK is derived from code written originally in C++, in which pointers are generally returned instead of the entire data structure. The following code is an example of a way to convert the pointer given by one of these function into an indexable array.

# If true, the next line would only return the elevation
# shaders that you have created (i.e. not the default ones)
only_custom_shaders = False
pointer, size = gm.GetElevationShaders(only_custom_shaders)
print("Size = ", size)

# At this point, type(pointer) will return that pointer is a single GM_CustomShader_t
# object, due to the way Python handles pointers
print(type(pointer))

shader_array = gm.GM_CustomShader_array_frompointer(pointer)
for i in range(size):
    print(shader_array[i])
# 'pointer' is still a pointer, but now shader_array is a GM_CustomShader_array

shader_list = gm.carray_to_list(shader_array, size)
print("The shaders are now also stored in a ", type(shader_list))

In this case, the GetElevationShaders function returns a pointer and the size of the array. The array exists and is stored sequentially in memory as you would expect, but since you only have the pointer, you can’t use Python indexing to see an arbitrary entry. As far as Python is concerned, pointer is just a single GM_CustomShader_t object. Trying to index an entry in it or any other pointer variable will result in TypeError: ‘SwigPyObject’ object is not subscriptable. The solution is the GM_CustomShader_array_frompointer function, which returns the information of the array starting at pointer to an indexable GM_CustomShader_array. There are several different *_array_frompointer functions for the types of arrays you might be given pointers to within Global Mapper (see below). If you want to put the data into a standard Python list, use the function carray_to_list(array, size).

For another case of converting a pointer to an array, see the example code for Opening and Closing a Layer. That code converts an array of layer handles using GM_LayerHandle_array_frompointer.

For more information about the array types, see: C++-Style Arrays for GM Types in Python.

All functions for getting an array object from a pointer to an array in the GlobalMapper SDK:

• COLORREF_array_frompointer(COLORREF *)

• double_array_frompointer(double *)

• float_array_frompointer(float *)

• GM_AreaFeature_array_frompointer(GM_AreaFeature_t *)

• GM_AttrValue_array_frompointer(GM_AttrValue_t *)

• GM_CustomShader_array_frompointer(GM_CustomShader_t *)

• GM_ElevColor_array_frompointer(GM_ElevColor_t *)

• GM_FoundFeature_array_frompointer(GM_FoundFeature_t *)

• GM_GroundControlPoint_array_frompointer(GM_GroundControlPoint_t *)

• GM_LayerHandle_array_frompointer(GM_LayerHandle_t32 *)

• GM_LidarAttrInfo_array_frompointer(GM_LidarAttrInfo_t *)

• GM_LidarPoint_array_frompointer(GM_LidarPoint_t *)

• GM_LidarReturnInfo_array_frompointer(GM_LidarReturnInfo_t *)

• GM_OnlineSourceInfo_array_frompointer(GM_OnlineSourceInfo_t *)

• GM_PaletteEntry_array_frompointer(GM_PaletteEntry_t *)

• GM_Point_t_array_frompointer(GM_Point_t *)

• GM_ValName_array_frompointer(GM_ValName_t *)

• uint8_array_frompointer(uint8 *)

• uint32_array_frompointer(unsigned long *)