Exercise 7
June 1, 2018
This must be somebody’s homework:
You are given an input file containing lines with three pipe-separated fields; the first field is a student number (a positive integer), the second field is a class name (a string), and the third field is the grade the student received for the class (a non-negative integer, no greater than 100):
22|Data Structures|45 23|English|52 22|English|51 26|Data Structures|72 23|Data Structures|61 21|English|81You are to output a list of class names along with the grade earned by the lowest-numbers student in each class. For instance, given the above data, the output for Data Structures is 45 corresponding to student 22 (with student number lower than 23 or 26, who also took Data Structures) and the output for English is 81 corresponding to student 21 (with student number lower than 22 or 23, who also took English).
Your task is to write a program to produce the requested output. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.
Programming Praxis 
Here is a hashing-based solution in standard R7RS Scheme and a few
popular SRFIs.
(import (scheme base) (scheme write) (scheme file) (only (srfi 1) first second third) (only (srfi 8) receive) (only (srfi 13) string-tokenize) (only (srfi 14) char-set-complement char-set) (only (srfi 69) make-hash-table hash-table-update!/default hash-table-walk)) (define parse-line (let ((token-char-set (char-set-complement (char-set #\|)))) (lambda (str) (let ((toks (string-tokenize str token-char-set))) (list (string->number (first toks)) (second toks) (string->number (third toks))))))) (define (ex7 in-file-name) (let ((ht (make-hash-table string=?))) (with-input-from-file in-file-name (lambda () (let loop ((line (read-line))) (if (eof-object? line) (hash-table-walk ht (lambda (k v) (for-each display (list k " " (second v))) (newline))) (receive (id cname score) (apply values (parse-line line)) (hash-table-update!/default ht cname (lambda (id+score) (if (< id (first id+score)) (list id score) id+score)) (list id score)) (loop (read-line))))))))) (ex7 "input101.txt")Here’s a solution using Unix utilities.
sort data.txt -t'|' -k2,2 -k1,1n | awk -F'|' '{if (a[$2]++ == 0) print $2,$3;}'Output:
Here’s a solution in Python.
import sys if len(sys.argv) != 2: sys.stderr.write("Usage: {} <FILE>".format(sys.argv[0])) sys.exit(1) with open(sys.argv[1]) as f: D = dict() for line in f.readlines(): student_id, class_name, grade = line.strip().split("|") student_id = int(student_id) grade = int(grade) if not class_name in D or student_id < D[class_name][0]: D[class_name] = (student_id, grade) for class_name, (_, grade) in D.items(): print(class_name, grade)Output:
Bash (no Awk) solution.
output:
Data Structures 45
English 81
Clojure.
(doseq [[_ class grade] (->> (line-seq (clojure.java.io/reader "data.txt")) (map #(clojure.string/split % #"\|")) ;; sort and then partition by class (sort-by second) (partition-by second) ;; keep row with smallest student number in each partition (map (fn [part] (first (sort-by #(read-string (first %)) part)))))] (println class grade))Output:
Data Structures 45
English 81
Kotlin.
https://pastebin.com/p57zd2qu
A Haskell solution.
import Data.List (groupBy, sortBy) import Data.List.Split (splitOn) import Data.Monoid ((<>)) import Data.Ord (comparing) data Grade = Grade { stuId :: Int , course :: String , grade :: Int } gradeFrom :: String -> Grade gradeFrom s = let [sId, sCourse, sGrade] = splitOn "|" s in Grade (read sId) sCourse (read sGrade) showGrade :: Grade -> String showGrade g = course g ++ " " ++ show (grade g) courseGradesForLowestStuId :: [Grade] -> [Grade] courseGradesForLowestStuId = map head . groupBy sameCourse . sortBy courseStuId where courseStuId g1 g2 = comparing course g1 g2 <> comparing stuId g1 g2 sameCourse g1 g2 = course g1 == course g2 main :: IO () main = do gs <- courseGradesForLowestStuId . map gradeFrom . lines <$> getContents mapM_ (putStrLn . showGrade) gsHere’s a solution in C.
