KAIST
EE 209: Programming Structures for EE

Assignment 2: String Manipulation

(Acknowledgment: This assignment is borrowed and modified from Princeton's COS 217)

Purpose

The purpose of this assignment is to help you learn/review (1) arrays and pointers in the C programming language, (2) how to create and use stateless modules in C, (3) the "design by contract" style of programming, and (4) how to use the GNU/UNIX programming tools, especially bash, emacs, gcc, and gdb.

Rules

Implement the string functions listed by the Table in Part 1. and Part 2 is the "on your own" part of this assignment, which is worth 50% of this assignment.

You will get an extra 10% of the full score if you implement the Part 1 only with pointer notation when you access the character. See the Extra credit section below (Extra Credit for Part 1).

Background

As you know, the C programming environment provides a standard library. The facilities provided in the standard library are declared in header files. One of those header files is string.h; it contains the declarations of "string functions," that is, functions that perform operations on character strings. Appendix D of the King textbook, Appendix B3 of the Kernighan and Ritchie textbook, and the UNIX "man" pages describe the string functions. The string functions are used heavily in programming systems; certainly any editor, compiler, assembler, or operating system created with the C programming language would use them.

Overview of Your Task

Your task in this assignment is to use C to create the "Str" module that provides string manipulation functions. Specifically, design your Str module so that each function behaves the same as described below. Your task in this assignment is threefold.

[Part 1] Read the description of the basic string library functions carefully, and implement each function. The basic functions are most commonly used standard string functions. Each function should behave the same as its corresponding standard C function.

[Part 2] Implement a simplified version of grep using Str functions. Read the provided file that contains skeleton code carefully, edit the file to make it process the required functionalities: find, replace, diff.

Part 1: The Basic Str Function Implementation

Your task for the first part is to use C to implement five basic string manipulation functions: StrGetLength(), StrCopy(), StrCompare(), StrSearch(), StrConcat(). Those five functions should follow the format of the corresponding standard C library functions. You can easily find the function's description and operation in the UNIX "man" page.

The following table shows the required basic string functions in Part 1 and their corresponding function names in the standard C library.

Str Function Standard C Function Man page link
StrGetLength strlen strlen man page
StrCopy strcpy strcpy man page
StrCompare strcmp strcmp man page
StrSearch strstr strstr man page
StrConcat strcat strcat man page

Use the Str module's interface in a file named str.h, and place your Str function definitions in str.c.

Note that your Str functions should not call any of the standard string functions. In the context of this assignment, pretend that the standard string functions do not exist. However, your functions may call each other, and you may define additional (non-interface) functions.

Design each function definition so it calls the assert macro to validate the function's parameters. In that way, your Str functions should differ from the standard C string functions. Specifically, design each function definition to assert that each parameter is not NULL. See the note below for more information of the assert() macro function.

Beware of type mismatches. In particular, beware of the difference between type size_t and type int: a variable of type size_t can store larger integers than a variable of type int can. Your functions should (in principle) be able to handle strings whose lengths exceed the capacity of type int. Also beware of type mismatches related to the use of the const keyword.

Extra Credit for Part 1: Implement functions with pointer notation

There are various ways to implement the functions in Part 1. Especially, you can access the character by pointer dereferencing like *pcSrc or by using an array notation such as pcSrc[uiLength] .

Here are two examples of StrGetLength() implementation. The first code implements the StrGetLength() function with the array notation; it traverses each given string or accesses the character using an index relative to the beginning of the string. However, with the pointer notation, the second version traverses each given string using an incremented pointer. 

size_t StrGetLength(const char pcSrc[]) /* Use array notation */
{
   size_t uiLength = 0U;
   assert(pcSrc != NULL);
   while (pcSrc[uiLength] != '\0')
      uiLength++;
   return uiLength;
}

size_t StrGetLength(const char *pcSrc) /* Use pointer notation */
{
   const char *pcEnd;
   assert(pcSrc != NULL);
   pcEnd = pcSrc;
   while (*pcEnd != '\0') /* note that *(pcSrc + uiLength) is valid but is "NOT" acceptable as pointer notation */
      pcEnd++;
   return (size_t)(pcEnd - pcSrc);
}

You can freely implement Part 1. However, if you implement part 1 only with pointer notation, you can get extra 10% of the full score. Note: if you simply convert array notation to pointer notation (e.g., *(p+i) instead of p[i]), that does not count as pointer notation. That is, pointer notation should not deal with an index. We will strictly apply the criteria for pointer notation, so please avoid using notation like (p+i) to get the full credit. p++/-- and *p are just fine. Please write your choice of implementation in the readme file. That is, specify if you used only the pointer notation for Part 1 in the readme file.

