name：Yulu Yang（or use name“Ruby”）account：yuy113pw：960531Wdxxm
more details on：https://canvas.usask.ca/courses/9771/assignments/20294
The University of Saskatchewan Saskatoon, Canada Department of Computer Science CMPT 214– Programming Principles and Practice Assignment 3 Date Due: October 5, 2020 Total Marks: 15 Submission Instructions • Assignments must be submitted using Canvas. • Programs must be written in C conforming to the C11 standard. • Always include the following identification in your solutions: your name, NSID, student ID, instruc- tor’s name, course name, and course section number. • No late assignments will be accepted. See the course syllabus for the full late assignment policy for this class. • VERY IMPORTANT: Canvas is very fragile when it comes to submitting multiple files. We insist that you package all of the files for all questions for the entire assignment into a ZIP archive file. This can be done with a feature built into the Windows explorer (Windows), or with the zip terminal command (LINUX and Mac). We cannot accept any other archive formats. This means no tar, no gzip, no 7zip. Non-zip archives will not be graded. We will not grade assignments if these submission instructions are not followed. • Instructions on "how to create zip archives" can be found here https://canvas.usask.ca/courses/ 9771/pages/how-to-zip-slash-compress-your-files https://canvas.usask.ca/courses/9771/pages/how-to-zip-slash-compress-your-files https://canvas.usask.ca/courses/9771/pages/how-to-zip-slash-compress-your-files 1 Background 1.1 Reading and Writing Files in C A file is a container in computer storage devices used for storing data. You can access the contents of a file using a few functions in C. Reading input data from a file is very often much more convenient than reading input interactively from the console, especially when more than a handful of input data values are needed. 1.1.1 Types of Files There are two types of files you should know about: • Text files • Binary files 1.1.2 Text files Plain text files or just text files are files that are created using any simple text editors such as Notepad. The contents within the file is stored as plain text and the contents of the files are readable by humans without any special decoding. Generic text files often have the extension .txt, but many other types of files are plain text files, including program source code files (.c, .py, .java), and many system configuration files in LINUX. On some LINUX systems, even passwords are stored in plain text files (although the passwords are stored as encrypted plain text strings). Plain text files store everything as text strings. In particular, numbers are stored in plain text files as strings of digit characters ’0’ through ’9’, and possibly a ’.’ for a decimal point, or a ’-’ character to indicate a negative number. 1.1.3 Binary files Binary data files store data values in a raw binary format identical to their binary representations in computer memory. Such files are not readable by humans. Data stored in binary form (0’s and 1’s) instead of plain text. For example, a 32-bit signed integer is stored in a binary file using exactly the same bit pattern as it has in the computer’s memory and every such representable number takes up 4 bytes (32 bits) in the binary file, regardless of how many digits it has. Whereas, a 32-bit integer in a plain text file takes up a number of bytes equal to the number of digit characters needed to represent it as a string. As a result, binary files that store numeric data generally take up less space than storing the same data in a text file. For now, however, we are only going to concern ourselves with reading and writing text files in C. We might come back to this topic and work with binary files on a future assignment. 1.2 Basic File Operations In C, the basic operations on files are: • Opening a file: – Creating a new file (usually for writing). – Opening an existing file (usually for reading). • Reading data from a file. • Writing data to a file. • Closing a file. Page 2 1.2.1 Opening a File Before C can work with a file, the file has to be opened. When a file is opened, it must be specified whether it is to be opened for reading or writing. A file opened for reading cannot be written to, and a file open for writing cannot be read from. To open a file in C, the function fopen() must be called. fopen() is declared in stdio.h, so we must include this header. The function prototype for fopen() is: FILE* fopen(char* path , char* mode); It’s behaviour is that fopen() opens the file name specified by the string path, and whether it is opened for reading or writing is determined by the string mode. Once the file is opened, fopen() returns a pointer to something with data type FILE. The exact nature of type FILE is not important right now1, but this pointer that we get to the data entity of type FILE is how we refer to an open file. We will pass this pointer to the functions that read from, write to, and close files so that those functions know which open file we want to perform the operation on, since we could potentially have more than one open file at a time! Type FILE is also defined in stdio.h. So to open a file in C, the