In November 2015 Iggy Fernandez has published The Best Job Interview of My Life about a job interview for C programming. In the same article he has published following C code anonymous.c and has asked for a "good code explanation":

$ cat >helloworld.c
int i;main(){for(;i["]<i;++i){--i;}"];read('-'-'-',i+++"hello, world!\n",'/'/'/'));}read(j,i,p){write(j/p+p,i---j,i/i);}
$ cc -o helloworld helloworld.c
$ ./helloworld
hello, world!

Here is my answer (for this blog post I have used Fabrice Bellard TCC compiler for Windows 64 bit on Windows 8.1).

Arrays and pointers in C

In C langage there is special relationship between arrays and pointers.
If a is an array variable name and i an integer variable name used to index this array, the following expressions to access array entry designated by variable i are equivalent:

a[ i ]

*( a + i )

i[ a ]

*( i + a )

See C Pointer Arithmetic on Wikipedia.

I have written 7 C programs and explained in each step what I have changed with respect to the previous one. The last one is the same as anonymous.c (ignoring spaces and line breaks).

test0.c

The following code displays the 13 characters of "hello, world!" string:

>type test0.c
int i = 0;
char s[] = "hello, world!";
main()
{
for( ; *(s + i ); i++)
 {
	printf("i=%d ",i);
	printf("*(s+i)=%c\n",*(s+i));
 }
}
>tcc -o test0.exe test0.c

>test0
i=0 *(s+i)=h
i=1 *(s+i)=e
i=2 *(s+i)=l
i=3 *(s+i)=l
i=4 *(s+i)=o
i=5 *(s+i)=,
i=6 *(s+i)=
i=7 *(s+i)=w
i=8 *(s+i)=o
i=9 *(s+i)=r
i=10 *(s+i)=l
i=11 *(s+i)=d
i=12 *(s+i)=!

Remember also that a C string is a array of characters that starts at index 0 (like any C array) and must end with binary 0. In above code this is used to end the loop: when i is set to 13, the character pointed by *(s+i) is the the binary 0 and this ends the loop because 0 evaluates to false in C.

test1.c

In the expression *(s+i) I can swap array name and index name:

>type test1.c
int i = 0;
char s[] = "hello, world!";
main()
{
for( ; *( i + s ); i++)
 {
	printf("i=%d ", i);
	printf("*(i+s)=%c\n", *( i + s ));
 }
}
>tcc -o test1.exe test1.c

>test1
i=0 *(i+s)=h
i=1 *(i+s)=e
i=2 *(i+s)=l
i=3 *(i+s)=l
i=4 *(i+s)=o
i=5 *(i+s)=,
i=6 *(i+s)=
i=7 *(i+s)=w
i=8 *(i+s)=o
i=9 *(i+s)=r
i=10 *(i+s)=l
i=11 *(i+s)=d
i=12 *(i+s)=!

test2.c

At this step I use array expression i[s] to replace *( i + s ):

>type test2.c
int i = 0;
char s[] = "hello, world!";
main()
{
for( ; i[s]; i++)
 {
        printf("i=%d ", i);
        printf("i[s]=%c\n", i[s]);
 }
}
>tcc -o test2.exe test2.c

>test2
i=0 i[s]=h
i=1 i[s]=e
i=2 i[s]=l
i=3 i[s]=l
i=4 i[s]=o
i=5 i[s]=,
i=6 i[s]=
i=7 i[s]=w
i=8 i[s]=o
i=9 i[s]=r
i=10 i[s]=l
i=11 i[s]=d
i=12 i[s]=!

test3.c

Now I decide to:

• replace the s string with a different character string of 15 characters (actually the one used in anonymous.c) because I will add "\n" to "hello world!" string
• use a new pointer arithmetic expression to print "hello world!\n" characters one by one
• use the write function instead of printf:

>test3
hello, world!

>type test3.c
int i = 0;
char s[] = "]<++i){--i;}";
main()
{
for( ; i[s]; i++ )
 {
        write(1, "hello, world!\n" + i , 1);
 }
}
>tcc -o test3.exe test3.c

>test3
hello, world!

test4.c

Now I made following changes:

• I remove the s string variable and to replace it by its litteral value in the for loop
• I move the write function in the last part of the loop making the loop body empty
• I add in the last part of the loop the i incrementation:

>type test4.c
int i = 0;
main()
{
for( ; i["]<i;++i){--i;}"]; write(1, "hello, world!\n" + i++ , 1) )
 {

 }
}
>tcc -o test4.exe test4.c

>test4
hello, world!

test5.c

Now I modify the code to:

• swap "hello world\n" and variable i
• create a new function named read but that only calls write
• replace write call by read call in for loop
• delete empty loop body:

>type test5.c
int i = 0;
main()
{
 for( ; i["]<i;++i){--i;}"]; read(1, i++ +  "hello, world!\n" , 1) );
}
read(p1,p2,p3)
{
 write(p1,p2,p3);
}
>tcc -o test5.exe test5.c

>test5
hello, world!

test6.c

Now I:

• replace read first actual parameter from 1 to 0
• rename read function formal parameters from p1, p2, p3 to j, i p
• modify write function actual parameters like in final code from p1, p2, p3 to j+p/p, i - - - - j and i/i

Because:

- p is always set to 1 and j always set to 0, j+p/p always evaluates to 1,

- j is always set to 0, i - - - - j always evaluates to i

- parameter i contains the character to be displayed: it will not be 0 because string "hello world!\n" has only a binary 0 at string end: so i/i always evaluates to 1:

>type test6.c
int i = 0;
main()
{
 for( ; i["]<i;++i){--i;}"]; read(0, i++ +  "hello, world!\n" , 1) );
}
read(j , i , p)
{
 write(j + p/p , i - - - - j, i/i);
}
>tcc -o test6.exe test6.c

>test6
hello, world!

test7.c = anonymous.c

Last step is to change read actual parameters by arithmetic expression where characters are converted to integers:

• 0 is replaced by '-' - '-'
• 1 is replaced by '/' / '/':

>type test7.c
int i;
main()
{
 for( ; i["]<i;++i){--i;}"]; read('-'-'-', i++ +  "hello, world!\n" , '/'/'/') );
}
read(j , i , p)
{
 write(j + p/p , i - - - - j, i/i);
}
>tcc -o test7.exe test7.c

>test7
hello, world!

Except spaces and line breaks this is now the original C code.