I am trying to make a 2D array in LC3. So far I am thinking of initializing a block of memory using .BLKW and then loading into that another array into each entry. This doesn't seem like it will lead me on the right track though. Any suggestions?
You can definitely do it with .BLKW and also with .STRINGZ, though the latter is admittedly a bit unusual.
The bigger, usual question is around how you decide to "get" and "put" data into that specific area of memory. There are several ways to do that for sure (no one right answer).
Your initial thoughts are cool and valid, but definitely seem to me to be more complex, especially in LC3.
A more direct "row major" or "column major" form of storage - where successive memory locations represent the next entry in a row (row major), or alternatively the next entry in a column (column major)) - is the standard way to do this.
Basically you want to allocate that area of memory, and then write two functions: one to put an item at location (r, c), and get an item from location (r,c).
For this, you will hopefully only need to put an item that is small enough to fit into on 16bit memory location for LC3. That could be a number, or a character. (bigger than 16 bit is doable but adds more complexity for your program for sure).
If you want a fully roughed out sample, you can find that here: http://lc3tutor.org/#array2Dcolordersmp (or just go to lc3tutor.org and look at the 2D array sample).
If you are wanting to learn and try this on your own, you can just read the description there and ignore the sample code (best if you doing for homework and you want to be sure you learn it). Otherwise, the code there should run fine in the browser-based lc3 simulator you find referenced there.
Good luck!
Jeff
PS Here's the pre-amble to that code, if you want to just work from this... hopefully this example helps anchor the col major approach taken in the full code sample:
.ORIG x3000
BR MAIN; jump over storage below to the start of the main section
.STRINGZ "ABCDEFGHIJKLMNOPQRSTUVWZYZ"; slightly tricky - we are storing a sequence of letters in our 2D array for reference.
; The address of the above string STARTS AT x3001,
; which you will see is the same as the 2D_ARRAY label value below.
; This is essentially our 2D_ARRAY, starting at x3001 and taking up 26 locations,
; plus 1 (for the null terminator on the string).
; We will assume the 2D array has 13 rows and 2 columns.
; Two letters per row and 13 letters per column. 26 letters.
; So our NUM_ROW label will be 13 and our NUM_COL label will be 2. (see labels below)
; We will treat this array as a column-major stored array.
; Based on our string above, that means the cells of the first
; column (column #0 by our conventions) are: A-M.
; And the cells of the second column (column #1) are: N-Z.
; If we were storing the array in row-major form, then the cells of the first ROW
; would be A, B, and the second ROW would be C, D. Etc.
; Like this:
;
;R\C | 0 | 1
; ------------
; 0 | A | N
; 1 | B | O
; 2 | C | P
; 3 | D | Q
; 4 | E | R
; 5 | F | S
; 6 | G | T
; 7 | H | U
; 8 | I | V
; 9 | J | W
; 10 | K | X
; 11 | L | Y
; 12 | M | Z
; such that 2D_ARRAY[ROW=8, COL=1] would be the letter "V"
Related
Objective
I have a Microsoft Excel spreadsheet containing a price list that may change over time (B2:B5 in the example). Separately, I have a budget that too may change over time (D2). I am attempting to construct a formula for E2 to output the number of items that can be purchased with the budget in D2. Thereafter, I'll attempt to construct formulas to output any change that would be made (F2) and a comma-delimited list of purchasable items (G2).
Note: It unfortunately isn't possible to add an intermediate calculation column to the list, such as a running total. As such, I'm trying for formulas for single cells (i.e., E2, F2, and G2).
Note: I'm using Excel for Mac 2019.
A B C D E F G
+---------+---------+-----+---------+-------+---------+---------------------------+
1 | Label | Price | | Budget | Items | Change | Item(s) |
+---------+---------+-----+---------+-------+---------+---------------------------+
2 | Item #1 | $ 10.00 | | $ 40.00 | 3 | $ 4.50 | Item #1, Item #2, Item #3 |
+---------+---------+-----+---------+-------+---------+---------------------------+
3 | Item #2 | $ 20.00 | | | | | |
+---------+---------+-----+---------+-------+---------+---------------------------+
4 | Item #3 | $ 5.50 | | | | | |
+---------+---------+-----+---------+-------+---------+---------------------------+
5 | Item #4 | $ 25.00 | | | | | |
+---------+---------+-----+---------+-------+---------+---------------------------+
6 | Item #5 | $ 12.50 | | | | | |
+---------+---------+-----+---------+-------+---------+---------------------------+
For E2, I've attempted:
{=MAX(N(SUM(INDIRECT("$B$2:$B$"&ROW($B$2:$B$6)))<=$D2)*ROW($B$2:$B$6)-MIN(ROW($B$2:$B$6))+1)}
Though, the above values and this formula result in an output of -1.
