Computer Systems Organization: Recitation 6

Exercise 1 -- The Buggy String Join

//combine an array of strings (of length num_strings)
//into one string in which the constituent strings are separated by
//the separator string
char *
join(int num_strings, char *strings[], char *separator)
{
	int total_len = 0;
	for (int i = 0; i < num_strings; i++) {
		total_len += strlen(strings[i]); //see "man strlen"
	}

	//allocate a new array whose length is the sum of the length of each individual string
	char *result = (char *)malloc(total_len);

	char *start = result;
	for (int i = 0; i < num_strings; i++) {
		strcpy(start, strings[i]); //see "man strcpy"
		start += strlen(strings[i]);
		strcpy(start, separator);
	}
	//to make sure i have not copied more than the allocated number of bytes
	assert((start - result) <= total_len);
	return result;
}

int
main(int argc, char *argv[])
{
	printf("Joined arguments are: %s\n", join(argc, argv, "*"));

}

In the last recitation, we asked you to implement various common functions to manipulate string. The C's standard library already includes many useful string functions that one can readily use. Just type man string to see a list of such string functions. Say, you want to use strlen, add the line #include <string.h> and type man strlen to see its documentation.

C's string library is quite spartan, compared with the string libraries available in other modern languages, such as Python or Go. For example, there is no join facility whereby programmers can combine many strings together into a single one. Your professor implemented join function, shown above, but it does not seem to work right. For example, when she types ./a.out hello world The program prints out Joined arguments are: ./a.outhelloworld* whereas the correct output should be Joined arguments are: ./a.out*hello*world*. Rumor also has it that she's not doing malloc right. Please help her debug join.c.

Exercise 2 -- Binary Tree Mon Amour

You are starting at your first internship job at company C-Lover. Someone else has written a set of C functions that implements a binary tree. The function insert inserts a value into the binary tree. The function exists searches the tree for the specified value. If the value is found in the tree, it returns 1, otherwise it returns zero. These functions are available in tree.c

Please design and write some code to test the correctness of the binary tree operations. For simplicity, you can write these tests in a main function. If our tree.c does not pass your test, debug it until it does.

Exercise 3 -- The Incorrect If

In the file exercise3.c, we have written a function checkEquality thats checks the equality of two variables, and is broken. By using GDB, figure out the broken line of code, and the fix for it. Once you've figured out the broken line of code, can you think of why the code behaves that way?

Exercise 4 -- Modifying Strings in Place

Lenny found a celebrity he wishes to emulate on a social media platform, and wrote a string to send out. Unfortunately, he made a mistake while writing the string. In the file exercise4.c, Lenny has written code to fix his mistake. He wants to edit the nth character of a string (n is 0-based) to a certain character. Boilerplate code has been provided. Write the code for editCharAt. Don't forget to use GDB as a debugging tool if needed. The use of the following libraries is recommended: string.h, stdlib.h.