Homework 1: Table Slicing and Dicing Solution



In the `hw2041/hw1` directory, you will find a files named `tables.ml` and `slicendice.ml`.

Create a directory named `hw1` in your personal repository, and copy both of these files

to your `hw1` directory. Don’t forget to run `git add` on both the directory and

the files!

Background: Command Line programs

Every (but one) homework in this class will involve eventually creating a

program that can be run from the “command line” in a terminal. We will

typically include directions for building these programs in the homework. They

can usually be compiled with `ocamlc` or `ocamlopt` but sometimes (later in the

semester) we’ll use additional libraries and it will be easier to use

`ocamlbuild` to resolve the dependencies between files.

Command line programs typically take arguments on the command line, for example,

like `git` or `ssh`. They also often need to read or write outputs to files, or

in the case of many *nix utilities, can read an input file from the “standard

input” and write to the “standard output.” As you probably know, a command line

program’s standard input can be “redirected” to use a file with the `<`

operator, and its standard output can be redirected with the `>` operator. (So

for example, if you type `git status >git.out` in the terminal while your

working directory is within a git repo, the result will go to a file named

`git.out` that you can read with your favorite text editor, or `less`)

Background: Tables

A very common way of storing data for presentation or manipulation is as a table. While spreadsheet applications like Excel, Numbers, Google Sheets, and LibreOffice each have their own customized format for storing tables, it is also common to store tables as lines of a text file, with a “delimiter” character between the columns in each row. So the table:

Last Name | First Name | Birth Year | Email | House

— | — | — | — | —

Potter | Harry | 1980 | pott213@hogwarts.ac.uk | Gryffindor

Granger | Hermione | 1979 | gran371@hogwarts.ac.uk | Gryffindor

Malfoy | Draco | 1980 | malf091@hogwarts.ac.uk | Slytherin

Chang | Cho | 1979 | chan010@hogwarts.ac.uk | Ravenclaw

Diggory | Cedric | 1977 | digg131@hogwarts.ac.uk | Hufflepuff

Might be stored in a file named `students.csv` with contents:


Last Name, First Name, Birth Year, Email, House

Potter, Harry, 1980, pott213@hogwarts.ac.uk, Gryffindor

Granger, Hermione, 1979, gran371@hogwarts.ac.uk, Gryffindor

Malfoy, Draco, 1980, malf091@hogwarts.ac.uk, Slytherin

Chang, Cho, 1979, chan010@hogwarts.ac.uk, Ravenclaw

Diggory, Cedric, 1977, digg131@hogwarts.ac.uk, Hufflepuff


Here the delimiter is `,`; if you are reading this file in a text editor, you’ll

notice that markdown also stores tables as a collection of lines, with columns

delimited by `|`. Another common delimiter for storing tables in text files is

the tab character, `\t`.

This table has six *rows*, including the headings, and five *columns*. Often it is useful to be able to *rearrange* or *delete* some of the columns, or to *transpose* a table so that the rows become columns, and vice versa. It can also be useful to convert a table using one delimiter into a table using a different delimiter. For Homework 1, we’ll write a tool that supports these operations.

Homework 1: `slicendice`

Open `tables.ml` in your favorite text editor, and you’ll see that there’s some

implementation in the file already:

+ `read_lines : unit -> string list` reads from standard input and returns a

list of strings, one for each line in the input.

+ `make_row : string -> string -> string list` takes a delimiter and a line, and

returns a list of strings separated by the delimiter. It trims extra whitespace

from these strings as well.

+ `write_row : string list -> string -> unit` takes a list of strings `r` and a

delimiter `delim`, and writes each string in `r` to the standard output, with

the string `delim` as the delimiter.

+ `output_table : string -> string list list -> unit` takes a delimiter and a

list of lists of strings, and prints each string list as a separate line

(delimited by the first input) on the standard output.

There’s one more function fully implemented in `tables.ml`, which is `main`. This function handles the flow of work to be done by our tool:

+ Read the input from stdin, storing it as a list of strings.

+ Then convert the list of strings into “table format,” a list of lists of strings. The function `table_of_stringlist`, which you’ll fill in, does this.

