Week 8 Arrays and File I O BJ Furman 21OCT2009 The Plan for Today Arrays What is an array How do you declare and initialize an array How can you use an array Array Examples Array Practice File I O Learning Objectives Explain what an array is Declare and initialize an array Use an array in a program How to open and close files for reading and writing 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 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 Format 1D array type array name num elements Ex Array of 4 characters named test 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 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 Be careful The C compiler may not stop you from indexing beyond the boundary of your array What could happen Initializing Array Elements array practice3 c 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 practice4 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 Arrays of Multiple Dimensions 1 Arrays can have more than one dimension 2D matrices commonly used in linear algebra Declaration for 2D array 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 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 my array2 nx ny nz Enclose each row of initializers in its own set of braces Order of storage far right subscript increments first my array2 0 0 0 my array2 0 0 1 my array2 0 0 2 etc to my array2 nx 1 ny 1 nz 1 Find element number for my array2 x y z x JK y K z 1 Ex array practice6 c Initializes a 4x3x2 element array Can think of array2 as a 4 element array of a 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 Notes on Arrays An array name that is not followed by a subscript is interpreted as a pointer to the first element of the array test 0 is equivalent to test pointer to first element of test test 0 is equivalent to test first element of test 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 Notes on Arrays 2 Pointer arithmetic array n is equivalent to array n Using just the name array is equivalent to a 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 Array File I O Arrays are often used with data consisting of many elements Too tedious to handle I O by keyboard and monitor File I O is used instead File I O and array example Opening a File Key concepts Declare a pointer to FILE e g FILE fp Use fopen function with path to the file and file mode as arguments fp fopen file name txt r Some common modes see reference below for others r for reading from a file Returns NULL if the file does not exist w for writing to a file Creates the file if it does not exist or wipes out contents of the file if it already exists a to append to a file Creates the file if it does not exist or starts writing at the end of the file if it already exists Use the file pointer as an argument in functions like fscanf fprintf etc Good idea to test the file was opened without error Test the return value of fopen fopen will return NULL if there is an error opening the file Reference http www cppreference com wiki c io fopen Closing a File Function header int fclose FILE fp Good idea to test the file was closed without error Test the return value of fclose fclose will return EOF if there is an error closing the file Make sure you close all files that you opened somewhere in your program Reference http www cppreference com wiki c io fclose Review References Jensen T 2003 A Tutorial on Pointers and Arrays in C available from http home netcom com tjensen ptr point ers htm Visited 02OCT2009 http www asciitable com Visited 03OCT2009