” EIE110编程 写作、C/C++程序设计 写作Homework 3 — String FunctionsCS110/EIE110/LP101 2020 Fall1 IntroductionThere are many interesting functions on strings that we can implement, for example the famousCaesar Cipher can encrypt and decrypt strings, whose idea is simply adding or subtracting a fixedvalue from each character, as depicted by the above graph found from the internet[1].We will write a C program that will perform different operations on strings. The program is organizedusing multiple functions and files. The emphasized knowledge aspects in this homework include thefollowing:Function design and implementation.c and .h files. A program with multiple files.Friendly interaction between the computer and the user.Users menu of choicesShow friendly and clear messages to a user.Clear input queue when needed. This is a technique to avoid input confusion.When users input is wrong, show some error message, and ask the user to do input again.Algorithms. We will implement some simple encryption and decryption algorithms.2 Tasks of this homework2.1 prepare multiple source files.The program should have multiple files, whose names and brief descriptions are listed below.myStrLib.h : function declarations (prototypes) of Section 2.2.strShape.h : function declarations of Section 2.3.strCypher.h : function declarations of Section 2.4.strShape.c : function definitions of Section 2.2.myStrLib.c : function definitions of Section 2.3.strCypher.c : function definitions of Section 2.4.driver.c : function definitions of Section 2.5With these .h and .c files, a function defined in one .c file can be used by another .c file where thecorresponding .h file is included ( #included ).2.2 Implement several string library functionsC standard library provide many helpful functions on string computation, they can be used includingthe head string.h which contains the prototypes of these functions. In this homework we willimplement our versions of some string library functions, with the same parameters, return value, andcomputation process as the standard library functions. At least three library functions should beimplemented, which are described by their prototype and comments as follows:unsigned int my_strlen(const char str[]);/* my version of the strlen() function.*/char * my_strcpy( char *dest, const char *src);/* my version of the strncpy() function httpss://en.cppreference.com/w/c/string/byte/strcpy */int my_strcmp(const char * str1, const char * str2);/* my version of the strcmp() function. httpss://en.cppreference.com/w/c/string/byte/strcmp */In your program, the three library functions strlen , strcpy , and strcmp should not be used, theyshould all be replaced by your versions.In addition, you can implement other functions in string.h as you like Detailed documents of thestring library functions can be found online at some websites [2]like www.cppreference.com .2.3 Some design and implement several other tool functionsWe wll design several functions related to string input and output. Their prototypes and computationdescriptions are as follows.int input_long_str(char storage[], unsigned int sizeLimit, int endMark)The function will record users input from the keyboard. The input can contain white spacesand multiple lines.The recorded input is saved in the array storage as a true C string.sizeLimit should be the size (number of elements) of storage . So, at mostsizeLimit – 1 characters of users input can be recorded in storage , one more spacefor the null character ( \0 ) of a C string.Recording the input will end in two cases:a) The endMark signal appears (returned by getchar() ), which is chosen by user of thisfunction. It could be some special character , or the EOF signal.b) Or, the number of input characters recorded is more than sizeLimit – 1 .void clear_input_queue(void)This function will try to clear the input queue of the stream stdin , as discussed in class.void print_str_at_center(const char str[]);Given a string, which can contain multiple lines, print these lines in some centered way. Forexample, when the augment is a string of three lines:123456789abcgoodthe function will prints:123456789abcgoodThe longest line does not need to indent, but the shorter lines need to indent for some spaces.void print_str_in_rectangle(const char str[], unsigned int row_width);Given a string str , print its characters in a rectangle where each row contains row_widthcharacters.Each newline characters and Tab character in str is printed as a space in the printing. Thereason of this requirement is that, the newlines make the square less compact, not goodlooking; and the width of a Tab character on different systems is different, making theprinting effect less predictable.For example, given the argument stringlong long time agoI can still rememberhow the music used to make me smileWith row_width as 7, the printing will be :“`long long timeago Ican still remember how the music used to make mesmile“`You can design some other interesting functions about printing a string.The my versions of these standard library functions should behave exactly the same as the originallibrary functions.2.4 Implement cypher functions for strings.We will