code would look something like this: FILE *fp; fp = fopen ("file_name", "mode"); You can see that we first declare a variable to hold the pointer to the FILE. The variable fp is a pointer to a data entity of type FILE. Such a variable is called a file stream or sometimes a file handle. Think of it as a name for the open file. You can also see that we assign the result of fopen() to fp. If the file "file_name" was opened successfully, fp will now be a valid file stream. If it was not opened successfully, then fopen() will return NULL (the "null" pointer). It is always good practice to make sure that fopen() does not return NULL because any operation performed on a file descriptor whose value is NULL will cause a program crash. Finally, the "mode" determines whether the file is opened for reading or writing. The possible values for "mode" are summarized in Table 1. These file modes are for opening text files. Other modes exist that allow reading and writing of binary files, and also for opening files for both reading and writing at the same time, but we omit these details for now. The table also describes how fopen() behaves for each file mode, and the conditions under which opening will fail, causing fopen() to return NULL. 1Type FILE is actually a C Structure. We discuss C structures in topic 10. Mode Purpose "r" Open a file for reading. When a file opened for reading its contents cannot be modified. If the given path specifies a file that does not exist, the opening fails and fopen() returns NULL. "w" Open a file for writing. If the given path specifies a file that does not exist, then a new file with the name given is created. If the given path names a file that already exists, the file is deleted and a new empty file is created. If an existing file cannot be erased in this manner, or a new file cannot be created (for example, due to file permission settings) then the opening fails, and fopen() returns NULL. "a" Open a file in append mode. If the given path specifies an existing file, it is opened with its existing contents intact, and any future writes to the file are added to the end of the existing file. If the given path specifies a file that does not exist, a new file is created. If a file cannot be opened or created (for example, due to file permission settings) then the opening fails and fopen() returns NULL. Table 1: Mode specifiers that can be passed to fopen(). Page 3 Example 1 This code fragment illustrates how to open a file for writing. #include
int main() { FILE *fp; fp = fopen("file_name.txt", "w"); if(fp == NULL) { // File open failed. printf("file_name.txt␣could␣not␣be␣opened␣for␣writing .\n"); return 1; } // File opened successfully. return 0; } Since the file mode was "w" in this example, the file file_name.txt is created in the current working directory at the time of program execution (any existing file of that name in that directory is first deleted). If desired, you can specify the full relative or absolute path to where you want to create your file. Opening a file for reading is identical, except that the file mode passed to fopen() would be "r" instead of "w". 1.2.2 Reading and Writing to/from a Text File For reading and writing to a text file, we use the functions fprintf() and fscanf(). These functions are identical to printf() and scanf() except that fprint() and fscanf() take an additional argument of type FILE* that specifies the file stream of the file that should be read from or written to. Their prototypes are: fprintf(FILE *stream , char *format , ...); fscanf(FILE *stream , char *format , ...); The first argument is the file stream to read/write, the second argument is the format string (identical to printf(), scanf()), and any remaining arguments are variables that are matched with format specifiers in the format string. Indeed, printf() is identical to calling fprintf() with the file stream stdout (standard output — the console), and scanf() is identical to calling fscanf() with the file stream stdin (standard input — the console). stdin and stdout are pre-defined variables of type FILE* defined in stdio.h. fprintf() and fscanf() otherwise follow the same behaviour as printf() and scanf(). Example 2 This example shows how to write a number entered by the user to a text file called file_name.txt. #include #include int main() { int num; FILE *fp; fp = fopen("file_name.txt","w"); // Caution: we should make sure fp is not NULL here! Page 4 // Read a number from the console. printf("Enter␣num:␣"); scanf("%d",&num); // Write the same number to the file file_name.txt fprintf(fp,"%d",num); fclose(fp); return 0; } Example 3 Read from the text file created from the previous example and prints it onto the screen. #include #include int main() { int num; FILE *fp; fp = fopen("file_name.txt","r"); fscanf(fp,"%d", &num); printf("Value␣of␣n=%d", num); fclose(fp); return 0; } 1.2.3 Limitations of Reading Strings with fscanf() It is important to understand that fscanf() has the same limitations when reading strings as scanf(). fscanf() will stop reading a string as soon as any whitespace is encountered, including a newline. Thus, fscanf() cannot read strings containing spaces. 1.2.4 Closing a file In order to ensure that no data is