Strings and Multi-Dimensional Arrays

Strings andMulti-DimensionalArrays Ethan Cerami New York University Ethan Cerami, NYU

Tuesday at a Glance • Tuesday: • Introduction to Strings • scanf v. gets • Basic String functions: strln, strcmp, strcat, and strcpy. • Basic Encryption • Extra: PGP (Pretty Good Privacy) Ethan Cerami, NYU

Introduction to Strings Ethan Cerami, NYU

Introduction to Strings • Strings are arrays of characters. • Hence, everything you know about arrays can be applied to Strings. Example: char name[]="ETHAN" Ethan Cerami, NYU

String Storage char name[]="ETHAN" E T H A N \0 0 1 2 3 4 5 This is the NULL terminator. Indicates the end of a String. name therefore has 6 elements. Ethan Cerami, NYU

Understanding the \0 • \0 always represents the end of a string. • It is added automatically. • The following are therefore equivalent: • char name[]="ETHAN"; • char name[] = {'E', 'T', 'H', 'A' ,'N', '\0'}; • If you have a five character name, C will always allocate six elements. Ethan Cerami, NYU

Inputting Strings:scanf v. gets Ethan Cerami, NYU

Inputting Strings • To input strings, you can use scanf() or gets(). • But it’s important to understand the difference between the two. Ethan Cerami, NYU

%s represents the string format specifier. scanf() scanf ("%s", name); • Note that there is no & before name. When reading in chars, floats, etc., you always need the &. • When reading in Strings, there is no & (more on this when we learn Pointers.) Ethan Cerami, NYU

scanf() continued • scanf reads in character data until the first white space character. • Whitespace = space, tab, or new line character. Ethan Cerami, NYU

Understanding Whitespace • For example, given the following program: scanf ("%s", name); printf ("Hello, %s!", name); • If I enter "Ethan Cerami", the program will print: Hello, Ethan! • That’s because scanf reads up until the first space character. Ethan Cerami, NYU

gets() • Gets: reads in character data until the new line character. • For example: gets (name); printf ("Hello, %s!", name); • If I enter "Ethan Cerami", the program will print: Hello, Ethan Cerami! Ethan Cerami, NYU

String.h Basic String Functions strlen, strcmp, strcat, and strcpy Ethan Cerami, NYU

strlen • C provides a strlen() function. • located in <string.h> • strlen returns the length of a string (does not include the \0 in its calculation.) • For example: char name[100]="Gore"; printf ("%d", strlen (name)); Output: 4 Note, however, that the size of the array is 100 Ethan Cerami, NYU

Comparing Strings • C provides a built-in function to compare two strings. • strcmp (str1, str2) fucntion • If str1 and str2 are identical, the function returns 0. Ethan Cerami, NYU

Example: Password Protection #include <stdio.h> #include <string.h> #include <conio.h> main () { char password[255]; printf ("Enter password: "); scanf ("%s", password); while (strcmp (password, "bluemoon") != 0) { printf ("Access Denied. Enter Password: "); scanf ("%s", password); } printf ("Welcome!"); getch(); } Ethan Cerami, NYU

Other String Functions • String.h includes lots of other string manipulation functions: • strcat(): appends one string onto the end of the other. • strcpy(): copies one string to another. Ethan Cerami, NYU

Using strcat() #include <stdio.h> #include <string.h> main () { char opening[255] = "And, the Oscar goes to... "; char winner[255] = "American Beauty"; strcat (opening, winner); printf ("%s", opening); getchar(); } Output: And, the Oscar goes to... American Beauty When using strcat, be careful that you do not overrun the array size. For example, do not append a 255 char word to opening. Ethan Cerami, NYU

Using strcpy #include <stdio.h> #include <string.h> main () { char word1[] = "And, the winner is...."; char word2[255]; strcpy (word2, word1); printf ("%s", word2); getchar(); } Output: And, the winner is.... This copies the contents of word1 into word2. Ethan Cerami, NYU

