Strings: print_integer(…)

# compile your test together with your print_integer.c to create an executable called test_print_integer
gcc -o test_print_integer test_print_integer.c print_integer.c

# run your executable and send the output to a file called actual.txt (That filename is arbitrary.)
./test_print_integer > actual.txt

# compare the actual output with the expected output using the diff command
diff actual.txt expected.txt

How much work is this?

This is a little harder than it looks. Getting your print_integer(…) to work properly with any valid int is a puzzle that you need to solve. However, it does not require very much code. For example, the instructor's solution for print_integer(…) is only 20 sloc*. cloc reports 20 sloc for the whole file.

* sloc = "source lines of code" (excluding comments and blank lines)

Here's a screenshot of all three files that you will be turning in. (This screenshot was taken with a version of the code that had a bug. It didn't handle INT_MIN properly. Fixing the bug added another 4 lines to the code.)

Your test cases must be your own. Do not copy test cases from here or anywhere else. We are sharing a few test cases so you can see how this works. In general, our code is not fair game unless explicitly stated otherwise. See the Syllabus.

Again… Do not copy these or any other test cases.

Submit

Q&A

1. Is an int always in decimal (base 10)?
   No. See the next question.
2. Does my print_integer need to deal with inputs that are specified in number bases other than decimal? For example, does my print_integer need to be able to convert from hex to binary?
   
   These questions, while common, don't actually make sense. Here's why:
   An int does not have a number base. It is just a value. The base (or "radix") of a number only refers to the way it is written, i.e., in your C code, or in the output of a program you write. For example, if n is the number of fingers on two hands, you could write that as 10, 0b1010, or 0xa, but it the same quantity no matter how you write it.
   
   When gcc reads your code for the first time, it interprets any integer literals (raw numbers in your code) according to how you write your code. Once your code has been compiled, the base that you used to write it becomes completely moot.
   
   Example: To gcc, the following are equivalent and have exactly the same effect.

   int n = 65;
   printf("n can be written as %d in decimal, as 0x%x in hexadecimal, or as %c if interpreted as an ASCII character.", n, n, n);

   int n = 0x41;
   printf("n can be written as %d in decimal, as 0x%x in hexadecimal, or as %c if interpreted as an ASCII character.", n, n, n);

   int n = 0b1000001;  // non-standard, but accepted by gcc; do not use for ECE 264
   printf("n can be written as %d in decimal, as 0x%x in hexadecimal, or as %c if interpreted as an ASCII character.", n, n, n);

   They all print the following:

   n can be written as 65 in decimal, as 0x41 in hexadecimal, or as A if interpreted as an ASCII character.

   To answer these questions more directly: Yes, your print_integer(…) will work no matter how the inputs were specified, but you don't do anything special to make that happen.
3. How do I print a string?
   Remember that a string is just an array of characters with a null character ('\0') at the end. Just start with the first character, and keep printing characters using fputc(ch, stdout) until you encounter the '\0'.
4. Should I print the '\0'?
   No.
5. Should hex (or other base >10) digits above 10 be uppercase or lowercase?
   Lowercase.
6. Why do we need the prefix parameter to print_integer?
   Some number formats use a prefix (e.g., "$" for currency). For negative numbers, it is customary to put the prefix after the minus sign (e.g., "-$3.00", not "$-3.00"). This parameter allows you to print numbers in that way. It will be used further in strings.
7. Are there any other examples of number formats with prefixes?
   Yes. Some number formats use a prefix (e.g., "$" for currency). For negative numbers, it is customary to put the prefix after the minus sign (e.g., "-$3.00", not "$-3.00"). This parameter allows you to print numbers in that way. It will be used further in strings.
8. Will my print_integer be called directly by external code?
   Yes.
9. What are we turning in?
   The specification (above) lists three files. Your expected.txt is just a plain text file. When you compile and run your test_print_integer.c file, the output should match your expected.txt file exactly. It should work the same, even if we substitute in the solution print_integer.c or another classmates' correct print_integer.c.
10. Is an int the same as an "integer"?
    When we say int, we mean the C data type, which has a limited range (-2,147,483,648 to 2,147,483,647 on many 64-bit systems, but you may not assume those particular bounds). An "integer" is a mathematical concept; it is the same as a whole number.
11. What if my output doesn't match up with someone else's output perfectly?
    This assignment is tightly specified, so the output of any two correct submissions should match perfectly. Likewise, we should be able to mix and match your print_integer.c with someone else's test_print_integer.c and expected.txt, or vice versa.
12. What radixes (number bases) must my print_integer(…) support?
    Your print_integer should support any value for radix between 2 and 36 (inclusive). For radixes above 10, it should represent digits beyond than 9 with lowercase letters. For example, print_integer(11, 16, "") would print "b" because b is the (11-9)^th lowercase letter of the alphabet. Similarly, print_integer(20, 21, "") would print "k" because k is the (20-9)^th letter of the alphabet.
    
