Let's start with little coding, which will really make you computer programmer. I'm going to write a single-line computer program to write Hello, World! on your screen. Let's see how it can be written using different programming languages:
Hello World Program in C
Try to click Try It option to see the output of the following program. This Try it option is available at the top right-corner of the following code box. Try to change the content inside printf() instead of Hello World! and then check its result. It just prints whatever you keep inside two double quotes.
#include <stdio.h> main() { /* printf() function to write Hello, World! */ printf( "Hello, World!" ); }
This little Hello World program will help us in understanding various basic concepts related to C Programming.
PROGRAM ENTRY POINT
For now just forget about #include <stdio.h> statement, but keep a note that you have to put this statement at the top of a C program.
So, every C program starts with main(), which is called main function and then it is followed by a left curly brace. Rest of the program instruction is written in between and finally a right curly brace ends the program.
The coding part inside these two curly braces is called program body. The left curly brace can be in the same line as main(){ or in the next line like it has been mentioned in the above program.
FUNCTIONS
Functions are small units of programs and they are used to carry out a specific task. For example, above program makes use of two functions (a) main() and (b) printf(). Here, function main() provides the entry point for the program execution and another function printf() is being used to print an information on computer screen.
You can write your own functions which we will see in separate chapter, but C programming itself provides various built-in functions like main(), printf(), etc., which we can use in our programs based on our need.
Few programming languages use word sub-routine instead of function but their functionality is more or less same.
COMMENTS
A C program can have statements enclosed inside /*.....*/. Such statements are called comments and these comments are used to make program user friendly and easy to understand the program. Good thing about comments is that they are completely ignored by compilers and interpreters. So you can whatever language you want to write your comments.
WHITESPACES
When we write a program using any programming language, we use various printable characters to prepare programming statements. These printable characters are a, b, c,......z, A, B, C,.....Z, 1, 2, 3,...... 0, !, @, #, $, %, ^, &, *, (, ), -, _, +, =, \, |, {, }, [, ], :, ;, <, >, ?, /, \, ~. `. ", '. Hope I'm not missing any printable characters from your keyboard.
Apart from these characters, there are some characters which we use very frequently but they are invisible in your program and these characters are spaces, tabs (\t), new lines(\n). These characters are calledwhitespaces.
These three important whitespace characters are common in all the programming languages and they remain invisible in your text document having your program:
| Whitespace | Explanation | Representation |
|---|---|---|
| New Line | This will be used to create a new line. | \n |
| Tab | This will be used to create a tab. | \t |
| Space | This will be used to create a space. | empty space |
A line containing only whitespace, possibly with a comment, is known as a blank line, and a C compiler totally ignores it. Whitespace is the term used in C to describe blanks, tabs, newline characters and comments. So you can write printf("Hello, World!" ); as follows. Here all the created spaces around "Hello, World!" are useless and the compiler will ignore them at the time of compilation.
#include <stdio.h> main() { /* printf() function to write Hello, World! */ printf( "Hello, World!" ); }
Assuming, I make all these whitespace characters visible, then your above program will look like something below and you will not be able to compile it:
#include <stdio.h>\n \n main()\n { \n \t/* printf() function to write Hello, World! */ \n \tprintf(\t"Hello, World!"\t);\n \n }\n
SEMICOLONS
Every individual statement in C Program, must be ended with a semicolon ; For example, if you want to write "Hello, World!" twice, then it will be written as follows:
#include <stdio.h> main() { /* printf() function to write Hello, World! */ printf( "Hello, World!\n" ); printf( "Hello, World!" ); }
This program will produce the following result:
Hello, World! Hello, World!
Here, I'm using new line character \n in first printf() function to create a new line. Let us see what happens if I do not use this new line character:
#include <stdio.h> main() { /* printf() function to write Hello, World! */ printf( "Hello, World!" ); printf( "Hello, World!" ); }
This program will produce following result:
Hello, World! Hello, World!
I'm skipping explanation about identifiers and keywords and will take them in next few chapters.
PROGRAM EXPLANATION
Let's try to understand how the above C program to print "Hello, World!" works. First of above program is converted into a binary format using C compiler. So let's put this code in test.c file and compile it as follows:
$gcc test.c -o demo
If there is any grammatical error (Syntax errors in computer terminologies), then we fix it before converting it into binary format. If everything goes fine then it produces binary file called demo. Finally we execute produced binary demo as follows:
$./demo
Which produces following result:
Hello, World!
Here, when we execute binary a.out file, what computer does is, it enters inside the program starting from main() and encounters a printf() statements. Keep a note about line inside /*....*/ is a comment so it is filtered at the time of compilation. So printf() function instructs computer to print the given line at the computer screen. Finally it encounters a right curly brace which indicates the end of main() function and exit of the program.
Syntax Error
If you do not follow rules defined by the programing language then at the time of compilation you will get syntax error and program will not be compiled. From syntax point of view, even a single dot or comma or single semicolon matters and you should take care of such small syntax as well. Following is Hello, World! program but here I'm not using semicolon, let's try to compile following program:
#include <stdio.h> main() { printf("Hello, World!") }
This program will produce following result:
main.c: In function 'main': main.c:7:1: error: expected ';' before '}' token } ^
So bottom-line is that if you are not following proper syntax defined by the programming language in your program then you will get similar type of syntax errors and before trying next compilation you will need to fix them and then proceed.
Hello World Program in Java
Following is the equivalent program written in Java. This program will also produce same result Hello, World!.
public class HelloWorld { public static void main(String []args) { /* println() function to write Hello, World! */ System.out.println("Hello, World!"); } }
Hello World Program in Python
Following is the equivalent program written in Python. This program will also produce same result Hello, World!.
# print function to write Hello, World! */ print "Hello, World!"
Hope you noted that for C and Java examples, first we are compiling programs and then executing produced binaries but in Python program we are directly executing it. As I explained in previous chapter, Python is an interpreted language and it does not need intermediate step called compilation.
Python programming languages does not require a semicolon (;) to terminate a statement like we used in C and Java, rather a new line always means termination of the statement.
Conclusion
Not sure if you understood what I taught you above in this chapter, but if you did not understand then I will suggest to go through it once again and make sure you understood all the above concepts. But if you understood these concepts, then you are almost done and let's proceed to the next chapter, which you are going to enjoy a lot.
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