But first, before you try to receive input, you must have a place to store that input. In programming, input and data are stored in variables. There are several different types of variables; when you tell the compiler you are declaring a variable, you must include the data type along with the name of the variable. Several basic types include char, int, and float. Each type can store different types of data.
A variable of type char stores a single character, variables of type int store integers (numbers without decimal places), and variables of type float store numbers with decimal places. Each of these variable types - char, int, and float - is each a keyword that you use when you declare a variable. Some variables also use more of the computer's memory to store their values.
It may seem strange to have multiple variable types when it seems like some variable types are redundant. But using the right variable size can be important for making your program efficient because some variables require more memory than others. For now, suffice it to say that the different variable types will almost all be used!
Before you can use a variable, you must tell the compiler about it by declaring it and telling the compiler about what its "type" is. To declare a variable you use the syntax <variable type> <name of variable>;. (The brackets here indicate that your replace the expression with text described within the brackets.) For instance, a basic variable declaration might look like this:
int myVariable;
Note once again the use of a semicolon at the end of the line. Even though we're not calling a function, a semicolon is still required at the end of the "expression". This code would create a variable called myVariable; now we are free to use myVariable later in the program.
It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma. If you attempt to use an undefined variable, your program will not run, and you will receive an error message informing you that you have made a mistake.
Here are some variable declaration examples:
int x; int a, b, c, d; char letter; float the_float;
While you can have multiple variables of the same type, you cannot have multiple variables with the same name. Moreover, you cannot have variables and functions with the same name.
A final restriction on variables is that variable declarations must come before other types of statements in the given "code block" (a code block is just a segment of code surrounded by { and }). So in C you must declare all of your variables before you do anything else:
Wrong
#include <stdio.h> int main() { /* wrong! The variable declaration must appear first */ printf( "Declare x next" ); int x; return 0; }
Fixed
#include <stdio.h> int main() { int x; printf( "Declare x first" ); return 0; }
Reading input
Using variables in C for input or output can be a bit of a hassle at first, but bear with it and it will make sense. We'll be using the scanf function to read in a value and then printf to read it back out. Let's look at the program and then pick apart exactly what's going on. You can even compile this and run it if it helps you follow along.
#include <stdio.h> int main() { int this_is_a_number; printf( "Please enter a number: " ); scanf( "%d", &this_is_a_number ); printf( "You entered %d", this_is_a_number ); getchar(); return 0; }
So what does all of this mean? We've seen the #include and main function before; main must appear in every program you intend to run, and the #include gives us access to printf (as well as scanf). (As you might have guessed, the io in stdio.h stands for "input/output"; std just stands for "standard.") The keyword int declares this_is_a_number to be an integer.
This is where things start to get interesting: the scanf function works by taking a string and some variables modified with &. The string tells scanf what variables to look for: notice that we have a string containing only "%d" -- this tells the scanf function to read in an integer. The second argument of scanf is the variable, sort of. We'll learn more about what is going on later, but the gist of it is that scanf needs to know where the variable is stored in order to change its value. Using & in front of a variable allows you to get its location and give that to scanf instead of the value of the variable. Think of it like giving someone directions to the soda aisle and letting them go get a coca-cola instead of fetching the coke for that person. The & gives the scanf function directions to the variable.
When the program runs, each call to scanf checks its own input string to see what kinds of input to expect, and then stores the value input into the variable.
The second printf statement also contains the same '%d'--both scanf and printf use the same format for indicating values embedded in strings. In this case, printf takes the first argument after the string, the variable this_is_a_number, and treats it as though it were of the type specified by the "format specifier". In this case, printf treats this_is_a_number as an integer based on the format specifier.
So what does it mean to treat a number as an integer? If the user attempts to type in a decimal number, it will be truncated (that is, the decimal component of the number will be ignored) when stored in the variable. Try typing in a sequence of characters or a decimal number when you run the example program; the response will vary from input to input, but in no case is it particularly pretty.
Of course, no matter what type you use, variables are uninteresting without the ability to modify them. Several operators used with variables include the following: *, -, +, /, =, ==, >, <. The * multiplies, the / divides, the - subtracts, and the + adds. It is of course important to realize that to modify the value of a variable inside the program it is rather important to use the equal sign. In some languages, the equal sign compares the value of the left and right values, but in C == is used for that task. The equal sign is still extremely useful. It sets the value of the variable on the left side of the equals sign equal to the value on the right side of the equals sign. The operators that perform mathematical functions should be used on the right side of an equal sign in order to assign the result to a variable on the left side.
Here are a few examples:
a = 4 * 6; /* (Note use of comments and of semicolon) a is 24 */ a = a + 5; /* a equals the original value of a with five added to it */ a == 5 /* Does NOT assign five to a. Rather, it checks to see if a equals 5.*/
The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is extremely useful in many areas of C; for example, you will often use == in such constructions as conditional statements and loops. You can probably guess how < and > function. They are greater than and less than operators.
For example:
a < 5 /* Checks to see if a is less than five */ a > 5 /* Checks to see if a is greater than five */ a == 5 /* Checks to see if a equals five, for good measure */
