Hash Table structure in mathematica 7

Questions
1

Hello,

can anybody tell me how tp program a hash table data structure in mathematica 7?

I search in the Internet a little but, did not find much or did not understand the code. What I found so far:

Creation of a hash table:

TableName[key]=value

List all the keys?
List all the values?
Delete key ?
Delete value?

Thank you for your help

2

Check the documentation for DownValues, Set, and Unset. For things that are free of explicit patterns, something like

myTable[key1] = val1;
myTable[key2] = val2;

is effectively a hash table with key and key2 as keys, etc. (and they really are hashed, under the hood). To remove one of these use Unset; I show the infix version below.

myTable[key1] = val1;                                                   
myTable[key2] = val2;

We look up key:

In[9]:= myTable[key1]                                                           

Out[9]= val1

Now clear it and try that lookup. It will fail to find a value.

In[10]:=  myTable[key1] =.                                                      

In[11]:= myTable[key1]                                                          

Out[11]= myTable[key1]
3

Hello Daniel,

thank you for your reply.

Did I understand you right to get a list of all keys or values I have to use Downvalue?
I had a look in the documentation. In fact the output contains the keys and values, but also other stuff. So I need to filter/extract the keys/value, am I right?
Or is there a way to get a “pure” list of keys and/or values?

thx again
Murad

4

You are correct on all counts.

Version 10 has Association which provides a means to get Keys and Values separately, amongst numerous other features. It too has a hash table under the hood. One caveat is it might not be as useful if one plans to build it up or modify in many steps.

5

ok got it.

thank you Daniel…