Strings as Arrays of Characters

1
Strings as Arrays of Characters

• string Greeting "hello"
• vs.
• char Greeting "hello"

2
pointers

• imagine passing the following string to a
  function
• void printStr( string x )
• x "Four score and seven years ago our fathers
  brought forth on this continent a new nation,
  conceived in Liberty, and dedicated to the
  proposition that all men are created equal. Now
  we are engaged in a great civil war, testing
  whether that nation, or any nation, so conceived
  and so dedicated, can long endure. We are met on
  a great battle-field of that war. We have come to
  dedicate a portion of that field, as a final
  resting place for those who here gave their lives
  that that nation might live. It is altogether
  fitting and proper that we should do this. But,
  in a larger sense, we can not dedicatewe can not
  consecratewe can not hallowthis ground. The
  brave men, living and dead, who struggled here,
  have consecrated it, far above our poor power to
  add or detract. The world will little note, nor
  long remember what we say here, but it can never
  forget what they did here. It is for us the
  living, rather, to be dedicated here to the
  unfinished work which they who fought here have
  thus far so nobly advanced. It is rather for us
  to be here dedicated to the great task remaining
  before us that from these honored dead we take
  increased devotion to that cause for which they
  gave the last full measure of devotion that we
  here highly resolve that these dead shall not
  have died in vain that this nation, under God,
  shall have a new birth of freedom and that
  government of the people, by the people, for the
  people, shall not perish from the earth."
• printStr( x )

3
in computer memory
four
score
and
seven
years
ago
our

shall
not
perish
from
the
earth.
0
4
computer memory is ADDRESSED
100
four
104
score
109
and
112
seven
117
years
address
data
5
a shortcut

• we could pass our huge string "x" to any function
  simply by passing the address of the first
  character.
• the address of the first character is referenced
  like this " x ".
• If x is defined as x
• the x is a pointer to a string of characters.

6
value vs. reference

• a string defined as
• string x "hello" is passed around and used
  as a complete value.
• a string defined as
• char x "hello" is passed around and used as
  a reference, or pointer, to a string.
• much quicker

7
note x

• x refers to the actual address in memory of x
  in our example, x 100
• if a function expects a pointer, and all you have
  is a variable, sometimes you can send the address
  and it won't complain
• why? because a pointer IS an address.

8
we will use references where appropriate

• strcmp, strcpy, strcat library functions only
  take pointers as arguments
• char x "hawk"
• char y "eagle"
• strcmp( x, y ) yields a 1. why
• -1 if hawk comes before eagle (it doesn't)
• 0 if they're identical strings (they're not)
• 1 if hawk comes after eagle (it does)

9
let's alphabetize a list

• this may take a while
• const int arraySize 8
• char birds arraySize "eagles", "hawks",
  "bluejays", "robins", "cardinals", "owls",
  "doves", "ducks"

10
our menu of choices
11
we'll need to print the list
the call from main
12
Why is printAnyArray passed an array, rather than
just using the global birds array? - it is
more useful then, because it can print any array,
not just birds
birds is defined as a pointer
a pointer to an array is expected
13
we'll need to swap around array elements