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Understanding Arrays & Pointers in C Programming

Learn about arrays, pointers, memory allocation, pointer variables, accessing array elements, and practical examples in C programming. Includes practice exercises.

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Understanding Arrays & Pointers in C Programming

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  1. Lecture 7: Arrays BJ Furman 06OCT2012

  2. The Plan for Today • Announcements • Review of variables and memory • Arrays • What is an array? • How do you declare and initialize an array? • How can you use an array? • Array Examples • Array Practice

  3. Announcements • Midterm this week in lab • Bring: text, notes, HW, lab reports, calculator • Covers everything through pointers • Lab project 7 after midterm

  4. Learning Objectives • Explain what a pointer is (review) • Explain what an array is • Declare and initialize an array • Use an array in a program

  5. Memory (8-bit) Address 0x10FE 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0x10FF 0x1100 0x1101 Symbol Table Var name Var type Var address Var value var1 int 10FE 0 Bit 7 6 5 4 3 2 1 0 What is a Pointer? - 1 int var1 = 0; • Variables in general • Variable declaration informs compiler of two things: • Name of the variable • Data type of the variable • Actions caused: • Bytes allocated in memory • Symbol table with name, address, and value • Can think of a variable as having two "values" • Value of what is stored at the memory location (rvalue) • Value of the memory location (its address) (lvalue)

  6. Variables in general, cont. Assigning a value to a variable (i = 3;) Value is copied to the memory location at the address listed in the symbol table Using a variable in an expression (j = i;) Value of what is stored at the memory location is accessed What is a Pointer? - 2 Memory (8-bit) Address 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0x10FE 0x10FF Symbol Table Name Type Address Value i short int 10FE Bit 7 6 5 4 3 2 1 0 j short int 1100 short int i, j; i = 3; j = i;

  7. What is a Pointer? - 3 Pointer variable A variable thatcontains anaddress Declaring apointervariable type * varname int* ptr1; float *ptr2; char * ptr3; Read as: "ptr1 is a pointer to an integer" "ptr2 is a pointer to a float" "ptr3 is a pointer to a char" &num1 means:the address forthe variable num1 • #include <stdio.h> • int main() • { • int num1; • int *ptr1 = &num1, *ptr2; • num1 = 7; • printf("size of num1: %d\n",sizeof(num1)); • printf("value of num1: %d\n",num1); • printf("address of num1: %p\n",&num1); • printf("address in ptr1: %p\n",ptr1); • return 0; • } • What is the size of num1? • Is &num1 == ptr1? • What address is stored in ptr2? Always initialize pointers you declare! An uninitialized pointer is like a loaded gun pointed in a direction that you don’t know.

  8. Accessing What a Pointer Points To #include <stdio.h> int main() { int num1; int *ptr1 = &num1; num1 = 7; printf("value of num1: %d\n", num1); printf("value of num1: %d\n", *ptr1); return 0; } • The indirection operator * gets the value at the address stored in the pointer

  9. Pointer - Practice 1 • Declare: • x_ptr, a pointer to an integer • y_ptr, a pointer to a double • What do the following statements do? • char *my_ptr, my_char1 = 'c', my_char2; • my_ptr = &my_char1; • my_char2 = *my_ptr; What is in: • my_ptr (in declaration? in second line?) • my_char1 • my_char2 (in declaration? in last line?)

  10. ASCII Table

  11. Why Use Pointers? • Allows a function to modify variables in the calling function (without using global variables) • Return more than one value from a function call • Pass a pointer to a large data structure rather than copying the whole structure • Arrays (coming next)

  12. What is an Array? #include <stdio.h> int main() { int i; char test[4]; test[0]='M'; test[1]='E'; test[2]='3'; test[3]='0'; for(i=0; i<4; i++) { printf("test[%d]=%c",i,test[i]); printf(" @ 0x%p\n",&test[i]); } return 0; } • So far we've dealt with scalar variables • contain just one value • Arrays are collections of data of the same type that occupy contiguous memory locations • Individual values in the collection are called elements of the array • Need to declare before use: • Format (1D array) • typearray_name [num_elements]; • Ex. Array of 4 characters named, 'test'

  13. What is an array? - 2 int nums [10]; • 10 element array of integers Element no. 3 is accessed by: nums [2] because indexing begins at 0

  14. Accessing Array Elements #include <stdio.h> int main() { int i; char test[4]; test[0]='M'; test[1]='E'; test[2]='3'; test[3]='0'; for(i=0; i<4; i++) { printf("test[%d]=%c",i,test[i]); printf(" @ 0x%p\n",&test[i]); } return 0; } • Individual elements of an array are accessed by their index number ** Important Note** • Array indexing starts at zero(take note of this, it is easy to forget!) • char test[4]; • the first element is test [0] • the fourth element is test [3] • Be careful! • The C compiler may not stop you from indexing beyond the boundary of your array. (What could happen?) What is test[4]? array_practice2.c