    This converter may help you get a sense (and test your code).
13. What is the difference between a "radix" and a "base"?
    They are synonyms for the same thing.
14. Should I write #include "print_integer.h" (quotation marks) or #include <print_integer.h> (angle brackets)?
    Use quotation marks for header files in your project. Use angle brackets for standard header files. Thus, you should start with this:
    #include <stdio.h>
#include "print_integer.h"
    (You will need to include one or two other standard headers.)
15. May we modify print_integer.h?
    No.
16. May we turn in our own print_integer.h?
    Yes, you may turn it in, but that file will be ignored. We will copy your files into an empty directory and then copy our test files, including print_integer.h on top. Thus, any print_integer.h you submit will be overwritten.
17. May we #include <math.h>?
    No. See requirement #3 (above).
18. May we use sizeof(‥)?
    Yes. sizeof(‥) is unusual in that it is technically an operator, even though it looks like a function. You may use any valid C99 operators you like.
19. How can we be sure to test with the largest and smallest possible int on the platform?
    You may use INT_MAX and INT_MIN from #include <limits.h> in your test_print_integer.c. No matter where you compile or run your code, those will contain the largest and smallest possible values of an int on that platform.
20. How can I test converting from bases other than 10?
    You don't. print_integer(…) is not converting from one base to another. The parameter n has no base. Although you may use base 10, 16, or 8 to write the integer literals for your test cases in test_print_integer.c, that is just a superficial aspect of how you wrote your code. The compiler just reads your code and converts them to values. Within your print_integer(…) there is no way to know what notation was used to write the C code that caused those values to be passed to it.
21. Within print_integer(…) what is the number base of n?
    An int has no number base. A number base is only relevant when writing an integer literal in your code (e.g., the 768336 part of the code int n = 768336;) and when printing a number to the console (print_integer(…), printf(…), etc.).
22. How do I print the prefix?
    This is covered in Q3 (above). You may copy/adapt this if you wish:

    char* s = "La fu ta ish ma za ra";
    for(int idx_in_s = 0; s[idx_in_s] != '\0'; idx_in_s++) {
        fputc(s[idx_in_s], stdout);
    }

    This snippet was updated to use a for loop, as per the code quality standards.

    It simply prints "La fu ta ish ma za ra" (nonsense), with no new line at the end.
    
    We will talk more about strings soon, but the basic idea is that a string is an array of characters (char). When stored in memory, it always ends in the null terminator, a character with the the value 0. It should be written as '\0' in code, to make it clear what you mean. So when you have the code char* s = "La";, s takes you to an array of three (not two) characters: 'L' (76), 'a' (97), '\0' (0). If you had the code char* t = ""; // empty string, t takes you to an array of one character: '\0' (0).
23. When I try to print INT_MIN strange things happen. What's going on?
    You're probably getting an overflow error. Take the following example:

    int n     = INT_MIN;     // -2147483648 on our platform
    int m     = INT_MAX;     //  2147483647 on our platform
    int n_abs = -n;  // would be 2147483648, but it overflows!!!  It is too big for an int.

    The problem is that n_abs is greater than INT_MAX so it is too big for an int.
    
    If you are trying to store the absolute value of n, you may want to assign it to an unsigned int. This is another data type that has a larger range than an int. The code above could be written as follows:

    int n         = INT_MIN;  // -2147483648 on our platform
    int m         = INT_MAX;  //  2147483647 on our platform
    unsigned int n_abs = -n;  //  2147483648 ... no problem

24. Why can't I check if prefix == ""?
    The following code will not work like you expect:

    if(prefix == "") {
        // …
    }

    For this assignment, you probably don't want to do this anyway. Just use the snippet in Q22 above. If you really wanted to know if prefix is "", a right way is this:

    if(prefix[0] == '\0') {
        // …
    }

25. May I use an array to store the string?
    No. Since you are not allowed to make any assumptions about the size of an int, you don't know how big to make the array. Just figure out how to print the number left-to-right. First, make the code work perfectly for a one-digit number. Then, make it work for two-digit numbers. Then, make it work for three-digit numbers. By the time you get that far, you will see a pattern and a very easy way to print the number left-to-right.
26. May I use recursion?
    Yes, you may if you wish, but it is not necessary.
27. How do I add a newline between my test cases?
    You may copy/adapt this if you wish:

    fputc('\n', stdout);

28. What is EXIT_SUCCESS?
    EXIT_SUCCESS is a symbolic constant for the number 0. Your code would work just as well if you wrote return 0 but don't do that. Write return EXIT_SUCCESS instead. It makes your code more readable because "success" is what you meant. Only use 0 if you are actually referring to zero of something (e.g., zero bananas, zero passes through a loop, the zero'th element in an array, etc.).

