1 Array, Pointer and Reference ( I ) Ying Wu Electrical Engineering and Computer Science Northwestern University EECS 230 Lectures Series

2 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?

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

4 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 ]

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

6 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] x 10 bytes = 40 bytes char c[10] x 10 bytes =10 bytes Declaring multiple arrays of same type int b[ 100 ], x[ 27 ];

7 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

8 String: an Array of char s 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'

9 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

10 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' )

11 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?

12 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!

13 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' );

14 Let’s program! #include using std::cin; using std::cout; using std::endl; void main() { // problematic code chara[10]; cout << “\nInput: “; cin >> a; cout << “what you input is: “ << a; // right solution cin.ignore(); charbuffer[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 }

15 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( )

16 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!