Pointers, Arrays, and Strings

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Pointers, Arrays, and Strings

• CPSC 265
• Week 2

Based upon http//www.cs.usask.ca/grads/nic169/csh
ort/week2/week2.html KR (Your C language text)
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What is a pointer?

• Integers, and other objects are all stored
  somewhat in the computers memory
• at a memory address or location.
• A pointer allows us to access these memory
  addresses, and manipulate the addresses as well
  as their contents
• Creating a pointer int p
• creates a pointer whose target is an integer

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Operators

• Address-of operator If i is a variable, then i
  tells us the memory address in which the value of
  i is stored.
• Dereferencing operator (AKA indirection
  operator) The unary operator
• When applied to a pointer, it gives us access to
  what is stored in the memory address to which the
  pointer is pointing.

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Example

• /
• foo1c A simple pointer example
• /
• include ltstdio.hgt
• int main(void)
• int x 5
• int ptr
• ptr x
• printf("The value of x is d\n", ptr)
• printf("The location of x is p\n", ptr)
  
• printf("The location of x is p\n", x)
• ptr 10
• printf("d\n", x)
• printf("d\n", ptr)
• printf("d\n", (ptr))
• printf("d\n", ptr)
• return 0

• Output
• The value of x is 5
• The location of x is 0xbff6b2f4
• The location of x is 0xbff6b2f4
• 15
• 16
• 16
• 17

From http//www.cs.usask.ca/grads/nic169/cshort/we
ek2/week2.html
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Associativity

• Unary operators associate from right to left
• (ptr)
• increments and returns the value stored at the
  address pointed to by the pointer
• ptr
• increments the pointer, and returns the value
  stored at the next memory location

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Passing by Reference vs. Passing by Value

• / Pass by value /
• void swap1(int x, int y)
• int z
• z x
• x y
• y z

• / Pass by reference /
• void swap2(int x, int y)
• int z
• z x
• x y
• y z

Slight modification of http//www.cs.usask.ca/gra
ds/nic169/cshort/week2/week2.html
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Main method Output

• include ltstdio.hgt
• / function prototypes /
• void swap1(int x, int y)
• void swap2(int x, int y)
• int main(void)
• int a 3, b 5
• printf("a d, b d\n", a, b)
• swap1(a, b)
• printf("a d, b d\n", a, b)
• swap2(a, b)
• printf("a d, b d\n", a, b)
• return 0

• Output
• a 3, b 5
• a 3, b 5
• a 5, b 3

Slight modification of http//www.cs.usask.ca/gra
ds/nic169/cshort/week2/week2.html
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Pointers to pointers!

• /
• pointer2 Pointing to pointers
• /
• include ltstdio.hgt
• int main ()
• int x 5
• int ptr1 x / A pointer to x /
• int ptr2 ptr1 / A pointer to ptr1 /
• printf("The value of x is d\n", x)
• printf("The value of x is d\n", ptr1)
• printf("The value of x is d\n", ptr2)
• return 0

• Pointer variables have addresses of their own and
  can be pointed to

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Arrays

• Declaring an array of 10 integers
• int a10
• This declaration creates 10 consecutive integer
  elements named a0, a1, , a9
• Important Index numbers start at 0!

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Arrays (2)

• int a10
• int pnt
• pnt a3 / pnt now points to the 4th element
  of the array a /
• pnt a
• pa a0
• In each case, pnt is set to point to the initial
  element of the array. The name of an array is a
  synonym for its first element

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Example

• /
• foo4.c Pointers and Arrays
• /
• include ltstdio.hgt
• int main()
• int i
• int a5 1, 2, 3, 4, 5 / note this
  /
• int ptr, ptr1
• ptr a0
• ptr1 a / ptr1 points to the array a
  /
• for (i 0 i lt 5 i)
• printf("d d d d\n", ai,
  (ai), (ptri), ptr1)
• return 0

