Strings 20/11/2018. a.k.a. character arrays. Strings. Strings

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1 ECE 150 Fundamentals of Programming Outline 2 a.k.a. character arrays In this lesson, we will: Define strings Describe how to use character arrays for strings Look at: The length of strings Copying strings Concatenating strings Consider string operations, specifically distances Learn how to manipulate strings Look at other alphabets and Unicode Prof. Hiren Patel, Ph.D. Douglas Wilhelm Harder, M.Math. LEL hdpatel@uwaterloo.ca dwharder@uwaterloo.ca 2018 by Douglas Wilhelm Harder and Hiren Patel. Some rights reserved. 3 4 An array stores a list of values E.g., temperatures, voltages, positions, speeds, etc. Generally, each value has independent significance An array of characters, however, has the following properties: The significance comes from how the characters are strung together: post pots spot stop tops opts spto The characters come from a small alphabet If the characters of an array come from a fixed alphabet, the array is called a string of characters, or simply a string The alphabet for C++ strings is the set of all ASCII characters More inclusive strings use Unicode The length of a string is the number of characters One structure that could be used for strings is as follows: struct string_t { char *string; std::size_t length; std::size_t capacity; ; void string_init( string_t &str, std::size_t cap ) { str.capacity = cap; str.string = new char[str.capacity]; str.length = 0; The string itself may be shorter than the capacity of the array 1

2 5 6 C++ strings, however, are much simpler: They are an array of characters where the entry following the last character is the null character '\0' with a value of 0x00 '\0' must be included in the array length, but not the string length The string length is at least one less than the array length We will prefix all identifiers that are pointers to strings with "s_" Important: 'a' is a single null character "a" is an array occupying 2 bytes The first entry is 'a' and the second is '\0' Oddly enough, "\0" is an array occupying 2 bytes Both entries are '\0' We can determine the length of a string: std::size_t string_length( char *s_str ) { for ( std::size_t k{0; true; ++k ) { if ( s_str[k] == '\0' ) { return k; What happens if you forget the '\0'? 7 8 Suppose we have an argument: s_str E C E \0???? We initialize k to zero and step through the array s_str E C E \0???? Any characters after the '\0' are ignored k: 0 std::size_t string_length( char *s_str ) { for ( std::size_t k{0; true; ++k ) { if ( s_str[k] == '\0' ) { return k; std::size_t string_length( char *s_str ) { for ( std::size_t k{0; true; ++k ) { if ( s_str[k] == '\0' ) { return k; 2

3 9 10 When we get to the null character, we return: s_str E C E \0???? Question: What happens if you forget to include a null character? s_str E C E 1 5 0????? It will continue until it finds a '\0' (0x00) or it causes a segmentation fault k: 7 std::size_t string_length( char *s_str ) { for ( std::size_t k{0; true; ++k ) { if ( s_str[k] == '\0' ) { return k; std::size_t string_length( char *s_str ) { for ( std::size_t k{0; true; ++k ) { if ( s_str[k] == '\0' ) { return k; If we want to copy a string, we must determine its length and copy its characters over std::size_t length{string_length( s_str ); char *s_result{new char[length + 1]; Suppose we have an argument: s_str C + + \0 for ( std::size_t k{0; k <= length; ++k ) { s_result[k] = s_str[k]; std::size_t length{string_length( s_str ); char *s_result{new char[length + 1]; for ( std::size_t k{0; k <= length; ++k ) { s_result[k] = s_str[k]; 3

4 13 14 We find the length of the string s_str C + + \0 We allocate memory for a new array of the appropriate capacity s_str C + + \0 s_result length: 3 length: 3 std::size_t length{string_length( s_str ); char *s_result{new char[length + 1]; for ( std::size_t k{0; k <= length; ++k ) { s_result[k] = s_1[k]; std::size_t length{string_length( s_str ); char *s_result{new char[length + 1]; for ( std::size_t k{0; k <= length; ++k ) { s_result[k] = s_1[k]; Copy over the characters: s_str C + + \0 We return the address of the new array: s_str C + + \0 s_result C + + \0 s_result C + + \0 length: 3 length: 3 std::size_t length{string_length( s_str ); char *s_result{new char[length + 1]; for ( std::size_t k{0; k <= length; ++k ) { s_result[k] = s_1[k]; std::size_t length{string_length( s_str ); char *s_result{new char[length + 1]; for ( std::size_t k{0; k <= length; ++k ) { s_result[k] = s_1[k]; 4