Encryption Ethan Cerami, NYU

Encryption • Encryption has been around for centuries. • Basic premise is to take a string of characters, and scramble the characters up. • We will look at three topics today: • Crypt-o-Text • PGP (Pretty Good Privacy) • Basic Encryption Program Note: Crypt-o-Text is available for free from www.savard.com. Ethan Cerami, NYU

Extra: About PGP • Developed by Phil Zimmerman • Stands for Pretty Good Privacy • Represents the defacto standard for encrypting email messages. Ethan Cerami, NYU

History of PGP • Developed in 1991. • Exporting encryption programs is considered the equivalent of exporting “munitions.” • PGP was distributed throughout the world via the Internet, and Phill Zimmerman was therefore under criminal investigation for three years. • Charges were eventually dismissed in 1996. • http://www.pgp.com/phil/phil-quotes.asp Ethan Cerami, NYU

Sending Email • “Today, electronic mail is gradually replacing conventional paper mail, and is soon to be the norm for everyone, not the novelty it is today. Unlike paper mail, E-mail messages are just too easy to intercept and scan for interesting keywords. This can be done easily, routinely, automatically, and undetectably on a grand scale.” – Phill Zimmerman Note: PGPPhone enables secure voice communication via the Internet. It’s also free. Ethan Cerami, NYU

Getting PGP • PGP is available for free at: http://web.mit.edu/network/pgp.html Ethan Cerami, NYU

Basic Encryption • Our program is real easy to crack, but it does illustrate the basics of encryption. • The program also provides lots of practice in array/string manipulation. • Complete code is available in the Course Packet, Program 5.3 Ethan Cerami, NYU

Program Architecture main encrypt printMenu decrypt getKey Ethan Cerami, NYU

Print the Introduction main () { int choice; /* Print Introduction */ printf ("Simple Text Scrambler\n"); printf ("======================\n"); /* Prime the while loop with user data */ printMenu(); scanf ("%d", &choice); while (choice != 3) { switch (choice) { case 1: encrypt(); break; case 2: decrypt(); break; default: printf ("Invalid Menu Choice."); printf ("Please try again.\n\n"); break; } printMenu(); scanf ("%d", &choice); } return 0; } While Loop for Menu Options

How to Encrypt a Word • Our program uses “ASCII Shifting” to encrypt words. • Assume the person enters “HELLO”, and key =5. Char ASCII + Key New ASCII New Char H 72 +5 77 M E 69 +5 74 J L 76 +5 81 Q L 76 +5 81 Q 0 79 +5 84 T Encrypted Word Ethan Cerami, NYU

First, we prompt the user for the key, and the clear_text word. We then encrypt the word one letter at a time. We do this by copying each letter and shifting up the ASCII table. void encrypt (void) { int passkey, index; char clear_text[255]; char encrypted_text[255]; passkey = getKey(); printf ("Enter Word (ALL CAPS): "); scanf ("%s", clear_text); /* Encrypt word by "shifting up" the ASCII Table */ for (index=0; index < strlen (clear_text); index++) encrypted_text[index] = clear_text[index] + passkey; /* Make sure to terminate the encrypted_text with a NULL character */ encrypted_text[index] = '\0'; printLine(); printf ("Encrypted String: %s\n", encrypted_text); printLine(); } If we forget the \n terminator, the string may never end.

How to Decrypt a Word • To decrypt a word, we shift down the ASCII table. • Assume the person enters “MJQQT”, and key =5. Char ASCII + Key New ASCII New Char M 77 -5 72 H J 74 -5 69 E Q 81 -5 76 L Q 81 -5 76 L T 84 -5 79 O Decrypted Word Ethan Cerami, NYU

We prompt for the pass key and the encrypted word. void decrypt (void) { int passkey, index; char clear_text[255]; char encrypted_text[255]; passkey = getKey(); printf ("Enter Encrypted Word: "); scanf ("%s", encrypted_text); /* Decrypt word by "shifting down" the ASCII Table */ for (index=0; index < strlen (encrypted_text); index++) clear_text[index] = encrypted_text[index] - passkey; /* Make sure to terminate the clear_text with a NULL character */ clear_text[index] = '\0'; printLine(); printf ("Decrypted String: %s\n", clear_text); printLine(); } We then decrypt the word one letter at a time. We do this by copying each letter and shifting down the ASCII table.

