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Understanding Pointer Arithmetic in C: Definitions, Types, and Examples

This resource covers the concept of pointers in C programming, particularly focusing on pointer arithmetic. It defines pointers as variables referencing memory addresses and explains their associated types. The document elaborates on operations that can be performed on pointers, including addition and subtraction, and how these operations relate to the size of the data type being pointed to (e.g., int, float). Additionally, it discusses the differences between NULL, 0, and '', providing clarity on their uses. A range of examples illustrates how pointer arithmetic works.

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Understanding Pointer Arithmetic in C: Definitions, Types, and Examples

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  1. Pointer Arithmetic CSE 2541 Matt Boggus

  2. Pointer definition • A variable whose value refers directly to (or "points to") another value stored elsewhere in the computer memory using its address • Memory addresses • Z+ • Positive, whole number • Why do pointers have an associated type? Ex: • int *p float * p char * p

  3. From earlier material… int array[10]; int *ap = array + 2; // ap points at &array[2] 1. ap 2. *ap 3. ap[0] 4. ap+ 6 5. *ap+ 6 6. *(ap + 6) 7. ap[6] 8. &ap 9. ap[-1] 10. ap[9]

  4. Pointer arithmetic example intarray[] = { 1, 2, 3 }; int*array_ptr = array; // try different types printf(“First: %i\n”, *(array_ptr++)); printf(“Second: %i\n”, *(array_ptr++)); printf(“Third: %i\n”, *array_ptr);

  5. Pointer arithmetic • When you add to or subtract from a pointer, the amount by which you do that is multiplied by the size of the type the pointer points to. • For the previous slide, each increment adds 1 times sizeof(int) • p++ => value of p = old value of p + 4

  6. Examples

  7. NULL vs 0 vs ‘\0’ • NULL is a macro defined in several standard headers • Only used for pointers • Defined as ((void *) 0) in most stdlibararies • 0 is an integer constant • 0 acts as the generic symbol for every type’s zero value • '\0' is a character constant • Referred to as nul • Only exists if you #define it yourself

  8. NULL and void – ((void *) 0) • 0 – the value of a NULL pointer (using casting) • VOID – no type • Cannot use arithmetic on a void pointer (no sizeof) • Cannot dereference (no type to return / unable to determine how many bytes to consider) • NULL is defined as 0 cast to a void * pointer (NOT an uninitialized pointer) • Note that NULL and void are separate concepts • char *p=0; • char *t=NULL; • int i=0; • char *q=(char*)i; // char * not the same type/sizeof as int * • char *q=0 • char *q=(char*)0

  9. L and R values • L-value (appearing on the left side of an assignment) • A place (i.e. memory location) for a value to be stored • R-value (appearing on the right side of an assignment) • A value or expression that simplifies to a value a = b+25 vs b+25 = a inta[30]; a[b+10]=0; inta, *pi; pi = &a; *pi = 20;

  10. L and R values Given: char ch = ‘a’; char *cp = &ch;

  11. Pointers to Arrays • <data type> (*<name of ptr>)[<an integer>] • Ex: a pointer ptr to an array of 5 integers. • int(*ptr)[5]; • Ex: an array of 5 int pointers • int*ptr[5];

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