CPS 196 Introduction to Computer Programming: C

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CPS 196Introduction to Computer Programming C
addresses pointers examples
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Overview

• Addresses of variables
• Basic types Pass-by-value
• Arrays Pass-by-reference
• Pointers
• scanf
• a function that modifies an array
• a function that modifies an int
• more examples

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Addresses of Variables

• Variables allocate space in memory.
• Each memory location has an address.
• Different variables will have different addresses.

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Primitive (Basic) Data Types Pass-By-Value

• For basic data types such as int, float, char,
  double, only the value is sent to a function.
• This is called "pass-by-value".
• The function uses a local variable to hold this
  value.
• Caller can't access this local variable.
• If the function changes the value, the caller
  will never know.

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Arrays Pass-By-Reference

• Arrays are "passed-by-reference".
• The function shares the array sent it doesn't
  make a copy of the array for itself.
• Any change in the array is made directly in the
  caller's array.
• All changes will immediately and permanently
  affect the caller's array.

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Pointers

• Pointers hold memory addresses as their values.
• The address of a pointer variable and the address
  value it holds are different things.
• The type of the pointer variable, and the type of
  the thing it is pointing to are also different.
• C uses pointers everywhere.

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Pointer Example

• n an int (integer) variable
• p an int (integer pointer pointer to integer)
  variable.

int n 5 int p n
5
n
int
integer
0x0F00
0x0F00
integer pointer pointer to integer
0x0F04
p
int
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Adress-of Operator and Indirection Operator

• The address-of operator gives the address of a
  variable (or array element)
• int p
• p n // address of n an int
• Address is really a pointer.
• The indirection operator gives back the thing
  pointed to
• int m p // m gets n an int

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Pointer Arithmetic

• Pointer's view of the memory is as if the memory
  only contains the type of value that this pointer
  points to (int, double, char, etc).
• We can move to the next location or previous
  location by using , --
• This will go to the memory location of
  next/previous int, double, char, NOT the next
  byte in memory.

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Pointer Arithmetic Example

• If sizeof(int) is 4 (each int takes four bytes of
  memory)
• Code Value of p
• p n 0x0F00 (address of n)
• p 0x0F04 (4 bytes)
• p 0x0F08 (4 bytes)
• p-- 0x0F04 (-4 bytes)
• p 2 0x0F0C (8 bytes)
• p p - 3 0x0F00 (-12 bytes)

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Pointer Arithmetic and Arrays

• The pointer arithmetic matches with array access.
• This is actually how C handles arrays.
• An int array is a non-modifiable int (integer
  pointer).
• The elements of the array will be accessed using
  pointer arithmetic.

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Pointer Arithmetic and Arrays An Example
int ar3 1, 4, 9 int p ar / Now,
these are equivalent ar0 p addr
of ar0 (an int) ar1 p 1 addr
of ar1 (an int) ar2 p 2 addr
of ar2 (an int) ar0 p
ar0 (an int) ar1 (p 1)
ar1 (an int) ar2 (p 2)
ar2 (an int) /
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How scanf Works

• We have seen operator in scanf
• scanf("d", you) / you's address /
• This sends the address of you to the scanf
  function.
• This address is an integer pointer.
• scanf can find you and change you now, because...

It knows where you live !!
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A Function Without Pointers

• This function does not work

void reset(int n) n 0

• Here, n is a local variable in the reset
  function.
• Changing it does not affect the caller.

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Same Function With Pointers

• This one works

void reset(int p) p 0

• This function needs the address of an integer
  variable, or an integer pointer value.
• This affects the caller's variable.

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Functions With and Without Pointers

• These functions are called differently

void reset(int n) n 0 int main() int
n 5 reset(n) / n is still 5 /
printf("d", n) return n
void reset(int p) p 0 int main()
int n 5 reset(n) / n is now 0 /
printf("d", n) return n
5
0
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Array Addresses and Pointers

• Array, sent to a function is also handled with a
  pointer.
• Instead of int, you could use int, and your
  function will work exactly the same way.
• You can send pointer values to a function that
  takes an array.
• You can send an array to a function that takes a
  pointer.

