C Tutorial I: Basic Output, variables, includes and main()
There is no point to making a program if you cannot give it a command or at least make it display something –at least not for a beginner. In this tutorial I will show you how to display data, how to read it, how to store it in variables (for as long as the program is running at least) and how mathematics are done in C.
Basic Output. The most commonly used function for outputting data in C is printf(). The parenthesis is where arguments are passed, which means that whatever is to be printed (to the screen) is placed in there. The fact that we can pass arguments to printf(), and the very existence of the parenthesis, indicates that it is a function. A function is a set of commands, which are executed whenever it is called, so we don’t have to write them again. Later on, you will learn to make your own, but let’s stick to using ready ones for now.
Printf() can take many particular types of arguments. These are generally the basic types of variables: characters, strings, integers, floats, doubles, and their variations.
Variables store data, but they must be declared before being used. That way C can keep track of what variables are around.
Characters: they are declared with the keyword “char”. A keyword, or reserved word, is a word the compiler uses to identify information the programmer passes. Printf() is also a keyword, and should always be written with lowercase letters (the naughty text editor auto-corrects this right here unfortunately). Back to characters then. Characters are what they say, a single character. This can be any letter of the alphabet, any digit, and many symbols. There are character sets with varying abilities, ASCII needs 3 bits (binary digits) and has only English letters, digits and some symbols, mostly used by the operating system. Note that symbols do not have to be represented in a particular visible way. ASCII’s 000 value (whether in binary, hexadecimal or decimal) is the EOF, End Of File symbol, which is represented as \0, or in programming, occasionally it is represented by -1. See section character sets for more information.
Strings: I know that you’re laughing, but get used to it, if you study computer science you’ll probably meet this word 500 times per semester as a professor once said. They are arrays of chars, which means they are many chars put in a row. Strings are interesting as they resize according to your needs. You will soon use strings as names, and possibly as other stuff as well.
Integers are variables that store numbers, like 1, 2, 135, -1762 etc.
Floats are floating point numbers, which means they can also store values that are between integers, such as 0.1, 3.7, -7.625 etc.
Doubles: they are the same as floats but can store twice as much information.
Variable sizes: in general this depends on the OS and compiler. However there are some general rules. Chars are usually 2 bytes long, integers twice as much, which means 4 bytes, floats twice as much as integers, 8 bytes, and doubles are 16 bytes long. However you can modify all except the first by placing the prefix long or short, doubling the size and cutting it in half accordingly. Strings are generally the number of characters * 2.
Now to start coding. The first thing you have to do is include a header or two. Headers contain functions and other data, so you don’t have to write all that code manually but rather use these ready tools. The most important one is stdio.h. After including the header (it is done with # symbol in front of the include command, to which we will refer to as the #include command) we have to tell the program where to start from. It always seeks out main() to start. Main() is a function, right? If you have a little code ready, you might see arguments in main(), these are arguments passed in the same way you type “gcc myfile.c –o filename”, by using a dash when running from the command line.
Below main are located two curly braces: {}. The area between these is called a code block. What happens in a code block, remains in a code block, with some exceptions. One such exception is the return command, which gives a value to the above block, the one through which this block was accessed. It is good programming practice to place a “return 0;” at the end of main, or “return 1;”. The semicolon ; is how you tell C that your command is complete. It is the #1 most common reason you will be getting compiler errors, there must be one in the end of every functioning line.
Finally, before we start, comments are used to remind yourself or tell others about what your code is doing. To add comments you either use a double slash // or a slash followed by an asterisk at the beginning of the comment and an asterisk with a slash at the end. The difference is that the first is used for a single line, while the later for multiple lines. Commenting out parts of code is common debugging practice. If you comment out code (with the last method mentioned) you reduce the code that may possibly be creating the problem, so you debug less code. However this code may indirectly effect the rest of the program, for example there might be a variable declaration in it, so the compiler will complain you have not declared that variable.
Let’s now see our “Hello World!” program, the first program every programmer writes in his learning:
#include <sdtio.h>
int main()
{
printf(“Hello World!”); //printing Hello World! to the screen
return 0;
}
Notice that there is a semicolon in all commands other than the #include. #include is a compiler command, so it is processed separately and essentially is not executable, but rather tells the compiler to add the code from stdio.h to the program.
Another thing to notice and keep in mind is that what printf() prints is in quotes (opening and closing quotes should appear identical in your editor or IDE). Later you will see how we can make it print variables and enhance it with other features.
This is all for this tutorial, continue to the next one so you can also learn about basic input and arithmetic operations, as well as how printf() handles numbers and other data. Understand that if one tutorial has a lot of theory in it, it is so you don’t have to read it later on and so you don’t have to try and integrate it to your knowledge, I try to present it in a usable form.
What now? Well, while in later sections I will tell you what you can make with what you have learned (as experimenting is essential in learning), there is little you can do now, other than just display a message. However, by the end of the next tutorial you will be able to make a basic calculator and probably also make something that will calculate the output of various mathematical functions, after that you will be introduced to branching, where you will be able to make a question game and if you have a little imagination a text adventure. If you read even further, you will be introduced into loops, where you can create something with a continuous gameplay which can start over from the beginning without exiting, and can have much more enhanced playability. After that the form of the games will not be much different until you reach a good enough level to go into graphics tutorials, but you will learn methods and techniques that will take care of much of the code and facilitate adding features. Do not ignore the advanced tutorials and do not jump to the GDI tutorials or others, as, if you do so and then turn back and read them, you will understand that you goofed up enormously. Thank you for reading!