Thursday at a Glance • Thursday: • Introduction to Multi-Dimensional Arrays • Declaring, initializing and referencing multi-dimensional arrays. • Putting it all together: Basic Example • Hunt the Wumpus: Advanced Example Ethan Cerami, NYU

Introduction to Multi-Dimensional Arrays Ethan Cerami, NYU

Why Use Multi-Dimensional Arrays? • So far we have only looked at one dimensional arrays. • Example: float stock[5]; • But, what if we want to track stocks for more than one company? • Answer: use multi-dimensional arrays. Ethan Cerami, NYU

Arrays and SpreadSheets • Arrays are used to represent tables of data. • They are therefore very similar to spreadsheets. Columns Rows Ethan Cerami, NYU

Examples • There are lots of potential applications for multi-dimensional arrays: • tracking stock portfolios. • tracking the temperature in multiple cites. • tracking all students grades or registered courses. • tracking news preferences on a web site. Ethan Cerami, NYU

How many Dimensions? • In C, you can create arrays with more than two dimensions. • For example, you can create a six dimensional arrays. • For this course, however, we will stick to one and two dimensional arrays. Ethan Cerami, NYU

Declaring / Initializing Multidimensional Arrays Ethan Cerami, NYU

Declaration • Template: • data_type var_name [number of rows][number of columns] • Examples: • float stocks[3][5]; • Track three different companies over five days. • int temp [10][5]; • Track ten different city temperatures over five days. Ethan Cerami, NYU

Initialization • To initialize a multi-dimensional arrays, use embedded brackets. • Examples: int a[2][2] = {{1,2}, {3,4}}; will create this array 1 2 3 4 Ethan Cerami, NYU

Initialization (cont.) • If there are not enough initializers for a row, the remaining elements are set to 0. • Example: int a[2][2] = {{1}, {3,4}; will create this array 1 0 3 4 Ethan Cerami, NYU

Referencing Multi-Dimensional Arrays Ethan Cerami, NYU

Referencing Elements • To reference a single dimensional array, you provide a single index value. • printf ("%f", stock[3]); • To reference a multi-dimensional array, you provide two index values: • printf ("%f", stock[1][2]); • Again, it is useful to think of spreadsheets. Ethan Cerami, NYU

Passing Multi-Dimensional Arrays to Functions Ethan Cerami, NYU

Passing Arrays • To pass a multidimensional array: • indicate the array with double brackets. • must indicate the number of columns (number of rows is optional, ignored) • Example: • void makeHot (int temp[][5]); Ethan Cerami, NYU

Putting it all together Ethan Cerami, NYU

Putting it all together • Let’s look at an example of creating, initializing, and passing a multi-dimensional array. • This program tracks temperatures in three cities over five days. • The function modifies the array by adding 10 degrees to each temperature value. Ethan Cerami, NYU

#include <stdio.h> #define NUMCITIES 3 #define NUMDAYS 5 void makeHot (int [][NUMDAYS]); main () { int i,j; int temp[NUMCITIES][NUMDAYS] = { {67, 68, 69, 70, 71}, {44, 45, 46, 47, 48}, {31, 32, 33, 34, 35}}; makeHot (temp); for (i=0 ; i<NUMCITIES; i++) { for (j=0; j< NUMDAYS; j++) printf ("\t%d", temp[i][j]); printf ("\n"); } } Function Prototype: We must indicate the number of columns. Double brackets tell the compiler that this is a two dimensional array. Array declaration and initialization. Whenever printing out the contents of a multi-dimensional array, you must use a nested for loop.

void makeHot (int thermo[][NUMDAYS]) { int i,j; for (i=0; i< NUMCITIES; i++) for (j=0; j< NUMDAYS; j++) thermo[i][j] += 10; } To iterate through all elements in a multi-dimensional array, we use a nested for loop. Output: 77 78 79 80 81 54 55 56 57 58 41 42 43 44 45

Strings and Multi-Dimensional Arrays