Array, Pointer and Reference ( I )

EECS 230 Lectures Series Array, Pointer and Reference ( I ) Ying Wu Electrical Engineering and Computer Science Northwestern University yingwu@ece.northwestern.edu

How do you store your data? • We have learnt: • Declare a variable • A variable in memory stack • Get input and store the value in a variable • Output the value of a variable int a; cin >> a; cout << “the value of a is: “ << a << endl; char c; cin >> c; cout << “the value of c is: “ << c << endl; cout << “the value of c is: “ << (int)c << endl; Question: what if I want to input/store a data set?

What we want … • Input/store a “word”, rather than a “letter”? • Input/store a dataset, rather than a single datapoint? ‘h’ ‘h’ ‘e’ ‘l’ 25 ‘l’ ‘o’ 3.1415

Good news! • Array • Consecutive group of memory locations • Same name and type • To refer to an element, specify • Array name and position number • Format: arrayname[ position number ] • First element is located at position 0 • n element array c: c[ 0 ], c[ 1 ]…c[ n - 1 ] • Array elements are like normal variables c[ 0 ] = 3; cout << c[ 0 ]; • Performing operations in subscript. If x = 3, c[ 5 – 2 ] == c[ 3 ] == c[ x ]

Core Concept of Array Name of array (Note that all elements of this array have the same name, c) c[0] -45 c[1] 6 c[2] 0 c[3] 72 c[4] 1543 c[5] -89 c[6] 0 c[7] 62 c[8] -3 c[9] 1 c[10] 6453 c[11] 78 Position number of the element within array c

Declaring Arrays • Declaring an array: • Name? Type? Number of elements? • Examples int c[ 10 ]; float hi[ 3284 ]; • What will happen once you declare an array? • The O/S will allocate (reserve) a group (consecutive) of memory units for this array. • You need to specify the # of elements • Once the #of elements is specified, you can not change it.Why? … • Let’s see how many memory are allocated: • int a[10] ------- 4 x 10 bytes = 40 bytes • char c[10] ------- 1 x 10 bytes =10 bytes • Declaring multiple arrays of same type int b[ 100 ], x[ 27 ];

Initializing an Array • Initializers int n[ 5 ] = { 1, 2, 3, 4, 5 }; • If not enough initializers, rightmost elements are set to be 0. • If too many initializers, a syntax error is generated int n[ 5 ] = { 0 } • Sets all the elements to 0 • If size omitted, the initializers determine it int n[] = { 1, 2, 3, 4, 5 }; • 5 initializers, therefore n is a 5 element array

String: an Array of chars • Strings • Arrays of characters (including letters, digits, special characters +, -, * …) • How do we know the end of a string? • All strings end with NULL ('\0') • Examples: char string1[] = "hello"; char string1[] = { 'h', 'e', 'l', 'l', 'o‘, '\0’}; • Subscripting is the same as for a normal array string1[ 0 ] is 'h' string1[ 2 ] is 'l'

Confusion: ‘a’  a, “a”  ‘a’ • Question: ‘a’ ?= a • Answer: NO, unless … • Why? • ‘a' means the letter a, which is 97, numerically. • Question: “a” ?= ‘a’ • Answer: NO, NO • Why? • “a” means a string, i.e., a set of characters • “a” actually is ‘a’ and ‘\0’ • So, size(“a”) is 2 bytes, while size(‘a’) is 1 byte • So, to hold a word with length N, you need a string of N+1 bytes

Initializing a String • String assignment • Character array: char color[] = "blue"; • What is the # of elements of color? • 4 or 5? • Answer: Creates 5 element char array, color, (last element is '\0')

Input a sentence? • Assign input to character array word[20] cin >> word • Reads characters until whitespace or EOF • Question: how many chars can word hold? • Answer: 20 or 19? • Question: what if I input more than enough? • Answer: exceed array size, and it may crash your program! • Question: how to solve this problem? • Answer: cin >> setw(20) >> word • Question: can this method read a sentence? • Answer: NO • Question: so, how can I solve it?

A Closer look at cin/cout • cin/cout work with strings or char arrays char str[10]; cin >> str; cout << str; • But … • cin keeps getting data until it meets a space, a return or a ‘\0’ • If I input more char than the size of the array, cin will still keep inputting, while it may be DANGEOUS! • cout keeps outputting data until it meets a ‘\0’ • It does not check the size of the array! • If there is no ‘\0’ in the array, what can you imagine? • So, be careful!

cin.getline() • cin.getline() • Prototype: cin.getline( array, size, delimiter character); • Copies input into specified array until either • One less than the size is reached • The delimiter character is input • Examples char sentence[ 80 ]; cin.getline( sentence, 80); cin.getline( sentence, 80, '\n' );

Let’s program! #include <iostream> using std::cin; using std::cout; using std::endl; void main() { // problematic code char a[10]; cout << “\nInput: “; cin >> a; cout << “what you input is: “ << a; // right solution cin.ignore(); char buffer[500]; cout << "\nInput a command line: "; // prompt for input cin.getline(buffer, 500); // get input from keyboard cout << “what you input is: “ << buffer << endl; // echo }

What have we learnt today? • The core concept of array • An array in memory • Initializing an array • String – char array • Why string is quite special? • cin/cout • cin.getlint( )

Q. for today: A Bad Example int main( ) { int size; cin >> size; int arr[size] for(int i=0;i<size;i++) arr[i] = i; return 0; } • I know what you want to do. But this method won’t work. • So, how can we do that? • Keep it and we will see in later lectures!

Array, Pointer and Reference ( I )