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Exercises

  1. Write a function, char* strdup(const char*), that copies a C-style string into memory it allocates on the free store. Do not use any standard library functions. Do not use subscripting; use the dereference operator * instead.
  2. Write a function, char* findx(const char* s, const char* x), that finds the first occurrence of the C-style string x in s. Do not use any standard library functions. Do not use subscripting; use the dereference operator * instead.
  3. Write a function, int strcmp(const char* s1, const char* s2), that compares C-style strings. Let it return a negative number if s1 is lexicographically before s2, zero if s1 equals s2, and a positive number if s1 is lexicographically after s2. Do not use any standard library functions. Do not use subscripting; use the dereference operator * instead.
  4. Consider what happens if you give strdup(), findx(), and strcmp() an argument that is not a C-style string. Try it! First figure out how to get a char* that doesn’t point to a zero-terminated array of characters and then use it (never do this in real — non-experimental — code; it can create havoc). Try it with free-store-allocated and stack-allocated “fake C-style strings.” If the results still look reasonable, turn off debug mode. Redesign and re-implement those three functions so that they take another argument giving the maximum number of elements allowed in argument strings. Then, test that with correct C-style strings and “bad” strings.
  5. Write a function, string cat_dot(const string& s1, const string& s2), that concatenates two strings with a dot in between. For example, cat_dot("Niels", "Bohr") will return a string containing Niels.Bohr.
  6. Modify cat_dot() from the previous exercise to take a string to be used as the separator (rather than dot) as its third argument.
  7. Write versions of the cat_dot()s from the previous exercises to take C-style strings as arguments and return a free-store-allocated C-style string as the result. Do not use standard library functions or types in the implementation. Test these functions with several strings. Be sure to free (using delete) all the memory you allocated from free store (using new). Compare the effort involved in this exercise with the effort involved for exercises 5 and 6.
  8. Rewrite all the functions in §18.7 to use the approach of making a backward copy of the string and then comparing; for example, take "home", generate "emoh", and compare those two strings to see that they are different, so home isn’t a palindrome.
  9. Consider the memory layout in §17.4. Write a program that tells the order in which static storage, the stack, and the free store are laid out in memory. In which direction does the stack grow: upward toward higher addresses or downward toward lower addresses? In an array on the free store, are elements with higher indices allocated at higher or lower addresses?
  10. Look at the “array solution” to the palindrome problem in §18.7.2. Fix it to deal with long strings by (a) reporting if an input string was too long and (b) allowing an arbitrarily long string. Comment on the complexity of the two versions.
  11. Look up (e.g., on the web) skip list and implement that kind of list. This is not an easy exercise.
  12. Implement a version of the game “Hunt the Wumpus.” “Hunt the Wumpus” (or just “Wump”) is a simple (non-graphical) computer game originally invented by Gregory Yob. The basic premise is that a rather smelly monster lives in a dark cave consisting of connected rooms. Your job is to slay the wumpus using bow and arrow. In addition to the wumpus, the cave has two hazards: bottomless pits and giant bats. If you enter a room with a bottomless pit, it’s the end of the game for you. If you enter a room with a bat, the bat picks you up and drops you into another room. If you enter the room with the wumpus or he enters yours, he eats you. When you enter a room you will be told if a hazard is nearby:

    • “I smell the wumpus”: It’s in an adjoining room.
    • “I feel a breeze”: One of the adjoining rooms is a bottomless pit.
    • “I hear a bat”: A giant bat is in an adjoining room.

    For your convenience, rooms are numbered. Every room is connected by tunnels to three other rooms. When entering a room, you are told something like “You are in room 12; there are tunnels to rooms 1, 13, and 4; move or shoot?” Possible answers are m13 (“Move to room 13”) and s13–4–3 (“Shoot an arrow through rooms 13, 4, and 3”). The range of an arrow is three rooms. At the start of the game, you have five arrows. The snag about shooting is that it wakes up the wumpus and he moves to a room adjoining the one he was in — that could be your room.

    Probably the trickiest part of the exercise is to make the cave by selecting which rooms are connected with which other rooms. You’ll probably want to use a random number generator (e.g., randint() from std_lib_facilities.h) to make different runs of the program use different caves and to move around the bats and the wumpus. Hint: Be sure to have a way to produce a debug output of the state of the cave.

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