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CMPT 102 Introduction to Scientific Computer Programming

CMPT 102 Introduction to Scientific Computer Programming. Pointers. Pointer Introduction. Pointer A special type of variable that holds the memory address of another variable Think about the memory in your computer

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CMPT 102 Introduction to Scientific Computer Programming

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  1. CMPT 102Introduction to Scientific Computer Programming Pointers

  2. Pointer Introduction • Pointer • A special type of variable that holds the memory address of another variable • Think about the memory in your computer • Conceptually, consider memory as a sequence of bytes (eight bit chunks, that can hold 8 zeros or ones) • All bytes are numbered, the first byte would be byte 0, the second byte 1 and so on • Variables of different types can have different sizes (use different numbers of bytes of memory) • The address of the variable is byte number in memory where the stored variable starts • You’ve seen pointers already • Call-by-reference parameters: Address of actual argument was passed

  3. What is a pointer? • Each variable is stored at some memory address, • Each location in memory has an address, that address can be represented as an integer • A pointer is a special type of variable that holds a memory address • A pointer is printed with a %p • A printed memory address looks like 0x22ccdc • The integer includes characters because it is shown in hexadecimal (base 16) • A pointer containing the address of a variable 'points to' that variable

  4. Addresses and Numbers • Pointers have their own types, these types • Hold addresses (represented by integers) • Point to variables of a particular type • There is a different pointer type to point to each type of variable • Even though an address is represented by an integer it is not of type integer • Therefore print with special conversion %p

  5. Data Types, pointers and integers • int is a data type • a set of objects, the integers • … -10, -9,-8, … ,123, 124, … • A set of operations that can be done on those objects • +, -, *, /, % … • A pointer to an integer is a data type • A set of objects, integers that represent the addresses of each location in the computers memory • A set of operations that can be done on those objects • +, - ,++, -- … • Does not make sense to %, * or / an address, %,*, / are not valid operations on addresses

  6. Declaring Pointer Variables • Pointers declared like other types • Add "*" before variable name • Produces "pointer to" that type • "*" must precede each variable that is to have a pointer type • int *v1p, *v2p, v1, v2; • v1p, v2p hold pointers to int variables • v1, v2 are ordinary int variables • Note that each pointer variable (reference) must have the * as the first character, the * is associated with the variable identifier not the type

  7. Declaring pointer variables • Pointer variables can point to only one type of variable int *v1p; // pointer to an integer double *v2p; // pointer to a double char *v3p; // pointer to a char myStruct *v5p; //pointer to a structure of // type myStruct

  8. Pointers and Style • It is useful to easily be able to see which variables in your function are pointers • C does not enforce any particular structure on your variable and pointer identifiers • To distinguish pointers from other variables a number of conventions are used in different coding standards • In this course the coding standard suggested is to assure that all pointer identifiers end in p to indicate they are pointers. • double myVariable, *myVariablep;

  9. Pointer Variables • Similarity of pointer variables and other types of variables • Can store value of the variable in a memory location • For pointer variables the address of the variable pointed to is contained in the associated memory location • For non pointer types the value of the variable is contained in the associated memory location • Not int, double, etc. • Instead: A POINTER to int, double, etc.!

  10. Pointer Variables • Example:1: double *v3p, *v5p, v1, v2, v3, v4, v5; 2: v1 = 76.2; v2 = 23.0; v3 = 11.9; 3: v4 = -23.7; v5 = -44.567; • v3p, v5p are declared as "pointer to double" variables • v3p can hold pointers to variables of type double • Cannot hold pointers to other types! • v1, v2, v3, v4, v5 are double variables • each double takes 8 bytes

  11. Declared variables in memory Value of variable Identifier of variable address 76.2 10001 v1 23.0 1008 v2 11.9 1016 v3 -23.7 1024 v4 -44.567 1032 v5

  12. Where do declared pointers point • When you declare a pointer int *v1p, v1; • Pointer variable v1p points to a random location in memory. • Why? v1p has not been initialized, it contains whatever was in the memory location associated with v1p before it was associated with v1p • This can cause serious problems, accessing and changing values that are not even associated with your program. • It is good programming practice to initialize all pointer to NULL. This means initialize the pointer to point nowhere. v1p = NULL;