Test your Str Functions

We provide a test client (client.c) that compares the results of the Str and C standard library functions with various input. Please use this code for testing and debugging. You can compile the test client with gcc209 as follows. 

gcc209 -o client client.c str.c

Then, test each of your Str functions separately by providing a function name as an argument. For example, 
./client StrCopy
will test StrCopy. Actually, the client accepts any one of the Str function names as a command-line argument: 

./client [StrGetLength|StrCopy|StrCompare|StrSearch|StrConcat]

Note that passing all tests provided by the client does not mean that your function always behaves correctly. Please devise your own testing (e.g., by changing the client code) for more confidence. Note that we may use a different test client for grading.

Part 2: Simple Grep

grep is a popular UNIX tool that manipulates input strings. In this part, you implement a simplified version of grep called sgrep. sgrep provides these three functionalities.

Here are some usage examples of sgrep (you can also see the test files (google_wiki.txt and microsoft.txt) which are used in the following example): 

$ ./sgrep -f Google < google_wiki.txt
Google Inc. is an American multinational technology company specializing 
Google was founded by Larry Page and Sergey Brin while they were Ph.D. 
 supervoting stock. They incorporated Google as a privately held company on 
 be evil".[9][10] In 2004, Google moved to its new headquarters in Mountain 
View, California, nicknamed the Googleplex.[11] In August 2015, Google 
Alphabet Inc. When this restructuring took place on October 2, 2015, Google 
became Alphabet's leading subsidiary, as well as the parent for Google's 

$ ./sgrep -r Google Microsoft < google_wiki.txt > microsoft.txt
$ cat microsoft.txt (output is abbreviated with ...)
Microsoft Inc. is an American multinational technology company specializing
...
Microsoft was founded by Larry Page and Sergey Brin while they were Ph.D.
...
 supervoting stock. They incorporated Microsoft as a privately held company on
...
 be evil".[9][10] In 2004, Microsoft moved to its new headquarters in Mountain 
...
View, California, nicknamed the Microsoftplex.[11] In August 2015, Microsoft  
...
Alphabet Inc. When this restructuring took place on October 2, 2015, Microsoft 
...
became Alphabet's leading subsidiary, as well as the parent for Microsoft's 

$ ./sgrep -d google_wiki.txt microsoft.txt
google_wiki.txt@1:Google Inc. is an American multinational technology company specializing
microsoft.txt@1:Microsoft Inc. is an American multinational technology company specializing
google_wiki.txt@6:Google was founded by Larry Page and Sergey Brin while they were Ph.D.
microsoft.txt@6:Microsoft was founded by Larry Page and Sergey Brin while they were Ph.D.
google_wiki.txt@9: supervoting stock. They incorporated Google as a privately held company on
microsoft.txt@9: supervoting stock. They incorporated Microsoft as a privately held company on
google_wiki.txt@15: be evil".[9][10] In 2004, Google moved to its new headquarters in Mountain
microsoft.txt@15: be evil".[9][10] In 2004, Microsoft moved to its new headquarters in Mountain 
google_wiki.txt@16:View, California, nicknamed the Googleplex.[11] In August 2015, Google
microsoft.txt@16:View, California, nicknamed the Microsoftplex.[11] In August 2015, Microsoft  
google_wiki.txt@18:Alphabet Inc. When this restructuring took place on October 2, 2015, Google
microsoft.txt@18:Alphabet Inc. When this restructuring took place on October 2, 2015, Microsoft 
google_wiki.txt@19:became Alphabet's leading subsidiary, as well as the parent for Google's 
microsoft.txt@19:became Alphabet's leading subsidiary, as well as the parent for Microsoft's

Rules:

Tips:

Logistics

Develop on lab machines using emacs to create source code and gdb to debug.

For part 1, you implement the code str.c and str.h, which should contain the definitions of required functions.

For part 2, a skeleton C file is available here: sgrep.c. It implements the basic I/O and argument parsing, so all you need is fill out the rest of the functionalities with Str functions. Feel free to use this file as your starting point (of course, you don't have to use it), and change any part in the code if you'd like.

Create a readme text file that contains:

Submission

Use KAIST KLMS to submit your assignments. Your submission should be one gzipped tar file whose name is

YourStudentID_assign2.tar.gz

For example, if your student ID is 20161234, please name the file as

20161234_assign2.tar.gz

the single tar file which includes following files:

Grading

We will grade your work on quality from the user's point of view and from the programmer's point of view. To encourage good coding practices, we will deduct points if gcc209 generates warning messages.

From the user's point of view, your module has quality if it behaves as it should.

In part, style is defined by the rules given in The Practice of Programming (Kernighan and Pike), as summarized by the Rules of Programming Style document. These additional rules apply:

Names: You should use a clear and consistent style for variable and function names. One example of such a style is to prefix each variable name with characters that indicate its type. For example, the prefix c might indicate that the variable is of type char, i might indicate int, pc might mean char*, ui might mean unsigned int, etc. But it is fine to use another style -- a style which does not include the type of a variable in its name -- as long as the result is a readable program.

