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

1. 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.
2. Mathematical expression that are assigned to variable must with the "@" sign instead of the keyword "set".
3. 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