C for Java Programmers: Basics

Write a program that stores a temperature in degrees Fahrenheit using a variable and converts it to Kelvins using the following formula:
If the formula isn’t displayed, you can look it up at Wikipedia. Your output should be of the form “212 degrees F is 373.15 K”.

Add code to test whether the temperature is less than or equal to 32 °F, in which case it should print “That’s freezing!”, or is greater than or equal to 212 °F, in which case it should print ”That’s boiling!”, otherwise it should print “That’s nice.”

Write code to print the numbers from 1 to 10, and to print the numbers from 10 down to 1 counting by twos.

C is not an object-oriented language, so it doesn’t have methods defined in classes. Instead, it has the more general idea of a “function”. This is covered in the C for Java Programmers document. Object-oriented programming in C is covered in the next lesson in this tutorial.

For now, write the following two functions:

• A function that has a single integer parameter and that prints that many stars (asterisks).
• A function that computes and returns the temperature in Kelvin of a temperature given in degrees Fahrenheit.

Include code to demonstrate your functions being used.

Let’s talk input and output (I/O).

In Java, you use the Scanner class to read input from the console (there are other possibilities also). It’s not quite so straightforward in C, but the basic patterns are not hard to learn.

In C, the basic building block for input is the C standard library function scanf (defined in <stdio.h>). It’s sort of like printf in reverse. You give it a format string with % specifiers, and it reads the input according to that specification. For example, %d means “read an integer,” %f means “read an floating-point number,” and so on. There can be more than one % specifier, in which case scanf tries to read more than one thing.

An important point is that scanf usually needs to “return” more than one thing. There’s the value or values specified in the format string, and there’s an overall return value indicating whether the values were present in the input. The latter is returned as the return value of the function. To receive the individual values, you must also pass scanf references (pointers) to where you want them stored. For primitive types, this will be a pointer to a variable of that type, for example an int*. For reference types, including strings which are char*s, you just just give scanf the pointer.

FYI: The C for Java Programmers document uses scanf as an example in its discussion of pointers. Check it out.

Then write a function that reads, separately, an integer, a floating-point value, and a string and prints out what it read.

The manual entry for scanf says the following about the %s specifier:

“Matches a sequence of non-white-space characters; the next pointer must be a pointer to char, and the array must be large enough to accept all the sequence and the terminating NUL character. The input string stops at white space or at the maximum field width, whichever occurs first.”

This is sort of similar to what Scanner.next returns.

If you just want to read the next line of input (up to a newline), like Scanner.nextLine, the function you want is fgets. The incantation fgets(str, 255, stdin); reads into the character array str at most 255 characters from the standard input (stdin). The array str must be big enough to hold that many characters and a terminating NUL character, and it’s YOUR problem to make sure that it is.

Note: There is an apparently simpler function named gets which reads from the console into the given character array with no length limit. The manual entry for gets and fgets says the following:

“The gets() function cannot be used securely. Because of its lack of bounds checking, and the inability for the calling program to reliably determine the length of the next incoming line, the use of this function enables malicious users to arbitrarily change a running program’s functionality through a buffer overflow attack. It is strongly suggested that the fgets() function be used in all cases.”

So don’t use gets. Use fgets and give an appropriate size limit for the buffer you’re reading into. Also note that fgets includes the newline that the user typed.

Finally, for this step, write a program that reads lines from the console and prints them out as they are read. The program should stop reading when it reads an empty (blank) line.

More I/O: In Java, you use File objects and various flavors of InputStreams, OutputStreams, Readers, Printers, and Scanners to read and write files. In C it’s quite a bit simpler, but the standard library provides functions to get the job done. You will need to #include <stdio.h> to use these functions.

• The function fopen opens a file identified by name. The name is relative to the “current working directory” of the process running your code. The format of pathnames varies by platform. You can ask for the file to be opened for reading (input) or writing (output), and there are other possibilities. If successful, fopen returns a FILE* (a “file pointer”, a pointer to a value of type FILE) representing the open file. If the open fails, fopen returns NULL and the global variable errno is set to a value that indicates what was the problem. You will need to #include <errno.h> to use errno if you need it.
• The function fprintf is just like printf, except that instead of printing to the console (“standard output”), it prints to the output FILE* given as first argument.
• The function fscanf is just like scanf, except that instead of reading from the console (“standard inpout”), it reads from the input FILE* given as first argument.
• We’ve already seen fgets, which reads text up to a newline. Its third argument is a FILE*.
• The function feof returns true if the input FILE* given as argument has been completely read (“end of file” or “EOF”).
• The function fclose is used to close a file when you are finished with it.
• The variable stdin used in our fgets example in the previous step is defined in <stdio.h> and refers to the console input (there is also stdout).

With this background, write a function that takes two filenames as parameters and copies the contents of the file with the first name into a file with the second name (creating it if it doesn’t exist, overwriting it if it does exist, both of which are the default behavior of fopen).