+ Next, convert from this table format to a more convenient representation for the job we want to do, *associative format*. When we’re manipulating a table, we’ll store it as a `(int*int*string) list`: a single list of triples, where if the triple `(r,c,e)` is in the list, this means that the string `e` is in the table at row `r` and column `c`. This is accomplished by the function `make_assoc`.

+ Next apply the operations specified on the command line in the order they were specified.

+ Finally, convert the associative representation back into a table representation, e.g. a `string list list`, in `table_of_assoc`, and pass the result to `output_table`.

A separate file, `slicendice.ml` contains code to read the command-line arguments and call the `main` function in `tables.ml`:

+ `slice_and_dice` handles the job of understanding the command-line arguments

to our tool, converting the operations to a format `Tables.main` will understand and

+ evaluating the line below it, `let () = …` is what causes all of the functions to be called with appropriate arguments.

The files as given will compile (with `ocamlc -o slicendice str.cma tables.ml slicendice.ml`) but the resulting program won’t produce useful output until you fill in all of the incomplete functions:

**NOTE: If an error in one function causes your submission to fail to compile, _all_ of your automated testing scores will be 0. If you cannot implement one of these functions in a way that will compile, leave the original version in place and put your best attempt in a comment.**

# 1. `table_of_stringlist`

The function `table_of_stringlist : string -> string list -> string list list` takes as input a delimiter `d` and a list of strings `l`, and converts each string in the list `l` into a list of strings using `make_row` using the first parameter `d` as a delimiter. So `table_of_stringlist d [r1;r2;…;rN]` should evaluate to `[(make_row d r1); (make_row d r2); …; (make_row d rN)]`. Some example evaluations:

+ `table_of_stringlist “,” [“a”]` should evaluate to `[[“a”]]`

+ `table_of_stringlist “,” [“a,b”]` should evaluate to `[[“a”; “b”]]`

+ `table_of_stringlist “|” [“a|b|c”; “d”]` should evaluate to `[[“a”;”b”;”c”]; [“d”]]`.

# 2. `make_assoc`

The function `make_assoc : string list list -> (int*int*string) list` should translate a list of lists of strings into *associative form*: rather than storing a table as a list of lists, we store it as a list of triples, where the triple `(r,c,”data”)` means that the string `”data”` was in the original table in row `r` and column `c`. Thus the list (of lists) `[[s11;…;s1k]; [s21;…;s2m]; …; [sN1;….;sNl]]` should be translated to `[(1,1,s11); …; (1,k,s1k); (2,1,s21); … ;(2,m,s2m); …; (N,1,sN1); …; (N,l,sNl)]`. Some concrete evaluations:

+ `make_assoc []` and `make_assoc [[]]` should both evaluate to `[]`

+ `make_assoc [[“a”]]` should evaluate to `[(1,1,”a”)]`

+ `make_assoc [[“a”;”b”];[“c”]]` should evaluate to (a permutation of) `[(2,1,”c”); (1,2,”b”); (1,1,”a”)]`

# 3. `move_column`

The function `move_column : int -> int -> (int * int * string) list -> (int * int * string) list` takes as input a “source” index `s`, a “destination” index `d`, and a table in associative format and moves the column at index `s` to index `d` in its output. If index `d` is to the right of `s`, all of the columns between `s` and `d` are moved one index to the left; otherwise all of these columns are moved one index to the right.

So for example, evaluating `move_column 1 3 [(1,1,”a”); (1,2,”b”); (1,3,”c”); (1,4,”d”)]` should result in (possibly a permutation of) `[(1,1,”b”); (1,2,”c”); (1,3,”a”); (1,4,”d”)]` (because the `”a”` in column 1 of the original table moves to the right in the output table, shifting columns 2 and 3 left.) For another example, evaluating `move_column 2 1 [(1,1,”hickory”); (1,2,”dickory”); (1,3,”dock”); (2,1,”three”); (2,2,”blind”); (2,3,”mice”)]` should result in (possibly a permutation of) `[(1,1,”dickory”); (1,2,”hickory”); (1,3,”dock”); (2,1,”blind”); (2,2,”three”); (2,3,”mice”)]`.

If the source or destination column are out of range (less than 1 or greater than the maximum column index appearing in the table) then `move_column` should fail by calling `failwith “column index out of range”`.

