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

Thursday December 8, 1994

Assigned Reading: 16.1 - 16.6

Handouts (available on-line): none

Topics Covered:

• Attendance was taken.

• Discussed the difference between using pointers to access the elements of an array by dereferencing and by array notation. For example, if p is a pointer to integers and A is an array of integers, then after the assignment p = A, we can access the first element of A using *p or p[0]. If we intend to use p as an array, then it is much clearer to use p[0]. Some people argue that this clearer method may be less efficient. However, any modern optimizing compiler should be able to generate code that removes these inefficiencies. So, we should opt for clarity.

• Speaking of clarity, we have a very unclear program which produces an interesting output. Unfortunately the author of the program misspelled "eighth" as "eigth" in this program. Just imagine having to figure out how to correct the program!

• We looked at the standard C string library. A complete description of these functions can be found in the manual pages on UNIX by issuing the man string command. The functions in the standard string library offer some of the same capabilities as the course library, but do not check for errors as rigorously as the string functions from the course library. Also, to use the string functions in the standard C library, you will have to allocate space for the strings yourself. We looked at the source code for some of the string functions in the course library strilib.c. We discussed how the functions: IthChar, Concat, StringLength, SubString and CreateString work. Competent programmers should avoid using functions like IthChar, StringLength and Concat because they are much slower than the standard routines.

• Next, we looked at how the function scanf from the stdio.h interface works. This is the function you would use if you did not use GetInteger, and GetReal from the course library. The function scanf takes pointers to variables as parameters (parameters passed by reference) so it can store the user's input in those variables.
  • Our first program using scanf simply reads in an integer and a double and prints them out. Sample run.
  • Our second example shows the consequence of forgetting the & in front of the variable passed to scanf: you get a core dump.
  • Our third example shows two alternate forms of scanf called sscanf and fscanf. These functions respectively read the input from a given string or a file instead of the keyboard.
  • Our fourth example shows that scanf can be used to read in several integers per line. The value returned by scanf is actually the number of variables it was able to fill in, and not the value stored in the variable. If scanf encounters the end-of-file character (control-D on UNIX), then it returns a -1. Sample runs.
  • Our fifth example shows that using scanf to read in several variables can be confusing to the user. In this program, we try to read in an integer followed by a double. The results could be confusing to the user because it is not always clear where the integer ends.

• Now that we have seen how scanf works we can look at how GetInteger is implemented in the source code of the simpio library. We also examined the strategy used for the GetLine function that allows us to read in a string without setting a limit on the length of the input.