Note: The formula for F2 and G2 seemingly easily follow E2; e.g. {=$D2-SUM(IF((ROW($B$2:$B$6)-MIN(ROW($B$2:$B$6))+1)<=$E2,$B$2:$B$6,0))} and {=TEXTJOIN(", ",TRUE,INDIRECT("$A$2:$A$"&(MIN(ROW($B$2:$B$6))+$E2-1)))} seem to work well, respectively.
Observations
{="$B$2:$B$"&ROW($B$2:$B$6)} evaluates to {"$B$2:$B$2";"$B$2:$B$3";...;"$B$2:$B$6"} (as desired);
{=INDIRECT("$B$2:$B$"&ROW($B$2:$B$6)) should evaluate to the equivalent of {{$B$2:$B$2},{$B$2:$B$3},...,{$B$2:$B$6}}; though, as a 1x5 multi-cell array formula, evaluates to the equivalent of {#VALUE!,#VALUE!,#VALUE!,#VALUE!,#VALUE!} and, with F9 does to {10;#N/A;#N/A;#N/A;12.5};
{=SUM(INDIRECT("$B$2:$B$"&ROW($B$2:$B$6)))<=$D2}, as a 1x5 multi-cell array formula, evaluates to the equivalent of {TRUE;TRUE;TRUE;FALSE;FALSE} (as desired); though, with F9 does to #VALUE!;
{=N(SUM(INDIRECT("$B$2:$B$"&ROW($B$2:$B$6)))<=$D2)}, as a 1x5 multi-cell array formula, evaluates to the equivalent of 1;1;1;0;0 (as desired); though, with F9 does again to #VALUE!;
{=N(SUM(INDIRECT("$B$2:$B$"&ROW($B$2:$B$6)))<=$D2)*ROW($B$2:$B$6), as as 1x5 multi-cell array formula, evaluates to the equivalent of {2,3,4,0,0} (as desired); though, with F9 does to {#VALUE!,#VALUE!,#VALUE!,#VALUE!,#VALUE!};
{=N(SUM(INDIRECT("$B$2:$B$"&ROW($B$2:$B$6)))<=$D2)*ROW($B$2:$B$6)-MIN(ROW($B$2:$B$6))+1}, as a 1x5 multi-cell array formula, evaluates to the equivalent of {1,2,3,-1,-1} (as desired); though, with F9 does again to {#VALUE!,#VALUE!,#VALUE!,#VALUE!,#VALUE!}; and,
{=MAX(N(SUM(INDIRECT("$B$2:$B$"&ROW($B$2:$B$6)))<=$D2)*ROW($B$2:$B$6)-MIN(ROW($B$2:$B$6))+1)} evaluates to -1
Interestingly:
If {=N(SUM(INDIRECT("$B$2:$B$"&ROW($B$2:$B$6)))<=$D2)*ROW($B$2:$B$6)-MIN(ROW($B$2:$B$6))+1} is placed as the multi-cell array formula in, say, E10:E14, a =MAX($E$10:$E$14) results in 3 (as desired).
Speculation
At present, I'm speculating that, when entered as a single cell array formula, the INDIRECT is not being assessed to be array producing and/or the SUM, as part of a single cell array formula, is not producing an array result.
Please assist. And, thank you in advance.
Solutions (Thanks to Contributors Below)
For E2, {=IF($B$2<=$D2,MATCH(1,0/(MMULT(N(ROW($B$2:$B$6)>=TRANSPOSE(ROW($B$2:$B$6))),$B$2:$B$6)<=$D2)),0)} (thank you Jos Woolley);
For F2, =IF($E2=0,MAX(0,$D2),$D2-SUM($B$2:INDEX($B$2:$B$6,$E2))) (thank you P.b); and,
For G2, =IF($E2=0,"",TEXTJOIN(", ",TRUE,$A$2:INDEX($A$2:$A$6,$E2))) (thank you P.b).
The first point to make, as I mentioned in the comments, is that it must be understood that piecemeal evaluation of a formula - via highlighting subsections of that formula and committing with F9 within the formula bar - will not necessarily correspond to the actual evaluation.
Evaluation via F9 in the formula bar always forces that part to be evaluated as an array. Though this is misleading, since the overall construction may not actually evaluate that part as an array.
The second point to make is that SUM cannot iterate over an array of ranges, though SUBTOTAL, for example, can, so replacing SUM with SUBTOTAL (9, in your current formula should work.