Line lengths: Limit line lengths in your source code to 72 characters. Doing so allows us to print your work in two columns, thus saving paper.

Comments: Each source code file should begin with a comment that includes your name, the number of the assignment, and the name of the file.

Comments: Each function should begin with a comment that describes what the function does from the caller's point of view. The function comment should:

Parameter Validation: Validate function parameters via asserts whenever possible.

Note: assert and NDEBUG

assert is a macro implementations of assertion, used for verifying the conditions. If the condition is true, it does nothing. However, if the conditions is FALSE, it displays an error messages and aborts the running program.

In this assignment, you should validate the function's parameters with assert. When you try to check whether your Str functions validate the given parameters correctly or not, the aborted program may annoy you. In that case, you can add NDEBUG macro in your source file to ignore the assert functions. Otherwise, you can also add -D NDEBUG argument to the gcc. The -D NDEBUG argument commands gcc209 to define the NDEBUG macro, just as if the preprocessor directive #define NDEBUG appeared in the specified .c file(s). Following OBcommands are example of disabling assert using -D option

gcc209 -D NDEBUG sgrep.c str.c -o sgrep

If NDEBUG is defined as a macro name in the source file, the assert macro is defined as ((void)0), which means that the assert macro will be ignore.

Note: Using Idioms

C programmers sometimes use idioms that rely on the fact that the null character ('\0'), the NULL address, the integer 0, and the logical concept FALSE have the same representation. You may use those idioms. For example, you may define StrGetLength like this:

size_t StrGetLength(const char pcSrc[])
{
   size_t uiLength = 0U;
   assert(pcSrc); /* NULL address, 0, and FALSE are identical. */
   while (pcSrc[uiLength]) /* null character and FALSE are identical. */
      uiLength++;
   return uiLength;
}

or like this:

size_t StrGetLength(const char *pcSrc)
{
   const char *pcEnd;
   assert(pcSrc); /* NULL address, 0, and FALSE are identical. */
   pcEnd = pcSrc;
   while (*pcEnd) /* null character and FALSE are identical. */
      pcEnd++;
   return (size_t)(pcEnd - pcSrc);
}

But you are not required to use those idioms. In fact, we recommend that you avoid the use of idioms that adversely affect understandability.

Note: The const Keyword and StrSearch

The use of the const keyword within StrSearch is tricky, as this question-and-answer sequence indicates.

Question

According to the man pages, the parameters of strstr are of type const char*. That implies that the parameters of StrSearch also should be of type const char*. Why are they not of type char*?

Answer

Suppose you were to define StrSearch like this:

char *StrSearch(char *pcHaystack, char *pcNeedle)
{
   ...
}

Further suppose the client then calls StrSearch like this:

const char *pcBig = "hello";
const char *pcSmall = "lo";
 ...
 ... StrSearch(pcBig, pcSmall) ...
 ...

(Note that's a perfectly reasonable way to call the function.) In that case the compiler, noting that pcBig is of type const char* and that pcHaystack is of type char*, would generate a warning on the function call. Thus pcHaystack (and pcNeedle) should be of type const char*.

Question

According to the man pages, the return type of strstr is char*. That implies that the return type of StrSearch also should be of type char*. Why is the return type not const char*?

Answer

Suppose you were to define StrSearch like this:

const char *StrSearch(const char *pcHaystack, const char *pcNeedle)
{
   ...
}

Further suppose the client then calls StrSearch like this:

char *pcBig = "hello";
char *pcSmall = "lo";
char *pc;
 ...
pc = StrSearch(pcBig, pcSmall);
 ...

(Note that's a perfectly reasonable way to call the function.) In that case the compiler, noting that pc is of type char* and that the value returned by StrSearch is of type const char*, would generate a warning on the assignment statement. Thus the return type of StrSearch should be char* and should not be const char*.

Question

Within the definition of StrSearch, I decided to define a local variable named pc that "points into" the first of the two given strings, as indicated by this code:

char *StrSearch(const char *pcHaystack, const char *pcNeedle)
{
   ...
   pc = pcHaystack;
   ...
   /* Increment pc so it points to the appropriate character. */
   ...
   return pc;
}

If I define pc to be of type char*, then the assignment statement generates a warning. If I define pc to be of type const char*, then the return statement generates a warning. How can I resolve that problem?

Answer

Unfortunately, C provides no elegant solution. We recommend that you define pc to be of type const char* so the assignment statement is warningless. Then use a cast operator in the return statement:

return (char*)pc;

to explicitly inform the compiler to not generate a warning. C programmers refer to that solution as "casting away the constness" of the variable. Sadly, often that inelegant technique is unavoidable.