Output 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 5 5
5
From http//www.cs.usask.ca/grads/nic169/cshort/we
ek2/week2.html
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Strings

• Strings are stored as arrays of type char
• The last character of a string should be NULL
  (written as \0)

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Strings - example

• /
• foo5.c string constants
• Illustrates string constants array of chars
  another way of declaring a char array
• /
• include ltstdio.hgt
• int main()
• char a "I am a constant" / a pointer /
• char b "Hello" / an array /
• char c 'H', 'e', 'l', 'l', 'o', '\0'
• printf("s\ns\ns\n",a, b, c)
• return 0

From http//www.cs.usask.ca/grads/nic169/cshort/we
ek2/week2.html
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String Storage
From http//www.cs.usask.ca/grads/nic169/cshort/we
ek2/week2.html
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Strlen ()

• /
• foo6.c the Strlen() function
• /
• include ltstdio.hgt
• int Strlen (char s) / prototype /
• int main()
• char str1100
• char ptr "Hello World"
• / what happens if Hello World is the input /
• printf("Enter a string")
• scanf("s", str1)
• printf("d\n", Strlen(ptr))
• printf("d\n", Strlen("Hello World"))
• printf("d\n", Strlen(str1))
• return 0

• int Strlen (char s)
• int n
• for (n 0 s ! '\0' s)
• n
• return n

From http//www.cs.usask.ca/grads/nic169/cshort/we
ek2/week2.html
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String functions

• Stdio.h defines some useful functions
• gets(), puts(), getchar(), putchar()
• Other functions require another header file
• include ltstring.hgt
• For a handy list of functions look at under the
  string.h section of http//refcards.com/refcards/c
  /
• Further information about many functions can be
  found on lab machines by executing
• Eg. man 3 strlen

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Multi-dimensional arrays

• int b325 / 3-dimensional array /
• int a54
• A 2-dimensional array
• Compiler allocates space for 54 20 chars
• Individual integers can be accessed in a variety
  of ways

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Example

• /
• arr Multi-dimensional array referencing example
  
• /
• include ltstdio.hgt
• int main ()
• int a54 1,2,3,4, 5,6,7,8,
  9,10,11,12,
• 13,14,15,16,
  17,18,19,20
• int i 2
• int j 3
• printf("d\n", aij)
• printf("d\n", (ai j))
• printf("d\n", ((ai))j)
• printf("d\n", (a00 4i j))
• return 0

Output 12 12 12 12
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Pointer arrays

• static char name January, February,
  March, , December
• Different month names are different length, and
  here each is only allocated space as needed

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Multi-dimensional vs. Pointer

• int a1020
• A 2-dimensional array. 200 int-sized locations
  allocated
• int b10
• Allocates 10 pointers, but leaves them
  uninitialized
• Initialization must be done explicitly
• If each pointer in a points to a 20 element
  array, then 10 pointer spaces 200 int-sized
  spaces are allocated

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Command line arguments

• Declare main as a method with two parameters
• argc the number of parameters
• argv pointer to a list of strings that are the
  arguments
• int main (int argc, char argv)

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Example

• Execute
• ./a.out This is an example of a long list of
  command line arguments
• Output
• Argument 0 ./a.out
• Argument 1 This
• Argument 2 is
• Argument 3 an
• Argument 4 example
• Argument 5 of
• Argument 6 a
• Argument 7 long
• Argument 8 list
• Argument 9 of
• Argument 10 command
• Argument 11 line
• Argument 12 arguments

• /
• args print out command line arguments
• /
• include ltstdio.hgt
• int main (int argc, char argv)
• int i
• for (i 0 i lt argc i)
• printf ("Argument 3d\ts\n", i, argvi)
• return 0

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Function pointers

• Functions in C arent variables
• but we can still point to them
• int func (char s) / function prototype /
• (in code)
• int (ptr) (char s) func / function pointer
  /

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Example