CSC 209 lab 04 exercises, week of 31 May 2022

Attendance

Remember that you can check that your attendance has been recorded properly at https://wwwcgi.teach.cs.toronto.edu/~ajr/cgi-bin/auth/present (this will work immediately after the recording, whether the attendance was recorded by you or by the TA)

Problem 1: Simple first task

Compile it with "gcc −Wall". You should always compile with "gcc −Wall".

Problem 2: rot13 (for credit)

So you add 13 to characters between 'a' and 'm' inclusive, as well as to characters between 'A' and 'M' inclusive; and you subtract 13 from characters between 'n' and 'z' inclusive, as well as from characters between 'N' and 'Z' inclusive.

Your program reads from the standard input with getchar() and writes to the standard output with putchar(). So you will probably want to begin with the "cat"-like program shown in lecture, which you can find in /u/csc209h/summer/pub/lab/04/cat.c

Remember that characters are just small integers in C, so for example 'a' is just an integer expression with value 97 (the ASCII character value), and 97 and 'a' can be used interchangeably (of course, you would use whichever one expressed your meaning most clearly — probably 'a').

Call your program rot13.c, and submit it for lab04 in the usual way.

Problem 3: Integer square root

3b) Write a loop in sh which uses your above program to find the largest integer whose integer square root is 12. (answer: 168)

3c) Modify your 3a program to print a prompt. Try it interactively. Then run your 3b shell script to demonstrate why we don't print prompts when writing software tools!

Problem 4: Separate compilation

Make sure it compiles with no warnings or error messages, and performs exactly correctly (this is simple enough that you should be able to make it perfect).

Problem 5: "Add"

Note that scanf("%d",&x) returns 1 for a successful input; EOF (i.e. −1) for end-of-file; and 0 if you are trying to read non-numeric input. In the non-numeric-input case, you should write "add: non-numeric input\n" to the standard error and not write anything to the standard output.

Otherwise, you should write the total (and nothing else except a terminating \n) to the standard output.

Note that "add </dev/null" should produce the output "0".

To write to stderr, do this:

	fprintf(stderr, "add: non-numeric input\n");

Problem 6: For credit

Your program declares an array of ten ints.

It calls a function named "fillarray" to read up to ten ints into that array from stdin using scanf() in a loop. Make this function reasonably general; it should not hard-code the number ten. If there are more than the specified number of ints on the standard input, excess values are ignored.

Write a function named "stats" which takes an array and outputs the min, max, and mean values in that array. You may use integer arithmetic throughout (thus truncating the mean to an integer).

Your main() function, after calling fillarray(), will call stats() on the whole array, then the first half of the array, then the second half of the array, labelling the output appropriately. Note that it is not necessary to copy or subdivide the array in any way. Just pass in (to your stats() function) the appropriate base address of where to find integer values from (using pointer arithmetic), and the number of integers that your stats() function should find starting at that location.

Example run, where '$' is the shell prompt (and remembering that the output of "seq 0 2 10" is 0 2 4 6 8 10):

$ seq 0 2 10 | ./a.out
Full array:
min: 0
max: 10
mean: 5
First half of the array:
min: 0
max: 4
mean: 2
Second half of the array:
min: 6
max: 10
mean: 8
$ 

Call your file "stats.c".
Please use exactly the above fixed strings for easier automated testing.

As usual, your file must compile on the teach.cs machines with "gcc −Wall" with no warning or error messages.

Problem 7: Calculator (not for credit)

Output an error message if you attempt to operate on more numbers on the stack than are currently on the stack.

To declare an array of ten integers, write:

	int stack[10];

Also recall:

        int c;
        ...
        while ((c = getchar()) != EOF) {
            ...

To submit

First of all, during the lab time, you must run "/u/csc209h/summer/present" on the console of a lab workstation, or get the TA to mark you as present.

Then, by the end of Friday June 3, you must submit your C files for problems 2 and 6. Submit with commands like

	submit -c csc209h -a lab04 rot13.c stats.c