#include <search.h> #include <stdio.h> #include <stdlib.h> #include <string.h> typedef unsigned int uint; typedef struct { uint id; uint grade; } datum_t; int main(int argc, char* argv[]) { if (argc != 2) { fprintf(stderr, "Usage: %s <FILE>\n", argv[0]); return EXIT_FAILURE; } FILE* fp = fopen(argv[1], "r"); if (!fp) { fprintf(stderr, "%s: Error reading file\n", argv[1]); return EXIT_FAILURE; } size_t n_lines = 0; char c; while ((c = fgetc(fp)) != EOF) { n_lines += c == '\n'; } rewind(fp); hcreate(n_lines); char** courses = NULL; datum_t* data = NULL; size_t n = 0; char* line = NULL; size_t linecap = 0; ssize_t linelen; ENTRY item; ENTRY* found_item; while ((linelen = getline(&line, &linecap, fp)) > 0) { char* _line = line; uint id = atoi(strsep(&_line, "|")); char* course = strsep(&_line, "|"); uint grade = atoi(strsep(&_line, "\n")); item.key = course; if ((found_item = hsearch(item, FIND)) == NULL) { ++n; courses = realloc(courses, n * sizeof(char*)); data = realloc(data, n * sizeof(datum_t)); courses[n-1] = strdup(course); data[n-1] = (datum_t){id, grade}; item.key = courses[n-1]; item.data = data + n - 1; hsearch(item, ENTER); } else { datum_t* datum = (datum_t*)found_item->data; if (id < datum->id) { datum->id = id; datum->grade = grade; } } } for (size_t i = 0; i < n; ++i) { printf("%s %d\n", courses[i], data[i].grade); } hdestroy(); free(courses); free(data); return EXIT_SUCCESS; }Example Usage:
Solution in Ruby.
require 'csv' def fn(csv) CSV .parse(csv, col_sep: '|') .group_by { |_, c_name, _| c_name } .map do |c_name, row| [c_name, row.min_by { |s_num, _| s_num }.last] end endTest Code
def test(target, output, input) puts "target: #{target}" puts "output: #{output}" puts "pass: #{output == target ? '✅' : '❌'}" end scores = <<-SCORES 22|Data Structures|45 23|English|52 22|English|51 26|Data Structures|72 23|Data Structures|61 21|English|81 SCORES target = [ ['Data Structures', '45'], ['English', '81'] ] test target, fn(scores), scoresOutput
target: [["Data Structures", "45"], ["English", "81"]]
output: [["Data Structures", "45"], ["English", "81"]]
pass: ✅
A solution in Racket.
(define (x7 class-records)
(let recurse ((records class-records) (results null))
(if (null? records)
(reverse
(map
(lambda (x) (string-append (cadr x) "|" (number->string (caddr x))))
results))
(let* ((items (string-split (car records) "|"))
(id (string->number (car items)))
(class (cadr items))
(grade (string->number (caddr items)))
(new-result (list id class grade)))
(recurse
(cdr records)
(if (equal? #f (member class (map (lambda (x) (cadr x)) results)))
(cons new-result results)
(map
(lambda (x)
(if
(and (equal? class (cadr x)) (< id (car x)))
new-result
x))
results)))))))
A solution in Mumps
; Load data into 1-dimensional array GTM>f r !,x:5 q:'$t s arr($i(arr))=x 22|Data Structures|45 23|English|52 22|English|51 26|Data Structures|72 23|Data Structures|61 21|English|81 ; Reformat data into 2-dimensional array (arr2) GTM>k arr2 f i=1:1:6 s arr2($p(arr(i),"|",2),$p(arr(i),"|"))=$p(arr(i),"|",3) ; This is how the data looks now GTM>s glo="arr2" f s glo=$q(@glo) q:glo="" w !,glo," = ",@glo arr2("Data Structures",22) = 45 arr2("Data Structures",23) = 61 arr2("Data Structures",26) = 72 arr2("English",21) = 81 arr2("English",22) = 51 arr2("English",23) = 52 ; For each class, get first student number and corresponding score GTM>s c="" f s c=$o(arr2(c)) q:c="" w !,c,": ",arr2(c,$O(arr2(c,""))) Data Structures: 45 English: 81 GTM>