5 17 18 If we want to concatenate two strings, we must determine the lengths and then copy them to a dynamically allocated array std::size_t length_1{string_length( s_1 ); std::size_t length_2{string_length( ); char *s_result{new char[length_1 + length_2 + 1]; Suppose we have two arguments: std::size_t k{0; for ( std::size_t i{0; i < length_1; ++i, ++k ) { for ( std::size_t i{0; i <= length_2; ++i, ++k ) { s_result[k] = [i]; std::size_t length_1{string_length( s_1 ); std::size_t length_2{string_length( ); char *s_result{new char[length_1 + length_2 + 1]; std::size_t k{0; for ( std::size_t i{0; i < length_1; ++i, ++k ) { for ( std::size_t i{0; i <= length_2; ++i, ++k ) { s_result[k] = [i]; We determine their lengths We allocate a new array length_1: 6 legnth_2: 10 std::size_t length_1{string_length( s_1 ); std::size_t length_2{string_length( ); char *s_result{new char[length_1 + length_2 + 1]; std::size_t k{0; for ( std::size_t i{0; i < length_1; ++i, ++k ) { for ( std::size_t i{0; i <= length_2; ++i, ++k ) { s_result[k] = [i]; str length_1: 6 legnth_2: std::size_t length_1{string_length( s_1 ); std::size_t length_2{string_length( ); char *s_result{new char[length_1 + length_2 + 1]; std::size_t k{0; for ( std::size_t i{0; i < length_1; ++i, ++k ) { for ( std::size_t i{0; i <= length_2; ++i, ++k ) { s_result[k] = [i]; 5

6 21 22 We copy over the first six characters: str length_1: 6 legnth_2: B j a r n e std::size_t length_1{string_length( s_1 ); std::size_t length_2{string_length( ); char *s_result{new char[length_1 + length_2 + 1]; We copy over all characters including '\0' from the second s_result length_1: 6 legnth_2: B j a r n e std::size_t length_1{string_length( s_1 ); std::size_t length_2{string_length( ); char *s_result{new char[length_1 + length_2 + 1]; std::size_t k{0; for ( std::size_t i{0; i < length_1; ++i, ++k ) { for ( std::size_t i{0; i <= length_2; ++i, ++k ) { s_result[k] = [i]; Notice we did not reset 'k' prior to starting the second loop We continue copying to where we left off std::size_t k{0; for ( std::size_t i{0; i < length_1; ++i, ++k ) { for ( std::size_t i{0; i <= length_2; ++i, ++k ) { s_result[k] = [i]; Printing of strings We return the address of the new array s_result length_1: 6 legnth_2: B j a r n e std::size_t length_1{string_length( s_1 ); std::size_t length_2{string_length( ); char *s_result{new char[length_1 + length_2 + 1]; std::size_t k{0; for ( std::size_t i{0; i < length_1; ++i, ++k ) { for ( std::size_t i{0; i <= length_2; ++i, ++k ) { s_result[k] = [i]; The std::cout object treats pointers to char as special The assumption if you want to print a pointer to a character, that that address is the address of a string If you want the address printed, you must indicate that it is a generic address: #include <iostream> int main(); Reinterpret the pointer to a char as a generic address type: void *p_generic_address; int main() { char *s_hi{"hello world!"; std::cout << s_hi << std::endl; std::cout << reinterpret_cast<void *>( s_hi ) << std::endl; return 0; Output: Hello world! 0x

7 25 26 Operations on strings Distances between strings There is a significant amount of work into strings Extracting or finding substrings Describing or finding patterns Matching case or not Defining whitespace and finding only whole words One important question is how similar are two strings? How close are two strings? Consider: "Et tu, Brute?" "t tu, Brute?" "Et ut, Brute?" "Et tu, Brune?" The Levenshtein distance is defined as the minimum number of edits required to convert one string to another One edit is defined as Inserting or removing a character Replacing a character Swapping two adjacent characters Distances between strings Distances between strings For example, you could use the Levenshtein distance to determine which words to suggest in a spell checker For example: incomprehssible is not a word, but What s wrong with this picture? incomprehssible incompressible This word is: One edit away from incompressible Two edits away from incomprehensible Recommend incompressible first incomprehssible incomprehesible incomprehensible The distance is context insensitive Ideas cannot be incompressible, so suggest the second first 7

