# Merge Sort in Algol68

Published on 29 January 2023 (Updated: 31 January 2023)

Welcome to the Merge Sort in Algol68 page! Here, you'll find the source code for this program as well as a description of how the program works.

## Current Solution

``````MODE PARSEINT_RESULT = STRUCT(BOOL valid, INT value, STRING leftover);
MODE PARSEINTLIST_RESULT = STRUCT(BOOL valid, REF []INT values);

PROC parse int = (REF STRING s) PARSEINT_RESULT:
(
BOOL valid := FALSE;
REAL r := 0.0;
INT n := 0;
STRING leftover;

# Associate string with a file #
FILE f;
associate(f, s);

# On end of input, exit if valid number not seen. Otherwise ignore it #
on logical file end(f, (REF FILE dummy) BOOL:
(
IF NOT valid THEN done FI;
TRUE
)
);

# Exit if value error #
on value error(f, (REF FILE dummy) BOOL: done);

# Convert string to real number #
get(f, r);

# If real number is in range of an integer, convert to integer. Indicate integer is valid if same as real #
IF ABS r <= max int
THEN
n := ENTIER(r);
valid := (n = r)
FI;

# Get leftover string #
get(f, leftover);

done:
close(f);
PARSEINT_RESULT(valid, n, leftover)
);

PROC count list items = (STRING s) INT:
(
INT count := 1;
FOR k TO UPB s
DO
IF s[k] = ","
THEN
count +:= 1
FI
OD;

count
);

PROC parse int list = (REF STRING s) PARSEINTLIST_RESULT:
(
BOOL valid := FALSE;
STRING leftover := s;
INT num list items = count list items(s);
HEAP [num list items]INT values;

# Repeat while valid value #
FOR k TO num list items
DO
# Get next integer value and update leftover string #
PARSEINT_RESULT result = parse int(leftover);
valid := valid OF result;
leftover := leftover OF result;

# Append the integer value to list #
values[k] := value OF result;

# Do nothing if end of string #
IF leftover = ""
THEN
SKIP
# Skip comma if leftover string starts with comma #
ELIF leftover = ","
THEN
leftover := leftover[2:]
# Otherwise indicate invalid #
ELSE
valid := FALSE
FI
UNTIL NOT valid
OD;

PARSEINTLIST_RESULT(valid, values)
);

PROC usage = VOID: (
printf((\$gl\$, "Usage: please provide a list of at least two integers to sort in the format ""1, 2, 3, 4, 5"""))
);

COMMENT
Source: https://en.wikipedia.org/wiki/Merge_sort#Top-down_implementation
COMMENT
PROC merge sort = (REF []INT values) VOID:
(
# Create temporary work array and copy the values into it #
INT n = UPB values;
HEAP [n]INT working values := values;

# Run the recursive merge sort #
merge sort rec(working values, 1, n + 1, values)
);

PROC merge sort rec = (REF []INT working values, INT low, INT high, REF []INT values) VOID:
(
# Only sort if enough values #
IF (high - low) > 1
THEN
# Get midpoint #
INT mid := (low + high) OVER 2;

# Sort the left half (low to midpoint - 1) from values into working values #
merge sort rec(values, low, mid, working values);

# Sort the right half (midpoint to high - 1) from values into working values #
merge sort rec(values, mid, high, working values);

# Merge the two halves from working values into values #
merge(working values, low, mid, high, values)
FI
);

PROC merge = (REF []INT src, INT low, INT mid, INT high, REF []INT dest) VOID:
(
# Copy the side that has the next largest value from the source to the destination #
# until a side is exhausted #
INT left index := low;
INT right index := mid;
INT combined index := low;
INT len;
WHILE left index < mid AND right index < high
DO
IF src[left index] <= src[right index]
THEN
dest[combined index] := src[left index];
left index +:= 1
ELSE
dest[combined index] := src[right index];
right index +:= 1
FI;

combined index +:= 1
OD;

# Copy the leftovers from the left side from the source to the destination #
len := mid - left index;
IF len > 0
THEN
dest[combined index:combined index + len - 1] := src[left index:mid - 1];
combined index +:= len
FI;

# Copy the leftovers from the right side from the source to the destination #
len := high - right index;
IF len > 0
THEN
dest[combined index:combined index + len - 1] := src[right index:high - 1]
FI
);

PROC show list values = (REF []INT values) VOID:
(
INT n = UPB values;
FOR k TO n
DO
IF k > 1
THEN
print(", ")
FI;

print(whole(values[k], 0))
OD;

IF n > 0
THEN
print(newline)
FI
);

# Parse 1st command-line argument #
STRING s := argv(4);
PARSEINTLIST_RESULT list result := parse int list(s);
REF []INT values := values OF list result;
IF NOT valid OF list result OR UPB values < 2
THEN
usage;
stop
FI;

# Do merge sort and show results #
merge sort(values);
show list values(values)

``````

Merge Sort in Algol68 was written by:

• rzuckerm

If you see anything you'd like to change or update, please consider contributing.

## How to Implement the Solution

No 'How to Implement the Solution' section available. Please consider contributing.

## How to Run the Solution

No 'How to Run the Solution' section available. Please consider contributing.