  13. & Operator • & is the unary "address of" operator • &myVariable • Used to determine the address of myVariable • The value of the expression &myVariable is an address • Example int *v3p, *v4p, v1, v2;v1 = 23; v3p = &v1; • Sets pointer variable v3p to "point to" int variable v1 • Read like: • "v3p is assigned the address of v1" • Or "v3p points to v1"

  14. Pointer Variables • Example:1: double *v3p, *v5p, v1, v2, v3, v4, v5; 2: v1 = 76.2; v2 = 23.0; v3 = 11.9; 3: v4 = -23.7; v5 = -44.567; 4: v3p = &v3; 5: v5p = &v5;

  15. After line 3 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address 76.2 10001 v1 ? v3p 23.0 1008 v2 11.9 1016 v3 v5p ? -23.7 1024 v4 -44.567 1032 v5

  16. After line 4 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address 76.2 10001 v1 1016 v3p 23.0 1008 v2 11.9 1016 v3 v5p ? -23.7 1024 v4 -44.567 1032 v5

  17. After line 6 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address 76.2 10001 v1 1016 v3p 23.0 1008 v2 11.9 1016 v3 v5p 1032 -23.7 1024 v4 -44.567 1032 v5

  18. & Operator • & is the unary "address of" operator • Also used to specify call-by-reference parameter • For example the function declared below has one parameter. v1. that is called by reference. • int funSample(int *v1); • In the code calling this function • variables v1 and v2 are defined • int v1; int v2, int solution; • The function is called twice • solution = funSample(&v1) + funSample(&v2); • The arguments of the function calls (&v1, &v2) are references to or “pointers to” the variables used as arguments

  19. * Operator: dereferencing • * The dereferencing operator • This operator can only be applied to a pointer variable • Consider the pointer variable f2p that points to variable f2 (f2 = &f2p) • f2p hold the address of the memory location in which f2 is stored • There are two ways to refer to the value of variable f2 in an expression • f2 • *f2p

  20. "Pointing to" Example • Consider: v1 = 0; p1p = &v1;printf(“%d\n”, v1); *p1p = 42; printf(“%d, %d\n”, v1, *p1p); • Produces output:042 42 • *p1p and v1 both refer to the value in the same location in memory

  21. Pointer Assignments • Pointer variables can be "assigned":int *p1, *p2;p2 = p1; • Assigns one pointer to another • "Make p2 point to where p1 points" • Do not confuse with:*p1 = *p2; • Assigns "value pointed to" by p1, to "valuepointed to" by p2

  22. Pointer Assignments Graphic: Display 10.1 Uses of the Assignment Operator with Pointer Variables

  23. Another example • int *v1p, *v2p, v1, v2; • v1p = &v1; • *v1p = 47; • v2p = v1p; • *v2p = 23; • v2p = &v2; • *v2p = 111;

  24. After line 1 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address ? 10001 v1 ? v1p ? 1004 v2 v2p ?

  25. After line 2 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address ? 10001 v1 1000 v1p ? 1004 v2 v2p ?

  26. After line 3 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address 47 10001 v1 1000 v1p ? 1004 v2 v2p ?

  27. After line 4 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address 47 10001 v1 1000 v1p ? 1004 v2 v2p 1000

  28. After line 5 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address 23 10001 v1 1000 v1p ? 1004 v2 v2p 1000 NOTE: this also changes the value of *v1p

  29. After line 6 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address 23 10001 v1 1000 v1p ? 1004 v2 v2p 1000

  30. After line 7 Value of variable Identifier of variable Value of pointer variable Pointer Identifier address 23 10001 v1 1000 v1p 111 1004 v2 v2p 1000

  31. Define Pointer Types • Can "name" pointer types • To be able to declare pointers like othervariables • Eliminate need for "*" in pointer declaration • typedef int* IntPtr; • Defines a "new type" alias • Consider these declarations:IntPtr p;int *p; • The two are equivalent

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