The goal for this week is to do structured programming in C, using functions and pointers to organize processing and update data.

• Readings: C text Chapters 4-6, Appendix B
• Tutorials: Continue working with unix and emacs.
• Compilation: gcc -ansi -std=c99 -pedantic -Wall -Wextra -O pg#.c
• Submission: Upload your source file pg2.c to the webCT assignment page. Make sure that you hit SUBMIT after uploading. You can retrieve and resubmit as many times as you want before the deadline. If you retrieve, it's as if you never submitted, so make sure you have time to resubmit! If you need to explain anything in general, include a README plain text file with your submission. Anything not on webCT by the deadline gets a 0 = no credit. Also bring a printout of your source code to class on the due date.
• Collaboration: This is an individual assignment. Remember that you may use or adapt code from your texts and lectures, but you MUST include a comment to cite the original source.
• Help: check the TA info course webpage for hours when help will be available in the CS lab. This will be updated with one more person over the next week.
• Grading: 10 points style & submission, 5 points reflection questions, 30 points functionality = 45 total. -5 points if there are any warnings using the required compilation options. 0 points if it doesn't compile (has errors, no a.out created). Any violation of the implementation details will result in a 0 grade for the assignment.

The Problem:

Write a program (in file pg2.c) that can be used to convert numbers from one base to another. In order to make it easier to test your program (and, hint hint, to write it), always output the original value as a decimal (base 10) also, even if it is not requested. If a requested base b is <= 10, then it uses the digits 0 through b-1. When a base is >10, then it also uses the lower case letters of the alphabet as digits. For example, something in base 20 would use digits 0-9 and letters a-j, corresponding to values 10-19. You are also required to do error checking on the input and stop processing when an error occurs, displaying a relevant error message. Your program is required to work (and should error check these inputs also) for bases 2 through 36. Here are some sample runs:

Enter original base: 13
Enter number in base 13: a3b50
(Decimal value is 294125)
Enter desired base: 6
Converted value is 10145405

Enter original base: 4
Enter number in base 4: 123456
ERROR: invalid characters in number

Enter original base: 4
Enter number in base 4: 2031
(Decimal value is 141)
Enter desired base: 1b6
ERROR: invalid base input

Implementation Details:

• You MUST NOT use any form of string variables (character arrays) to solve this problem. You may use string literals for output.
• You MUST use multiple functions to organize the processing.
• You MAY NOT use global (external) variables.
• You should get input in functions other than main.
• You can only use getchar() for input.
• You will probably want or need to use some recursion.
• You will probably need to use pointers for pass by reference.

Reflection: Are there any restrictions on the values that your solution will process correctly? If so, what and why? Could you eliminate the problem without violating the implementation details? Discuss in a comment at the start of your program.