hello_world.md

##Hello World

"The first program to write is the same for all languages:" Print the words hello, world

--- 'The C Programming Language', p. 5

###Instructions

"The only way to learn a new programming language is by writing programs in it. The first program to write is the same for all languages:

Print the words

hello, world

... In C, the program to print hello, world is

#include <stdio.h>

int main()
{
  printf("hello, world\n");
}

Just how to run this program depends on the system you are using. As a specific example, on the UNIX operating system you must create the program in a file whose name ends in .c, such as hello.c, then compile it with the command

cc hello.c

If you haven't botched anything, such as omitting a character or misspelling something, the compilation will proceed silently, and make an executable file called a.out. If you run a.out by typing the command

a.out

it will print

hello, world

Now, for some explanations about the program itself. A C program, whatever its size, consists of functions and variables. A function contains statements that specify the computing operations to be done, and the variables store values used during the computation. C functions are like subroutines and functions in Fortran or the procedures and functions of Pascal. Our example is a function named main. Normally you are at liberty to give functions whatever names you like but main is special - your program begins executing at the begininning of main. This means that every program must have a main somewhere.

main will usually call other functions to help perform its job, some that you wrote, and others from libraries that are provided for you. The first line of the program,

#include <stdio.h>

tells the compiler to include information about the standard input/output library; the line appears at the beginning of many C source files...

The function main contains only one statement,

printf("hello, world\n");

A function is called by naming it, followed by a parenthesized list of arguments, so this calls the function printf with the argument "hello, world\n". printf is a library function that prints output, in this case the string of characters between the quotes.

... The sequence \n in the string is C notation for the newline character, which when printed advances the output to the left margin on the next line. If you leave out the \n (a worthwhile experiment), you will find that there is no line advance after the output is printed.

... Notice that \n represents only a single character. An escape sequence like \n provides a general and extensible mechanism for representing hard-to-type or invisible characters. Among others that C provides are \t for tab, \b for backspace, \" for double quote and \\ for the backslash itself.

Exercise 1-1. Run the hello, world program on your system. Experiment with leaving out parts of the program, to see what error messages you get.

Exercise 1-2. Experiment to find out what happens when prints's argument string contains \c, where c is some character not listed above."

from 'The C Programming Language', p. 5-8

###Gotchas

• In the examples from 'The C Programming Language', they omit the int before the main function. You may see the following warning as a result:

 hello_world.c:3:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
 main()
 ^~~~

In that case, just add the missing type declaration:

int main() {
...

• Using curly quotes can lead to the following error:

  hello_world.c:5:10: error: non-ASCII characters are not allowed outside of literals and identifiers
    printf(“hello, world”);
           ^
  hello_world.c:5:13: error: use of undeclared identifier 'hello'
    printf(“hello, world”);
            ^
  hello_world.c:5:25: error: non-ASCII characters are not allowed outside of literals and identifiers
    printf(“hello, world”);

In that case I recommend using a different text editor.

###Bonus More context on Hello World.

Solution: Hello World