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Array Addresses and Pointers A Warning

• Warning If a function tries to access many
  "array elements" and you just send the address of
  a single variable, the function may access memory
  locations that don't hold variables!

n
0x0F00
?
0x0F04
?
0x0F08
?
0x0F0C
?
0x0F10
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Array Addresses and Pointers A Warning An
Example
void reset3(int ar) ar0 ar1 ar2
0 int main() int k 5 reset3(k)
// bad idea!! return 0
0
k
0xF200
0
?
0xF204
0
?
0xF208

• This buggy program accesses memory locations that
  are not defined.
• It may crash!

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Pointer Examples 1
int k 5 int p p k p 0 // k is now
0 p 3 // k is now 3 p p 2 // k is
5 p 4 // k is 20 p 5 // k is
25
double a 5.5 double dp dp a dp 0.5
// a is now 0.5 dp 3.3 // a is now
3.3 dp p 2.25 // a is 5.55 dp 4
// a is 22.2 dp 5.8 // a is 28.0
char ch 'A' char cp cp ch cp 'Z' //
ch is 'Z' cp - 2 // ch is 'X' cp ' ' //
ch is ' '
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Pointer Examples 2
int j 3 int k 5 int p // no init val p
j // p is j p -p // j is now -3 p
k // p is k p 0 // k is now 0 // (j,
k) (-3, 0)
char hi3 'h', 'i', '\0' char cp hi
// cp is hi0 printf("c", cp) // 'h' cp
// cp is hi1 printf("c", cp)
// 'i' cp // cp is hi2 if
(cp '\0') // true printf("!\n") //
"!\n"
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Pointer Examples 3
char hi3 'h', 'i', '\0' char cp hi
// cp is hi0 while(cp ! '\0')
printf("c", cp) cp printf("\n") //
"\n"
char hi3 'h', 'i', '\0' char cp hi
// cp is hi0 while(cp ! '\0') //
till '\0' char printf("c", cp) // print
char go 1 printf("\n") // "\n"
h
hi
hi
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Pointer Examples Errors
double d double dp int n int p p 0 p
n n p 5 p 5 p 2.5 dp d dp
2.5 p d d dp 5 p 25 dp d
Error p not initialized (points to random memory
location) OK. p now points to n Error p points
to n, p is n, but n was not initialized! OK.
now, n is initialized, to 5 Error p points to
int p is int (but 2.5 is not) OK. dp now
points to d OK. d now has the value 2.5 Error p
is int, not double Error (modulus) operator
only works with ints (dp is double) Error p is
int (address), not an int Error dp is double,
not a double
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Pointer Examples Functions

• Functions can only return one value.
• You can also return values using pointer
  arguments (parameters).

char getch() return getchar() int main()
char ch getch() printf("c", ch)
printf("c", getch()) return 0
void setch(char cp) cp getchar() int
main() char ch setch(ch)
printf("c", ch) return 0
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Pointer Examples Functions

• Here is a function that sets two values

/ gets next char into cp. If it is a digit,
also sets p, to that digit's integer value
/ void getdigit(char cp, int p) cp
getchar() if (cp gt '0' cp lt '9')
p cp - '0' int main() char ch
int digit -1 getdigit(ch, digit)
printf("c --gt d", ch, digit) return 0
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Pointer Examples Functions

• Here is a different version of the same function

/ gets and returns next char. If it is a
digit, also sets p, to that digit's integer
value / char getdigit2(int p) char c
getchar() if (cp gt '0' cp lt '9')
p cp - '0' return c int main()
char ch int digit -1 ch
getdigit(digit) printf("c --gt d", ch,
digit) return 0
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Pointer Examples Functions

• Does this version run the same way?

/ gets next char into cp. If it is a digit,
also returns that digit's integer value / int
getdigit(char cp) int dgt cp
getchar() if (cp gt '0' cp lt '9')
dgt cp - '0' return dgt int main()
char ch int digit -1 digit
getdigit(ch) printf("c --gt d", ch,
digit) return 0
No! There is a bug. If character is not a
digit dgt is never initialized!! (it will have a
random value)