HOW DO WE PRINT NUMBERS?

Example program ------> ONEINT.C

Load the file named ONEINT.C and display it on the monitor for our first example of how to work with data in a C program. The entry point main() should be clear to you by now as well as the beginning brace. The first new thing we encounter is line 5 containing int index; which is used to define an integer variable named index. The word int is a keyword in C, and can not be used for anything else. It defines a variable that can store a whole number within a predefined range of values. We will define an actual range later. The variable name, index, can be any name that follows the rules for an identifier and is not one of the keywords for C. The final character on the line, the semi-colon, is the statement terminator as discussed earlier. Note that, even though we have defined a variable, we have not yet assigned a value to it, so it contains an undefined value. We will see in a later chapter that additional integers could also be defined on the same line, but we will not complicate the present situation. Observing the main body of the program, you will notice that there are three statements that assign a value to the variable index, but only one at a time. The statement in line 7 assigns the value of 13 to index, and its value is printed out by line 8. (We will see how shortly. Trust me for the time being.) Later, the value of 27 is assigned to index, and finally 10 is assigned to it, each value being printed out. It should be intuitively clear that index is indeed a variable and can store many different values but only
one value at a time of course.Please note that many times the words "printed out" are used to mean "displayed on the monitor". You will find that in many cases experienced programmers take this liberty, probably due to the printf() function being used for monitor display.
To keep our promise, let's return to the printf() statements for a definition of how they work. Notice that they are all identical and that they all begin just like the printf() statements we have seen before. The first difference occurs when we come to the % character. This is a special character that signals the output routine to stop copying characters to the output and do something different, usually to output the value of a variable. The % sign is used to signal the output of many different types of variables, but we will restrict ourselves to only one for this example. The character following the % sign is a d, which signals the output routine to get a decimal value and output it. Where the decimal value comes from will be covered shortly. After the d, we find the familiar \n, which is a signal to return the video "carriage", and the closing quotation mark. All of the characters between the quotation marks define the pattern of data to be output by this statement. Following the output pattern, there is a comma followed by the variable name index. This is where the printf() statement gets the decimal value which it will output because of the %d we saw earlier. The system substitutes the current value of the variable named index for the %d and copies it to the monitor. We could add more %d output field descriptors anywhere within the brackets and more variables following the description to cause more data to be printed with one statement. Keep in mind however, that the number of field descriptors and the number of variable definitions must be the same or the runtime system will generate something we are not expecting. Much more will be covered at a later time on all aspects of input and output formatting. A reasonably good grasp of these fundamentals are necessary in order to understand the following lessons. It is not necessary to understand everything about output formatting at this time, only a fair understanding of the basics.
Compile and run ONEINT.C and observe the output. Two programming exercises at the end of this are based on this program.