C Shell Scripting/Guppies

Source code

#!/bin/csh

alias commas "echo \!:1 | sed -e :a -e 's/\(.*[0-9]\)\([0-9]\{3\}\)/\1,\2/;ta'"

# The program is to output a day-by-day account of the populations
# until one of them dies off or the end of the observation period is
# reached.
# All reproductions and deaths occur overnight.
# Any fractional fish are discarded.
# These sharks only feed during the day.

if ( $1 == 'default' ) then

	set sharks   = 54
	set guppies  = 1000
	set duration = 150
	
else

	printf "Please enter number of sharks: "
	set sharks = $<

	printf "Please enter number of guppies: "
	set guppies = $<

	printf "Number of days to observe: "
	set duration = $<

endif

set day = 0
while ( $day < $duration )

  	@ day = $day + 1

	#print
	printf "Start of %3s Day: %15s sharks  %15s guppies\n" $day `commas $sharks` `commas $guppies`

	if ( $sharks <= 0 || $guppies <= 0 ) then
		break
	endif
		
	# Each shark eats 5 guppies a day.
	@ guppies_eaten = ( $sharks * 5 )
	@ guppies = ( $guppies - $guppies_eaten )

	if ( $guppies < 0 ) then
		set guppies=0
	endif

	#print
	printf "End of %5s Day: %15s sharks  %15s guppies\n" $day `commas $sharks` `commas $guppies`

	if ( $sharks <= 0 || $guppies <= 0 ) then
		break
	endif
		
	# The guppies increase at a rate of 80% per day,
	# provided the shark population is less than 20 % of the guppy population.
	# Otherwise there is no increase in the guppy population.
	if ( ( $guppies / $sharks ) > 5 ) then
	    @ guppy_plus =  ($guppies * 80 ) / 100
	    @ guppies = ( $guppies + $guppy_plus )
	endif
	
	# The sharks increase at a rate of 5% of the guppy population per day,
	# provided there are 50 or more guppies per shark.
	# Otherwise, the sharks die off at a rate of 50% per day.
	if ( ( $guppies / $sharks ) > 50 ) then
	    @ shark_increase = ( $guppies / 20 )
	    @ sharks = ( $sharks + $shark_increase )
	else
	    @ sharks = ( $sharks / 2 )
	endif

end

exit 0

Lessons

C shell does not have functions so all logic must be in-lined. An "alias" can be created to reuse code such as formating a number with commas using the "sed" program.
Mathematical expression that are assigned to variable must with the "@" sign instead of the keyword "set".
Notice that the syntax is closer to BASIC than the C language with regards to control statements. You must include the key word "then" after the "if" statement expression.

Notes

Bourne shell version by Chris F.A. Johnson from which this version was ported.
The homework assignment that was the genesis of this program