implement some cypher functions. A simple encryption of a string is sorting its characters.Correspondingly, the decryption function is to recover the original string from the sorted string. Forexample, given a string:long long time agoAfter sorting, the sorted string (cyphertext) will look like the following, where the three spaces are onthe left end.aegggillmnnoooWe have the following observationsBy this kind of sorting, the ending null character is not moved, still appears at the last position inthe string.The sorted string and the original string have the same number of characters. No character isadded or removed.The operation of a sorting string can be understood as sorting its characters around.The sorting is in-space, which means changing the array in its space, no separate array isgenerated.2.4.1 Cypher by selection sortselection sort is an efficient and simple sorting algorithm. We can find good explanation of theselection sort at wikipedia .[3]We use the selection sort to do encryption. However, in order to recover the original string, we haveto remember the history of how a character is moved in the process of sorting.The following pseudo-code describe a modified version of selection sort so that the history ofmoving characters is recorded.Algorithm Name : Selection sort with historyParameters:arr : A character arrayarrlen : The number of elements in arr . Especially, if arr is a C string, then arrlen isthe string length (not counting the ending \0).history : An array of integers, whose space should be large enough to save arrlen – 1integers.Computation：j = 0 /* array index start from 0 to arrlen-1*/while j arrlen-1 /* only need to move arrlen – 1 times */find a smallest item in the inclusive range arr[j] … arr[arrlen-1] , which meansfrom the jth element of arr, and the last element of arr. Record the position v of thissmallest item.swap arr[j] and arr[v] .history[j] = vj = j + 1The following algorithm can recover a sorted array of characters back to its original sequenceaccording to some history of the sorting.Algorithm Name : Recover sorted array of selection sort by history of sortingParameters:arr : A character array, the sorted array to be recovered.arrlen : The number of elements in arr . Especially, if arr is a C string, then arrlen isthe string length (not counting the ending \0).history : An array of integers, whose space should be large enough to have arrlen – 1integers.Computation：Using the history, the the characters of arr move back to their original positions beforethe sorting. You can have to provide the details of doing so.You need to write two functions that implement the encryption and decryption algorithm describedabove.ss_encrypt(char arr[], unsigned int arrLen, unsigned int history[]);ss_dncrypt(char arr[], unsigned int arrLen, unsigned int history[]);Put their definitions in strCypher.c , and prototypes in strCypher.h .2.5 Design the user interface and test the functionsThe driver.c file will implement the user interface and test all the functions that you created.When the program is running, the user will repeatedly see a menu, and the program will respond tothe choice of the user, until the user choose to quit. The choice of the menu and the correspondingcomputation of the program is described as follows:a: Input two strings.The user is asked to provide two strings and they are recorded. This is to test theinput_long_str() function. Then the three functions of my_strlen() , my_strcpy() , andmy_strcmp() are called using the two recorded input strings, and the results are printed. Youcan decided the computation details. As long as the the three functions are called it will be fine.b: Print strings in shapesPrint the first string centered, and the second string in rectangle. This is to test the two functionsprint_str_centered() and print_str_in_rectangle() .c: Use the cypherCall the function ss_encrypt() with the recorded strings, and show the results of encryption.Call ss_decrypt() with the cypher text, show that the original string can be recovered.q: quit the program.If you have designed and implemented some other functions, make sure that they are also tested.SubmissionUpload your files at the webpage address of this homework on Moodle.The uploaded files can include only the following:The source files (.c and .h files),An optional document file (.txt, .md, .docx) describing your work, like: what features of thehomework are achieved; what are the remaining problems; how did you solve the difficultiesthat you met, some screen record of compiling and running the program …About homework submission in a group:at most 3 students can form a group to submit the homework. You can surely do thehomework alone.One group only need to submit the homework by once by one student. The other membersdo not need to submit anything, or just submit one .txt file saying who are the members ofthe group and who submitted the homework.In each file that you submit, record the names in Chinese (if you are not an internationalstudent) and English letters, classes of each student, last 5 digits of student ID, as如有需要，请加QQ：99515681 或WX：codehelp
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