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Pointers Character Values
- There are a fixed number of char values
Pointers Character Values There are a fixed number of char values - - PDF document
Pointers Character Values There are a fixed number of char values - - PDF document
Pointers Character Values There are a fixed number of char values Examples: a b \n Number of possibilities : the number of chars 1 A char Variable Pointer Values There are a fixed number of pointer values Assume a 32 bit
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Pointer Values(Constants) The & operator
& is the address operator
- Provides a pointer constant to any named
location
- Example: &aChar
- Use %p to print an address
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Printing Pointers
What does it print ? 142300 142301
Pointers : 4 byte Integers
Address of a variable is: Address of the first byte of the variable
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Pointer Variables
Possible to define variables holding pointers Pointer Variables:
– Contain a pointer constant (i.e., an address) – Can have value changed – “Point” to a specific type of data – Many variables can point to the same value!
Pointer Variables
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Schematic : Pointer Variables Accessing the Variables
The Operator * is the Indirection operator “Dereferencing” often read as “contents of” p = &a; c = *p + 43; Add 43 to (*p) what p points to Assign this number to variable c
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Examples : Get them right !! Declaring Pointer Variables
Examples:
char *p; int *q; float *r; long double *s; long long int *t;
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Uninitialized Pointers
As with all variables in C: If you don’t initialize you get whatever junk is found at that time !!!
BEWARE:
Uninitialized pointers contain some address
Uninitialized Pointers
p could be even pointing to your program ???
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Initializing : Be safe - to nowhere
What if a pointer variable shouldn’t point anywhere? Answer: set it to NULL int *p; p = NULL; NULL is pre-defined by “C” in stdio.h or stddef.h
Initialization to Somewhere
int a; int *p; p = &a; Remember :
- Never try to dereference a NULL pointer
- Gives run-time error (a segmentation fault ??)
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Example : Add using pointers
Work out the code to set r = a + b…
*pr = *pa + *pb;
Pointers as Parameters
C uses pass by value!! To modify a passed variable: Pass the address Modify the contents In other words : Pass the pointer !!!
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Returning Pointer Variables
Suppose we want to return a pointer? Then declare the return type to be a pointer Example:
int *smaller (int *p1, int *p2);
It returns a pointer to an integer...
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Returning Pointer Variables Local Pointers : A Serious Error
Never return a pointer to a local variable float *mistake() { float temp = 12; return &temp; } Points to space on stack that isn’t used !!
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Pointers to pointers
If a pointer variable “points” somewhere:
Why not to another pointer variable??
- Ans. Of course you can do it.
int **p;
This variable pointer to a pointer which points to an integer...
Pointers to Pointers
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Using Pointer to Pointers…
Declare code for r, q, p, a where a is a float float a, *p, **q, ***r ;
Using Pointer to Pointers…
Use scanf with each of these variables scanf(“%d”, &a) scanf(“%d”, p) scanf(“%d”, *q) scanf(“%d”, **r)
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Sizes and Compatibility
char c; char *pc; int a; int *pa; double x; double *px; Print sizeof(c), sizeof(pc), sizeof(*pc)…
Size and Compatibility
sizeof(c)= 1 sizeof(pc)= 4 sizeof(*pc)= 1 sizeof(a)= 4 sizeof(pa)= 4 sizeof(*pa)= 4 sizeof(x)= 8 sizeof(px)= 4 sizeof(*px)= 8
What can you learn :
- Generally speaking Pointers are all the same size
- They may be of diff. sizes for structures etc.
- The contents of a pointer know their size & type
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Types & Validity Type and Validity : Get it right
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Using Expressions
Expressions evaluate to a value Expressions are classified by: How they can be used There are two kinds of expressions : lvalue and rvalue expressions lvalue and rvalue Either Assigned or Evaluated An lvalue expression is to be used Whenever it is receiving a value Valid on the LHS LHS can ONLY have an lvalue An rvalue expression is to be used To supply a value for further use Valid on the RHS of an assignment
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The 7 lvalues
(Blue not covered yet)
If returning an address function call 7 Structure indirect selection expression->name 6 Structure selection expression.name 5 Dereference expression *expression *(A+7) 4 Expression must be lvalue (expression) (A+7) 3 Array indexing Identifier[] A[i] 2 Variable identifier Identifier x 1 Comments Expression Type
lvalue expressions
a = … a[5] = … (a) = … *p = …
All non-lvalue expressions are rvalue expressions
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Some Operators Require lvalues Anything that can be changed
x= 1 y +=4
Assignment (left operand)
++x --y
Prefix increment / decrement
x++ y--
Postfix increment / decrement
&score
Address Operator Examples Type of Operator
Mistakes
The following are rvalue expressions where there should be an lvalue.
a+2 is an rvalue can’t be modified a+2 is rvalue address requires lvalue Same as above Postfix inc. needs an lvalue Prefix inc. needs an lvalue a+2 = 6; &(a+2); &4; (a+2)++; ++(4-b); Problem Expression
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