UMBC CMSC201, Computer Science I, Fall 1994 Sections 0101, 0102 and Honors

Tuesday December 6, 1994

Assigned Reading: 13.5 - 14.3

Handouts (available on-line): Quiz 3

Topics Covered:

• Discussed Project 5 and the strategy you should implement.
• Continued discussion of memory allocation. Our first program used malloc to allocate an array of 26 characters, which we used to store the letters of the alphabet. Note that malloc returns a pointer value, but elements of the array can be accessed using array notation. Also, when we are done with the block of memory allocated by malloc we should free it using the function free. Program and sample run.
• Our second program using memory allocation uses malloc to allocate an array of integers. We use the casting operator (int *) to change the type of the value returned by malloc from pointer to void to pointer to int. We also check to see if malloc successfully allocated the requested amount of space. (If malloc failed, it returns the NULL value which is defined to be zero in stdio.h.) Program and sample runs.
• In the third program, we use the NewArray and FreeBlock functions from the course library instead of malloc and free. These functions do the error checking for you. We allocate a 27 character array to store the letters of the alphabet followed by the null character '\0'. This character signals the end of a string. In fact, the strings we have been using all along are really pointers to dynamically allocated arrays of characters. Thus, at the end of the program we can print out the characters in our array using %s. Program and sample run.
• Knowing that strings are really dynamically allocated arrays allows us to access the elements of the array and change them directly. For example, we can now write an implementation of the Captain Crunch secret decoder ring much easier. Program and sample run.
• Also we now have more efficient ways to examine every character in a string. The following program shows three ways of translating each character in a string to upper case. They all do the same thing, but some of the methods are much slower. Program and sample run.
• Quiz 3.