However, you would still be left with a construction which is volatile, so I would recommend this non-volatile alternative:
=MATCH(1,0/(MMULT(N(ROW(B2:B6)>=TRANSPOSE(ROW(B2:B6))),B2:B6)<=D2))
In E2 you can use:
=MATCH(TRUE,--SUBTOTAL(9,OFFSET(B2:B6,,,ROW(B2:B6)))>=D2,0)
In F2 you can use:
=D2-SUM(B2:INDEX(B2:B6,E2))
In G2 you can use:
=TEXTJOIN(", ",1,A2:INDEX(A2:A6,E2))
I'm looking for a way to create an excel array with n occurences of an x value, n and x being vectors.
Desired behaviour :
|---------------------|------------------|------------------|
| occurences | value | result |
|---------------------|------------------|------------------|
| 3 | 4 | {4;4;4;1;1} |
|---------------------|------------------|------------------|
| 2 | 1 |
|---------------------|------------------|
This is a question similar to this one, except that I want one more dimension. I'm not interested in a VBA answer, I'm looking for a formula.
I've tried playing around with index and concatenation like in the answer to the previously linked question but with no luck until now.
This result will be used in a bigger formula that will sum the m greatest values (I already have that part figured and working, the m value is irrelevant here). You can consider this question as if the occurences are the storage amounts, and I want the sum of the m greatest individual values.
Here's another approach in O365:
=INDEX(B:B,MATCH(SEQUENCE(SUM(A1:A3),1,0),
MMULT(N(ROW(A1:A3)>=TRANSPOSE(ROW(A1:A3))),A1:A3)-A1:A3))
where you're looking up the row number of the output array in the running total of the input counts.
I think it could be modified to work over an arbitrary range but would then be a fairly long formula.
If the inputs aren't in the sheet but coming from an array formula, then still possible but it would be a very long formula.
=FILTERXML("<t><s>" & TEXTJOIN("</s><s>",TRUE,SEQUENCE(3,,4,0),SEQUENCE(2,,1,0)) & "</s></t>","//s")
will return: {4;4;4;1;1} which can be used as part of a larger formula.
I need an excel formula that will look at the cell and if it contains an x will treat it as a 8 and add it to the total at the bottom of the table. I have done these in the pass and I am so rusty that I cannot remember how I did it.
Generally, I try and break this sort of problem into steps. In this case, that'd be:
Determine if a cell is 'x' or not, and create new value accordingly.
Add up the new values.
If your values are in column A (for example), in column B, fill in:
=if(A1="x", 8, 0) (or in R1C1 mode, =if(RC[-1]="x", 8, 0).
Then just sum those values (eg sum(B1:B3)) for your total.
A | B
+---------+---------+
| VALUES | TEMP |
+---------+---------+
| 0 | 0 <------ '=if(A1="x", 8, 0)'
| x | 8 |
| fish | 0 |
+---------+---------+
| TOTAL | 8 <------ '=sum(B1:B3)'
+---------+---------+
If you want to be tidy, you could also hide the column with your intermediate values in.
(I should add that the way your question is worded, it almost sounds like you want to 'push' a value into the total; as far as I've ever known, you can really only 'pull' values into a total.)
Try this one for total sum:
=SUMIF(<range you want to sum>, "<>" & <x>, <range you want to sum>)+ <x> * COUNTIF(<range you want to sum>, <x>)
I've been reading Learn C The Hard Way for a few days, but here's something I want to really understand. Zed, the author, wrote that char ** is for a "pointer to (a pointer to char)", and saying that this is needed because I'm trying to point to something 2-dimensional.
Here is what's exactly written in the webpage
A char * is already a "pointer to char", so that's just a string. You however need 2 levels, since names is 2-dimensional, that means you need char ** for a "pointer to (a pointer to char)" type.
Does this mean that I have to use a variable that can point to something 2-dimensional, which is why I need two **?
Just a little follow-up, does this also apply for n dimension?
Here's the relevant code
char *names[] = { "Alan", "Frank", "Mary", "John", "Lisa" };
char **cur_name = names;
No, that tutorial is of questionable quality. I wouldn't recommend to continue reading it.
A char** is a pointer-to-pointer. It is not a 2D array.
It is not a pointer to an array.
It is not a pointer to a 2D array.