  15. Initializing Array Elements /* array_practice3.c */ #include <stdio.h> int main() { int i; int nums[5]={0,1,2,3,4}; for(i=0; i<5; i++) { printf("nums[%d]=%d",i,nums[i]); printf(" @ 0x%p\n",&nums[i]); } return 0; } • Use braces to enclose the elements • Separate elements by commas • Can set number of elements in declaration: • Explicit: int nums[5] • Implicit: int imp[ ] = {1, 2}; • Read from the keyboard • Ex. array_practice3.c • Note addresses • Ex. array_practice4.c • Read from the keyboard

  16. Arrays of Multiple Dimensions - 1 /* array_practice5.c */ #include <stdio.h> int main() { int i,j; int my_array1[2][3]={{0,1,2},{3,4,5}}; for(i=0; i<2; i++) { printf("\n"); for(j=0; j<3; j++) printf("%d ", array1[i][j]); } return 0; } • Arrays can have more than one dimension • 2D matrices commonly used in linear algebra • Declaration for2D array: • int my_array1[number_of_rows][number_of_columns]; • Enclose each row of initializers in its own set of braces • Note method to print and access individual elements • Can think of array1 as a "2-element array of 3 elements"

  17. Arrays of Multiple Dimensions - 2 • Arrays of more than one dimension, cont. • Declaration for 3D array: • int Ary3[nx][ny][nz]; • Enclose each row of initializers in its own set of braces • Order of storage: far right subscript increments first (called, ‘row-major’ order) • Ary3[0][0][0] • Ary3[0][0][1] • Ary3[0][0][2] • etc. to Ary3[nx-1][ny-1][nz-1] • To find the element number for Ary3[x][y][z] declared to be of size [I] [J] [K]: • ex. If I=J=K=10, Ary3[2][0][7]--> 2(10*10)+0+7+1=208th • Ex. array_practice6.c • Initializes a 4x3x2 element array • Can think of array2 as a "4-element array of 3x2 element arrays"

  18. Arrays - Practice 1 • Declare: • an array of 100 characters named char100 • a 3x4 array of doubles named data1 • Given intmy_array1[2][3]={{0,1,2},{3,4,5}}; • Find • my_array1[1][2] • my_array1[0][0] • my_array1[0][3]

  19. Array Names #include <stdio.h> int main() { int i=0; char test[]={'M','E','3','0'} printf("test[%d]==%c",i,test[i]); printf(" @ 0x%p\t",&test[i]); printf("test[%d]==%c", i, *test); printf(" @0x%p\n", test); for(i=1; i<4; i++) { printf("test[%d]==%c",i,test[i]); printf(" @ 0x%p\n",&test[i]); } return 0; } • An array name by itself is interpreted as a constant pointer to the first element of the array. • &test[0] isequivalent to test • A constant pointer to the first element of test • test[0] is equivalent to *test • first element of test array_practice9.c

  20. Pointer Arithmetic • Pointer arithmetic • array[n] is equivalent to *(array + n) • Using just the name 'array' is equivalent to a constant pointer to the first element of array (i.e., base address) • Dereferencing the expression, where an integer n is added to 'array', is equivalent to indexing the array to its element at subscript n • array names cannot be changed, but pointer variables can be changed • may not have: array_name = &var; • the array name is a constant pointer and may not be changed

  21. Passing an Array to a Function #include <stdio.h> voidPrintArray(int elements, char array[]); int main() { /* initialize the array */ char test[]={'M','E','3','0'}; /* get the size of the array */ int num_elem=sizeof(test)/sizeof(char); /* pass array to function */ PrintArray(num_elem, test); return 0; } /* PrintArray() function definition */ voidPrintArray(int num_elem, char array[]) { int i; for(i=0; i<num_elem; i++) { printf("test[%d]==%c",i,array[i]); printf(" @ 0x%p\n",&array[i]); } } • Prototype and function header need: • data type • array name • [ ] • Function call with actual name of array • Note: in the function prototype and function header char *array would also work

  22. Review

  23. References Jensen, T. (2003). A Tutorial on Pointers and Arrays in C, available from: http://home.netcom.com/~tjensen/ptr/pointers.htm. .Visited 02OCT2009 Parlante, N. (1999) Pointer Basics, [online]. Available from: http://cslibrary.stanford.edu/106/ Passing Arrays to Functions, [online]. Available from: http://irc.essex.ac.uk/www.iota-six.co.uk/c/f3_passing_arrays_to_functions.asp. Visited 12MAR2010. http://www.asciitable.com/. Visited 03OCT2009.

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