29. GCC says: “…implicit declaration of function…”. What does it mean?

    You tried to call a function without first declaring it.

    Example:

    test_print_integer.c: In function ‘main’: test_print_integer.c:11:2: warning: implicit declaration of function ‘print_integer’ [-Wimplicit-function-declaration] print_integer(0, 10, ""); ^~~~~~~~~~~~~

    In this example, we called print_integer(…) on line 11 of test_print_integer.c, but the compiler (GCC) doesn't know what parameters that function expects and what it returns. That can happen if you forget to #include print_integer.h in test_print_integer.c.

    Solution: Make sure you #include "print_integer.h" in your test_print_integer.c.

30. GCC says: "… makes integer from …"  What does it mean?

    Example:
    test_print_integer.c:12:8: warning: passing argument 1 of ‘fputc’ makes integer from pointer without a cast [-Wint-conversion] fputc("\n", stdout); ^~~~

    This is one of the most confusing error messages you will see from GCC.

    It is telling you that you mistakenly used a string literal (e.g., "a") where a char (e.g., 'a') was expected.

    Remember: Use single quotes for a character literal (e.g., 'a'). Use double quotes for a string literal (e.g., "a").

31. GCC says: “▒.h: No such file or directory”  What does it mean?

    That can happen if one of your #include statements is not correct.

    Example:
    test_print_integer.c:5:10: fatal error: print_integer.h: No such file or directory #include <print_integer.h> ^~~~~~~~~~~~~~~~~

    In this case, the problem is that we used angle brackets (< and >) around one of our own header files. In general, you use #include <▒▒▒.h> for standard header files (e.g., stdio.h), and #include "▒▒▒.h" for header files that are part of your project. The difference in syntax tells the compiler (GCC) where to look for the specified ▒▒▒.h file.

32. GCC says: “multiple definition of `print_integer`”  What does it mean?

    It means GCC found two definitions of the same function. In the simplest case, that could happen if you had this:

    void print_integer(…) { } void print_integer(…) { }

    However, for this assignment, a common mistake is with the #include statements. If you mistakenly do this…

    #include "print_integer.c"   // ✘ INCORRECT
    

    … then that #include statement is replaced by the contents of the implementation file (print_integer.c), which contains the function definition of print_integer(…). You should only #include header files (e.g., print_integer.h), since they contain only function declarations (function signature but not function body).

    Solution: Make sure you have this…

    #include "print_integer.h"   // ✔ CORRECT
    

    … and not this…

    #include "print_integer.c"   // ✘ INCORRECT
    

33. GCC says: “undefined reference to `main'”  What does it mean?

    Example:
    /lib/../lib64/crt1.o: In function `_start': (.text+0x20): undefined reference to `main' collect2: error: ld returned 1 exit status

    Solution: Make sure you are including test_print_integer.c when you compile.

    Incorrect:
    ✘ $ gcc print_integer.c -o test_print_integer

    Correct:
    ✔ $ gcc print_integer.c test_print_integer.c -o test_print_integer

34. Diff says: “No newline at end of file”  What does it mean?

    Translation: One of the two things diff is comparing has a newline ('\n') at the end; the other does not. Your expected.txt almost certainly ends with a newline. Vim (like most/all other editors) adds a newline to the last line of any non-empty file. Thus, it is probably the output of your program that does not.

    Solution: Add fputc('\n', stdout) after each call to print_integer(…) in your main(…).

35. When I first log in—i.e., before running any commands—I get a message telling me that my disk quota is exceeded?

    Sometimes, this happens when a student has an infinite loop in their print_integer(…) that causes it to print a large number of some character (e.g., “00000000000000000000000000000000…”). When they run ./test_print_integer > actual.txt the actual.txt file fills up with a large number of some character. That file gets so big that it exceeds the maximum amount of storage that they are allowed to use on the system.

    To find out if this is your problem, look at the contents of your HW02 directory.

    you@eceprog ~ $ cd 264
    you@eceprog ~/264 $ cd hw02
    you@eceprog ~/264/hw02 $ ls -l

    If the size of actual.txt is very large (i.e., more than about 10,000 bytes or so), then delete it.

    you@eceprog ~/264/hw02 $ rm actual.txt