A classic logic puzzle.

A farmer has a fox, a chicken and a bag of grain. Left alone without the farmer, the fox would eat the chicken and the chicken would eat the grain. The farmer must transport all three to the other side of a river but can only take one item in his rowboat at a time. How does he get all three to the other side? In this puzzle, the position of the fox, chicken, grain and farmer can be represented with a number. Zero means they are on this side of the river, or one which means they are on the other side of the river. E.g. `farmer = 0` Means assign the farmer to be on this side of the river. `grain = 1` Means assign the grain to be on the other side of the river.

Assessment

These objectives get progressively harder. Attempt as much of the program as you can in the order presented below. Remember to use a comment to describe a subprogram, selection or iteration.

Success Criteria

Initialising the variables

In the main program:

• Output to the screen the statement, “A fox, chicken and a bag of grain wait by the side of a river.”
• Assign four integer variables called, fox, chicken, grain and farmer to be 0.
• Ask the user to input fox, chicken, grain or farmer. This is to be stored in a variable called choice.

3 marks

The output should now look like this:

A fox, chicken and a bag of grain wait by the side of a river.
Which item will you take in your rowboat to the other side?
fox, chicken, grain or farmer?
Choose:

Row across the river

In the main program write selection statements to change the variables:

• If the choice was farmer and the farmer is 0 then assign the farmer to be 1.
• If the choice was fox and the farmer is 0 and the fox is 0 then assign the farmer and fox to both be 1.
• If the choice was chicken and the farmer is 0 and the chicken is 0 then assign the farmer and chicken to both be 1.
• If the choice was grain and the farmer is 0 and the grain is 0 then assign the farmer and grain to both be 1.

4 marks

Show the state of the game

Note, if you are not confident with subprograms you can do this in the main program instead.

• Define a subprogram called, output that takes no parameters.
  • Output what is on this side of the river (a variable is 0).
  • Output what is on the other side of the river (a variable is 1).
• In the main program call the subprogram output to show the outcome of the user choice.

4 marks

The output should now look like this:

A fox, chicken and a bag of grain wait by the side of a river.
Which item will you take in your rowboat to the other side?
fox, chicken, grain or farmer?
Choose: chicken
-----------------------
This side of the river:
Fox
Grain

The other side of the river:
Farmer
Chicken
-----------------------

Losing the puzzle

Note, if you are not confident with subprograms you can do this in the main program instead.

• Define a subprogram called wrong_move that takes no parameters.
  • If the farmer is 1 and the fox and chicken are both 0, output “The fox ate the chicken.”
  • If the farmer is 1 and the chicken and grain are both 0, output “The chicken ate the grain.”
  • Return True if the puzzle is lost, or False if it is not.
• In the main program call the subprogram wrong_move to check if the puzzle is lost.

5 marks

The output should now look like this:

A fox, chicken and a bag of grain wait by the side of a river.
Which item will you take in your rowboat to the other side?
fox, chicken, grain or farmer?
Choose: grain
-----------------------
This side of the river:
Fox
Chicken

The other side of the river:
Farmer
Grain
-----------------------
The fox ate the chicken.

Apply the logic to the other side of the river:

• Add statements to the main program to enable the farmer with the fox, chicken or grain to return to this side of the river.
• Add statements to the wrong_move subprogram to check if the fox ate the chicken or the chicken ate the grain on the other side of the river.

2 marks

Puzzle is solved

Note, if you are not confident with subprograms you can do this in the main program instead.

• Define a subprogram called puzzle_solved that takes no parameters.
  • If the fox, chicken and grain are all 1, output “You solved the puzzle.”
  • Return True if the puzzle is solved or False if not.
• Add a call to puzzle_solved in the main program to check if the puzzle has been solved.
• Add an iteration to the main program so that the puzzle continues until it is either lost or solved.

5 marks

Good programming practices

• The subprograms and main program sections have comments.
• The subprograms have a comment to describe their purpose.

2 marks

Maximum mark: 25

If you score less than 20 you need more practice at levels 1-3 before you continue to the next level.