The author of the tutorial is likely confused because there is a wide-spread bad and incorrect practice saying that you should allocate dynamic 2D arrays like this:
// BAD! Do not do like this!
int** heap_fiasco;
heap_fiasco = malloc(X * sizeof(int*));
for(int x=0; x<X; x++)
{
heap_fiasco[x] = malloc(Y * sizeof(int));
}
This is however not a 2D array, it is a slow, fragmented lookup table allocated all over the heap. The syntax of accessing one item in the lookup table, heap_fiasco[x][y], looks just like array indexing syntax, so therefore a lot of people for some reason believe this is how you allocate 2D arrays.
The correct way to allocate a 2D array dynamically is:
// correct
int (*array2d)[Y] = malloc(sizeof(int[X][Y]));
You can tell that the first is not an array because if you do memcpy(heap_fiasco, heap_fiasco2, sizeof(int[X][Y])) the code will crash and burn. The items are not allocated in adjacent memory.
Similarly memcpy(heap_fiasco, heap_fiasco2, sizeof(*heap_fiasco)) will also crash and burn, but for other reasons: you get the size of a pointer not an array.
While memcpy(array2d, array2d_2, sizeof(*array2d)) will work, because it is a 2D array.
Pointers took me a while to understand. I strongly recommend drawing diagrams.
Please have a read and understand this part of the C++ tutorial (at least with respect to pointers the diagrams really helped me).
Telling you that you need a pointer to a pointer to char for a two dimensional array is a lie. You don't need it but it is one way of doing it.
Memory is sequential. If you want to put 5 chars (letters) in a row like in the word hello you could define 5 variables and always remember in which order to use them, but what happens when you want to save a word with 6 letters? Do you define more variables? Wouldn't it be easier if you just stored them in memory in a sequence?
So what you do is you ask the operating system for 5 chars (and each char just happens to be one byte) and the system returns to you a memory address where your sequence of 5 chars begins. You take this address and store it in a variable which we call a pointer, because it points to your memory.
The problem with pointers is that they are just addresses. How do you know what is stored at that address? Is it 5 chars or is it a big binary number that is 8 bytes? Or is it a part of a file that you loaded? How do you know?
This is where the programming language like C tries to help by giving you types. A type tells you what the variable is storing and pointers too have types but their types tell you what the pointer is pointing to. Hence, char * is a pointer to a memory location that holds either a single char or a sequence of chars. Sadly, the part about how many chars are there you will need to remember yourself. Usually you store that information in a variable that you keep around to remind you how many chars are there.
So when you want to have a 2 dimensional data structure how do you represent that?
This is best explained with an example. Let's make a matrix:
1 2 3 4
5 6 7 8
9 10 11 12
It has 4 columns and 3 rows. How do we store that?
Well, we can make 3 sequences of 4 numbers each. The first sequence is 1 2 3 4, the second is 5 6 7 8 and the third and last sequence is 9 10 11 12. So if we want to store 4 numbers we will ask the system to reserve 4 numbers for us and give us a pointer to them. These will be pointers to numbers. However since we need to have 3 of them we will ask the system to give us 3 pointers to pointers numbers.
And that's how you end up with the proposed solution...
The other way to do it would be to realize that you need 4 times 3 numbers and just ask the system for 12 numbers to be stored in a sequence. But then how do you access the number in row 2 and column 3? This is where maths comes in but let's try it on our example:
1 2 3 4
5 6 7 8
9 10 11 12
If we store them next to each other they would look like this:
offset from start: 0 1 2 3 4 5 6 7 8 9 10 11
numbers in memory: [1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]
So our mapping is like this:
row | column | offset | value
1 | 1 | 0 | 1
1 | 2 | 1 | 2
1 | 3 | 2 | 3
1 | 4 | 3 | 4
2 | 1 | 4 | 5
2 | 2 | 5 | 6
2 | 3 | 6 | 7
2 | 4 | 7 | 8
3 | 1 | 8 | 9
3 | 2 | 9 | 10
3 | 3 | 10 | 11
3 | 4 | 11 | 12
And we now need to work out a nice and easy formula for converting a row and column to an offset... I'll come back to it when I have more time... Right now I need to get home (sorry)...
Edit: I'm a little late but let me continue. To find the offset of each of the numbers from a row and column you can use the following formula:
offset = (row - 1) * 4 + (column - 1)
If you notice the two -1's here and think about it you will come to understand that it is because our row and column numberings start with 1 that we have to do this and this is why computer scientists prefer zero based offsets (because of this formula). However with pointers in C the language itself applies this formula for you when you use a multi-dimensional array. And hence this is the other way of doing it.
From your question what i understand is that you are asking why you need char ** for the variable which is declared as *names[]. So the answer is when you simply write names[], than that it is the syntax of array and array is basically a pointer.
