Lecture 7: Arrays BJ Furman 06OCT2012

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

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

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

What is a Pointer? - 1 Variables in general  Variable declaration informs compiler of two things: 1. Name of the variable 2. 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) int var1 = 0; 10FE Var address 0 intvar1 Var valueVar typeVar name Symbol Table Memory (8-bit) Address x10FE 0x10FF Bit x1100 0x1101

Bit Memory (8-bit) Address x10FE 0x10FF What is a Pointer? - 2 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 short int i, j; i = 3; j = i; 1100 short intj 10FE Address short inti ValueTypeName Symbol Table

What is a Pointer? - 3 Pointer variable  A variable that contains an address Declaring a pointer variable  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" #include 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; } &num1 means: the address for the variable num1 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.

Accessing What a Pointer Points To The indirection operator * gets the value at the address stored in the pointer #include 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; }

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?)

ASCII Table

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)

What is an Array? 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: #include 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]); 0x%p\n",&test[i]); } return 0; } Format (1D array)  type array_name [num_elements]; Ex. Array of 4 characters named, 'test'

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

Accessing Array Elements 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] #include 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]); 0x%p\n",&test[i]); } return 0; } 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

Initializing Array Elements 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 /* array_practice3.c */ #include 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]); 0x%p\n",&nums[i]); } return 0; }

Arrays of Multiple Dimensions - 1 Arrays can have more than one dimension  2D matrices commonly used in linear algebra /* array_practice5.c */ #include 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; }  Declaration for 2D 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"

Arrays of Multiple Dimensions - 2 Arrays of more than one dimension, cont.  Declaration for 3D array: int Ary3[n x ][n y ][n z ] ; 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[n x -1][n y -1][n z -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"

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

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

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

Passing an Array to a Function 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 #include void PrintArray(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 */ void PrintArray(int num_elem, char array[]) { int i; for(i=0; i<num_elem; i++) { printf("test[%d]==%c",i,array[i]); 0x%p\n",&array[i]); }

Review

References Jensen, T. (2003). A Tutorial on Pointers and Arrays in C, available from: m.. Visited 02OCT m Parlante, N. (1999) Pointer Basics, [online]. Available from: Passing Arrays to Functions, [online]. Available from: six.co.uk/c/f3_passing_arrays_to_functions.asp. Visited 12MAR six.co.uk/c/f3_passing_arrays_to_functions.asp Visited 03OCT