Arrays and Pointers.

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Vincent Whitehead
5 years ago
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1 Arrays and Pointers

2 What is an array? First element a a[0] a[1] a[2] a[3] a[4] a[5] a[6] a[7] a[8] a[9] Name of array Array index

3 Array Declaration const int MAXY = 20; double x[100], y[maxy], z[maxy+2]; char name[40]; These are also called as static arrays, why? Their size if specified before execution!

4 Bound problems in C/C++ arrays int b[10], r; r = b[10]; b[-1] = 100; Are these legal statements? Should it be allowed?

5 Arrays In C/C++, name of array is address of first element of array If you create an array a of int; int a[10]; Then a is address of a[0], that means, a is &a[0], and *a is a[0]

6 Arrays and Pointers a = &a[0] = base address of array = 1024 &a[9] = a+9*size of int = *4 = 1060 a[0] a[1] a[2] a[3] a[4] a[5] a[6] a[7] a[8] a[9] a This means a+i is &a[i], and (a+i) is a[i]

7 Arrays and Pointers We can navigate through arrays elements using pointers- Or Or

8 Arrays and Pointers Note that *p++, gives increment to p, if you want to increment data pointed by p, then write (*p)++! Can we have code like this? int a[10]; for (i=0; i<10; i++) cin >> *a++;

9 Exercise Write a function to copy one string to another string, for examplechar name1[30]= DAIICT ; char name2[30]= Gandhinagar ; char* name; I want to add name2 to name1, such that it becomes DAIICT, Gandhinagar and store it into string name. before doing this allocate enough memory to name. How do I do it?

10 Practice Pointers Char Array based strings and linked list are good exercises to get more confidence on pointers, and using heap based (dynamically storage allocated) variables

11 String as char Array Write functions for Determining length of a string Concatenation String copy, ensuring that target string does not run out of memory Safe String Concatenation, ensuring that target string does not run out of memory And so,

12 example: strncpy void strncopy(char*, char*, int); void main() { char name[30], buffer[80]; cin.getline(buffer); strncopy(name, buffer, 29); cout << name; } void strncopy(char* dest, char* source, int max) { int i = 0; while ( source[i]!= '\0' && i < max ) { dest[i] = source[i]; i++; } dest[i] = '\0'; }

13 example: strncpy void strncopy(char*, char*); void main() { char name[30], buffer[80]; cin.getline(buffer, 80); strcopy(name, buffer); cout << name; } void strcopy(char* target, char* source) { while ( *source ) *target++ = *source++; *target = '\0'; } //More explicitly you could write while condition //while ( *source!= \0 ) //Assumption: target has sufficient memory allocated

14 example: strncpy Two version; First one is compact while second is more verbose void strcopy(char* target, char* source) { while ( *source ) *target++ = *source++; *target = '\0'; } void strcopy(char* target, char* source) { while ( *source!= \0 ){ *target = *source; target++; source++; } *target = '\0'; }

15 const and pointers Pointers to cons objects Pointers, we have seen so far, is that value of object to which it points, can be changed However value of const can not be changed, therefore pointer to consts has to be different, it is declared likeconst double PI = 3.14; double p = &PI; //is invalid const double *p = &PI; //is ok (*p)++, or *p=12.45; //are invalid

16 const and pointers Pointers to cons objects Note that const can be used before or after data typeconst double PI = 3.14; double const PI = 3.14; const double *p = &PI; double const *p = &PI;

17 const and pointers Constant Pointers Pointers, which can point to only one object, they can not point to another object int x = 10, y = 100; int* const p = &x; //then p = &y; or p++ //are invalid

18 Dynamic Arrays Are the arrays whose size is bound at run time, and memory is allocated from heap Below is how you can create one, char* a = new char[n]; Thereafter you can treat them as these arrays in the same manner you treat static arrays You would be deleting memory allocated to dynamic arrays like this delete [] a;

19 Advantage: Advantages/Disadvantages of dynamic array You can have the array of size exactly required, which is normally becomes known at run-time Disadvantage: You may loose base address of array, leaving garbage, heap allocation, which is considered to have more overhead

20 Thanks

Dynamic Allocation in C

Dynamic Allocation in C C Pointers and Arrays 1 The previous examples involved only targets that were declared as local variables. For serious development, we must also be able to create variables dynamically,