16.14 Answers to Chapter Questions

Answer 16-1: The problem is that you are writing an ASCII file, but you wanted a binary file. In Unix, ASCII is the same as binary, so the program runs fine. In MS-DOS/Windows, the end-of-line issue causes problems. When you write a newline character (0x0a) to the file, a carriage return (0x0D) is added to the file. (Remember that end-of-line in MS-DOS/Windows is <carriage return><line feed>, or 0x0d, 0x0a.) Because of this editing, you get an extra carriage return (0x0d) in the output file.

To write binary data (without output editing) you need to open the file with the binary option:

    out_file.open("test.out", std::ios::out | std::ios::binary); 

Answer 16-2: The std::printf call does not check for the correct number of parameters. The statement:

std::printf("The answer is %d\n");

tells the std::printf to print the string "The answer is" followed by the answer. The problem is that the parameter containing the answer was omitted. When this happens, std::printf gets the answer from a random location and prints garbage.

Properly written, the std::printf statement is:

std::printf("The answer is %d\n", answer);

Answer 16-3: The std::printf call does not check the type of its parameters. You tell std::printf to print an integer number (%d) and supply it with a floating-point parameter (result). This mismatch causes unexpected results, such as printing the wrong answer.

When printing a floating-point number, you need a %f conversion. Properly written, our std::printf statement is:

std::printf("The answer is %f\n", result);

Answer 16-4: The problem is that std::fgets gets the entire line, including the newline character (\n). If you have a file named sam, the program reads sam\n and tries to look for a file by that name. Because there is no such file, the program reports an error.

The fix is to strip the newline character from the name:

name[strlen(name) - 1] = '\0';    /* Get rid of last character */ 

The error message in this case is poorly designed. True, you did not open the file, but the programmer could supply the user with more information. Are you trying to open the file for input or output? What is the name of the file you are trying to open? You don't even know whether the message you are getting is an error, a warning, or just part of the normal operation. A better error message is:

std::fprintf(stderr, "Error: Unable to open %s for input\n", name); 

Notice that this message would also help us detect the programming error. When you typed in "sam", the error would be:

Error: Unable to open sam 
for input 

This clearly shows us that you are trying to open a file with a newline in its name.