Question

Try to fix the following buggy program.

%%file buggy.c

# Create a function pointer type that takes a double and returns a double
double *func(double x);

# A higher order function that takes just such a function pointer
double apply(func f, double x)
{
    return f(x);
}

double square(double x)
{
    return x * x;
}

double cube(double x)
{
    return pow(3, x);
}

double mystery(double x)
{
    double y = 10;
    if (x < 10)
        x = square(x);
    else
        x += y;
        x = cube(x);
    return x;
}

int main()
{
    double a = 3;
    func fs[] = {square, cube, mystery, NULL}

    for (func *f=fs, f != NULL, f++) {
        printf("%d\n", apply(f, a));
    }
}

! clang -g -Wall buggy.c -o buggy

What other language has an annual Obfuscated Code Contest http://www.ioccc.org/ ? In particular, the following features of C are very conducive to writing unreadable code:

• lax rules for identifiers (e.g. _o, _0, _O, O are all valide identifiers)
• chars are bytes and pointers are integers
• pointer arithmetic means that array[index] is the same as *(array+index) whihc is the same as index[array] !
• lax formatting rules especially with respect to whitespace (or lack of it)
• Use of the comma operator to combine multiple expressions together with the ?: operator
• Recursive function calls - e.g. main calling main repeatedly is legal C

Here is one winning entry from the 2013 IOCCC entry that should warm the heart of statisticians - it displays sparklines (invented by Tufte).

main(a,b)char**b;{int c=1,d=c,e=a-d;for(;e;e--)_(e)<_(c)?c=e:_(e)>_(d)?d=e:7;
while(++e<a)printf("\xe2\x96%c",129+(**b=8*(_(e)-_(c))/(_(d)-_(c))));}

%%file sparkl.c
main(a,b)char**b;{int c=1,d=c,e=a-d;for(;e;e--)_(e)<_(c)?c=e:_(e)>_(d)?d=e:7;
while(++e<a)printf("\xe2\x96%c",129+(**b=8*(_(e)-_(c))/(_(d)-_(c))));}

! gcc -Wno-implicit-int -include stdio.h -include stdlib.h -D'_(x)=strtof(b[x],0)' sparkl.c -o sparkl

import numpy as np
np.set_printoptions(linewidth=np.infty)
print ' '.join(map(str, (100*np.sin(np.linspace(0, 8*np.pi, 30))).astype('int')))

%%bash

./sparkl 0 76 98 51 -31 -92 -88 -21 60 99 68 -10 -82 -96 -41 41 96 82 10 -68 -99 -60 21 88 92 31 -51 -98 -76 0

If you have too much time on your hands and really want to know how not to write C code (unless you are crafting an entry for the IOCCC), I recommend this tutorial http://www.dreamincode.net/forums/topic/38102-obfuscated-code-a-simple-introduction/