So when you write *names[] than that means you are pointing to an array. And as array is basically a pointer so that means you have a pointer to a pointer and thats why compiler will not complain if you write
char ** cur_name = names ;
In above line you are declaring a pointer to a character pointer and then initialinzing it with the pointer to an array (remember array is also pointer).
I'm going to create a program that can generate strings from L-system grammars.
Astrid Lindenmayer's original L-System for modelling the growth of algae is:
variables : A B
constants : none
axiom : A
rules : (A → AB), (B → A)
which produces:
iteration | resulting model
0 | A
1 | AB
2 | ABA
3 | ABAAB
4 | ABAABABA
5 | ABAABABAABAAB
that is naively implemented by myself in J like this:
algae =: 1&algae : (([: ; (('AB'"0)`('A'"0) #. ('AB' i. ]))&.>"0)^:[) "1 0 1
(i.6) ([;algae)"1 0 1 'A'
┌─┬─────────────┐
│0│A │
├─┼─────────────┤
│1│AB │
├─┼─────────────┤
│2│ABA │
├─┼─────────────┤
│3│ABAAB │
├─┼─────────────┤
│4│ABAABABA │
├─┼─────────────┤
│5│ABAABABAABAAB│
└─┴─────────────┘
Step-by-step illustration:
('AB' i. ]) 'ABAAB' NB. determine indices of productions for each variable
0 1 0 0 1
'AB'"0`('A'"0)#.('AB' i. ])"0 'ABAAB' NB. apply corresponding productions
AB
A
AB
AB
A
'AB'"0`('A'"0)#.('AB' i. ])&.>"0 'ABAAB' NB. the same &.> to avoid filling
┌──┬─┬──┬──┬─┐
│AB│A│AB│AB│A│
└──┴─┴──┴──┴─┘
NB. finally ; and use ^: to iterate
By analogy, here is a result of the 4th iteration of L-system that generates Thue–Morse sequence
4 (([: ; (0 1"0)`(1 0"0)#.(0 1 i. ])&.>"0)^:[) 0
0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0
That is the best that I can do so far. I believe that boxing-unboxing method is insufficient here. This is the first time I've missed linked-lists in J - it's much harder to code grammars without them.
What I'm really thinking about is:
a) constructing a list of gerunds of those functions that build final string (in my examples those functions are constants like 'AB'"0 but in case of tree modeling functions are turtle graphics commands) and evoking (`:6) it,
or something that I am able to code:
b) constructing a string of legal J sentence that build final string and doing (".) it.
But I'm not sure if these programs are efficient.
Can you show me a better approach please?
Any hints as well as comments about a) and b) are highly appreciated!
The following will pad the rectangular array with spaces:
L=: rplc&('A';'AB';'B';'A')
L^:(<6) 'A'
A
AB
ABA
ABAAB
ABAABABA
ABAABABAABAAB
Or if you don't want padding:
L&.>^:(<6) <'A'
┌─┬──┬───┬─────┬────────┬─────────────┐
│A│AB│ABA│ABAAB│ABAABABA│ABAABABAABAAB│
└─┴──┴───┴─────┴────────┴─────────────┘
Obviously you'll want to inspect rplc / stringreplace to see what is happening under the covers.
You can use complex values in the left argument of # to expand an array without boxing.
For this particular L-system, I'd probably skip the gerunds and use a temporary substitution:
to =: 2 : 'n (I.m=y) } y' NB. replace n with m in y
ins =: 2 : '(1 j. m=y) #!.n y' NB. insert n after each m in y
L =: [: 'c'to'A' [: 'A'ins'B' [: 'B'to'c' ]
Then:
L^:(<6) 'A'
A
AB
ABA
ABAAB
ABAABABA
ABAABABAABAAB
Here's a more general approach that simplifies the code by using numbers and a gerund composed of constant functions:
'x'-.~"1 'xAB'{~([:,(0:`(1:,2:)`1:)#.]"0)^:(<6) 1
A
AB
ABA
ABAAB
ABAABABA
ABAABABAABAAB
The AB are filled in at the end for display. There's no boxing here because I use 0 as a null value. These get scattered around quite a bit but the -.~"1 removes them. It does pad all the resulting strings with nulls on the right. If you don't want that, you can use <#-.~"1 to box the results instead:
'x'<#-.~"1 'xAB'{~([:,(0:`(1:,2:)`1:)#.]"0)^:(<6) 1
┌─┬──┬───┬─────┬────────┬─────────────┐
│A│AB│ABA│ABAAB│ABAABABA│ABAABABAABAAB│
└─┴──┴───┴─────┴────────┴─────────────┘