# 4. `delete_column`

The function `delete_column : int -> (int*int*string) list -> (int*int*string) list` takes as input a column index `c` and a table in associative format and removes the column at index `c`, moving all columns after `c` one index to the left. So for instance, `delete_column 2 [(1,1,”a”); (1,2,”b”); (1,3,”c”); (2,1,”X”); (2,2,”Y”); (2,3,”Z”)]` should evaluate to (possibly a permutation of) `[(1,1,”a”); (1,2,”c”); (2,1,”X”); (2,2,”Z”)]`. Deleting a column that is out of range should just leave the table unchanged, so `delete_column 2 [(1,1,”a”)]` should evaluate to `[(1,1,”a”)]`.

# 5. `transpose_table`

The function `transpose_table : (‘a*’b*’c) list -> (‘b*’a*’c) list` takes a table in associative form and returns the table with the rows and columns transposed, e.g. if `(r,c,e)` is in the input list, then `(c,r,e)` should be in the output list. `transpose_table` may output the list in a different order than the input list. Some example evaluations:

+ `transpose_table [(1,1,”a”); (1,2,”b”); (1,3,”c”); (2,1,”d”); (2,2,”e”); (2,3,”f”)]` should evaluate to (a permutation of) `[(1,1,”a”); (2,1,”b”); (3,1,”c”); (1,2,”d”); (2,2,”e”); (3,2,”f”)]`.

+ `transpose_table [(1,4,”h”)]` should evaluate to `[(4,1,”h”)]`

+ `transpose_table []` should evaluate to `[]`

# 6. `table_of_assoc`

The function `table_of_assoc : (int*int*’a) list -> ‘a list list` takes as input a table in associative form and returns the table as a list of lists of strings, where the first string list represents the first row, the second represents the second row, and so on. So `table_of_assoc [(1,1,”a”)]` should evaluate to `[[“a”]]`, `table_of_assoc [(1,1,”a”),(2,1,”b”)]` should evaluate to `[[“a”]; [“b”]]`, and `table_of_assoc [(1,1,”a”),(1,2,”b”)]` should evaluate to `[[“a”; “b”]]`. Missing rows should be represented by empty lists, e.g. `table_of_assoc [(1,1,”a”); (3,1,”c”)]` should evaluate to `[[“a”]; []; [“c”]]`, and missing column entries should be filled in with empty strings (`””`), so `table_of_assoc [(1,1,”a”); (1,3,”b”)]` should evaluate to `[[“a”; “”; “b”]]`.

Your solution may make some assumptions about the associative form:

+ No duplicate row, column pairs occur in the input. So your solution doesn’t need to consider inputs like `[(1,1,”a”); (1,1,”b”)]`.

+ All row and column indices in the input are positive.

You might find it useful to be able to sort lists for this function: in that case, the function `List.sort : (‘a -> ‘a -> int) -> ‘a list -> ‘a list` will be useful. The first argument to this function is a “comparison” function that should return 0 if its arguments are equal, a positive value if the first argument is greater than the second, and a negative value otherwise.

Other considerations

In addition to satisfying the functional specifications given above, your code should be readable, with comments that explain what you’re trying to accomplish. It must compile with `ocamlc -c str.cma tables.ml`. It must be committed in the `hw1` directory of your personal repository. Finally, solutions that pay careful attention to resources like running time and stack space (e.g. using tail recursion wherever feasible) and code reuse are worth more than solutions that do not have these properties.

Late grading

As described in the class syllabus, you may opt to have one homework turned in late but graded for full credit. If you would like to have turn in this homework late, then by 11:59pm on the original due date (Tuesday, February 12, 2019), add a file named `late_request` (the contents can be anything – e.g. your `umn.edu` username) to the `hw1` directory in your personal repository, and commit and push this to `github.umn.edu`. If this file is present in your repo, we will wait until *11:59pm on Friday, February 15, 2019* to pull your repo for grading. Note that choosing this option means you will not be able to submitting any other homeworks for late grading.

(If you created this file, pushed it, and then change your mind before the original deadline, you can remove the `late_request` file from your repo using `git rm late_request` followed by `git commit` and `git push`.)

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