8 29 30 Distances between strings Editing strings Recall the properties of the Euclidean distance: dist( A, B ) 0 dist( A, B ) = 0 if and only if A = B dist( A, B ) = dist( B, A ) dist( A, B ) dist( A, C ) + dist( C, A ) All of these properties hold for the Levenshtein distance between strings If we want to edit a string, it cannot be a literal string: #include <iostream> int main(); int main() { char *s_str{(char *)"Hello world."; s_str[12] = '!'; // segmentation fault!!! return 0; Editing strings Editing strings If we want to edit a string, it cannot be a literal string: #include <iostream> We can also swap two adjacent characters: #include <iostream> int main(); int main(); int main() { char *s_str{(char *)"Hello world."; std::cout << s_str << std::endl; int main() { char *s_str{(char *)"Hello wrold."; std::cout << s_str << std::endl; char *s_copy{string_copy( s_str ); // Replace the 11th character with '!' s_copy[11] = '!'; std::cout << s_copy << std::endl; delete[] s_copy; return 0; Output Hello world. Hello world! char *s_copy{string_copy( s_str ); // Swap characters 7 and 8 char ch{s_copy[8]; s_copy[8] = s_copy[7]; s_copy[7] = ch; std::cout << s_copy << std::endl; delete[] s_copy; return 0; Output Hello wrold. Hello world. 8

9 Editing strings 33 Editing strings 34 Erasing a character requires a for-loop: #include <iostream> int main(); int main() { char *s_str{(char *)"Hello world."; std::cout << s_str << std::endl; char *s_copy{string_copy( s_str ); // Remove the 5th character for ( std::size_t k{5; s_str[k]!= '\0'; ++k ) { s_copy[k] = s_copy[k + 1]; std::cout << s_copy << std::endl; delete[] s_copy; return 0; Output Hello world. Helloworld. Inserting a character requires a for-loop, but it also requires that the array is sufficiently large What happens if it is not? Exercise: What do you have to do to add two exclamation marks to the end of the string "Hello world!"? char *s_str{(char *)"Hello world!"; std::cout << s_str << std::endl; // How could you modify 'string_copy'? char *s_copy{string_copy( s_str ); Desired output: Hello world! Hello world!!! in other alphabets 35 in other alphabets 36 Other alphabets include: Morse code uses five characters: dot dash inter-character space inter-word space inter-sentence space Note: SOS is while the mayday SOS is inter-character spaces Western European alphabets often include additional characters on top of ASCII; however, Unicode allows for most alphabets German Swedish Italian Slovenian Polish Greek Russian Persian Gurmukhi ABCDEFGHIJKLMNOPQRSTUVWXYZÄÖÜß ABCDEFGHIJKLMNOPQRSTUVWXYZÅÄÖ ABCDEFGHILMNOPRSTUVZ ABCČDEFGHIJKLMNOPRSŠTUVZŽ AĄBCĆDEĘFGHIJKLŁMNŃOÓPQRSŚTUWXYZŹŻ ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩъыьЭЮ Я ی ه و ن م ل گ ک ق ف غ ع ظ ط ض ص ش س ژ ز ر ذ د خ ح چ ج ث ت پ ب ا ੳਅੲਸਹਕਖਗਘਙਚਛਜਝਞਟਠਡਢਣਤਥਦਧਨਪਫਬਭਮਯਰਲਵੜ 9

10 in nature 37 Summary 38 Even better, deoxyribonucleic acid (DNA) is a string with a four-characters alphabet: cytosine C guanine G adenine A thymine T All the algorithms developed by computer scientists for analyzing and manipulating strings were immediately transferable to the analysis and manipulation of DNA This is one of the beauties of abstraction Following this lesson, you now Know that strings are sequences of characters Those characters come from a fixed alphabet Know the most primitive means of storing strings are nullcharacter-terminated arrays of char Understand how to: Calculate the length of a string Copy a string Concatenate two strings Know that the last two require dynamic memory allocation Understand string distances Are aware that Simple strings are limited to ASCII Other languages require Unicode References 39 Colophon 40 [1] No references? These slides were prepared using the Georgia typeface. Mathematical equations use Times New Roman, and source code is presented using Consolas. The photographs of lilacs in bloom appearing on the title slide and accenting the top of each other slide were taken at the Royal Botanical Gardens on May 27, 2018 by Douglas Wilhelm Harder. Please see for more information. 10

11 Disclaimer 41 These slides are provided for the ECE 150 Fundamentals of Programming course taught at the University of Waterloo. The material in it reflects the authors best judgment in light of the information available to them at the time of preparation. Any reliance on these course slides by any party for any other purpose are the responsibility of such parties. The authors accept no responsibility for damages, if any, suffered by any party as a s_result of decisions made or actions based on these course slides for any other purpose than that for which it was intended. 11