Tips for golfing in C

Question

What general tips do you have for golfing in C? I'm looking for ideas that can be applied to code golf problems in general that are at least somewhat specific to C (e.g. "remove comments" is not an answer). Please post one tip per answer. Also, please include if your tip applies to C89 and/or C99 and if it only works on certain compilers.

Answer 1

Use bitwise XOR to check for inequality between integers:

if(a^b) instead of if(a!=b) saves 1 character.

Answer 2

• Abuse main's argument list to declare one or more integer variables:

  main(a){for(;++a<28;)putchar(95+a);}
  

  (answer to The alphabet in programming languages)

  This solution also abuses the fact that a (a.k.a. argc) starts out as 1, provided the program is called with no arguments.

• Use global variables to initialize things to zero:

  t[52],i;main(c){for(;i<52;)(c=getchar())<11?i+=26:t[i+c-97]++;
  for(i=27;--i&&t[i-1]==t[i+25];);puts(i?"false":"true");}
  

  (answer to Anagram Code Golf!)

Answer 3

Avoid catastrophic function-argument type declarations

If you're declaring a function where all five arguments are ints, then life is good. you can simply write

f(a,b,c,d,e){

But suppose d needs to be a char, or even an int*. Then you're screwed! If one parameter is preceded by a type, all of them must be:

f(int a,int b,int c,int*d,int e){

But wait! There is a way around this disastrous explosion of useless characters. It goes like this:

f(a,b,c,d,e) int *d; {

This even saves on a standard main declaration if you need to use the command-line arguments:

main(c,v)char**v;{

is two bytes shorter than

main(int c,char**v){

I was surprised to discover this, as I have not so far encountered it on PPCG.

Answer 4

The comma operator can be used to execute multiple expressions in a single block while avoiding braces:

main(){                                                                                     
                                                                                            
int i = 0;                                                                                  
int j = 1;                                                                                  
if(1)                                                                                       
    i=j,j+=1,printf("%d %d\n",i,j); // multiple expressions are all executed                                                  
else                                                                                        
    printf("failed\n");                                                                     
                                                                                            
}

Outputs: 1 2

Answer 5

Instead of >= and <= you can simply use integer division (/) when the compared values are above zero, which saves one character. For example:

putchar(c/32&&126/c?c:46); //Prints the character, but if it is unprintable print "."

Which is of course still shrinkable, using for example just > and ^ (a smart way to avoid writing && or || in some cases).

putchar(c>31^c>126?c:46);

The integer division trick is for example useful to decide whether a number is less than 100, as this saves a character:

a<100 vs 99/a

This is also good in cases when higher precedence is needed.

Answer 6

Use lambdas (unportable)

Instead of

f(int*a,int*b){return*a>*b?1:-1;}
...
qsort(a,b,4,f);

or (gcc only)

qsort(a,b,4,({int L(int*a,int*b){a=*a>*b?1:-1;}L;}));

or (clang with blocks support)

qsort_b(a,b,4,^(const void*a,const void*b){return*(int*)a>*(int*)b?1:-1;});

try something like

qsort(a,b,4,"\x8b\7+\6\xc3");

...where the string literal contains the machine language instructions of your "lambda" function (conforming to all platform ABI requirements).

This works in environments in which string constants are marked executable. By default this is true in Linux and OSX but not Windows.

One silly way to learn to write your own "lambda" functions is to write the function in C, compile it, inspect it with something like objdump -D and copy the corresponding hex code into a string. For example,

int f(int*a, int*b){return *a-*b;}

...when compiled with gcc -Os -c for a Linux x86_64 target generates something like

0:   8b 07                   mov    (%rdi),%eax
2:   2b 06                   sub    (%rsi),%eax
4:   c3                      retq

MD XF wrote a bash script that may assist in the writing of simple "lambda" functions.

Edit: This technique was previously published by Shinichiro Hamaji in this document.

GNU CC goto:

You can call these "lambda functions" directly but if the code you're calling doesn't take parameters and isn't going to return, you can use goto to save a few bytes. So instead of

((int(*)())L"ﻫ")();

or (if your environment doesn't have Arabic glyphs)

((int(*)())L"\xfeeb")();

Try

goto*&L"ﻫ";

or

goto*&L"\xfeeb";

In this example, eb fe is x86 machine language for something like for(;;); and is a simple example of something that doesn't take parameters and isn't going to return :-)

It turns out you can goto code that returns to a calling parent.

#include<stdio.h>
int f(int a){
 if(!a)return 1;
 goto*&L"\xc3c031"; // return 0;
 return 2; // never gets here
}
int main(){
 printf("f(0)=%d f(1)=%d\n",f(0),f(1));
}

The above example (might compile and run on Linux with gcc -O) is sensitive to the stack layout.

EDIT: Depending on your toolchain, you may have to use the -zexecstack (for gcc) or -Wl,-z,execstack (for clang) compile flag.

If it isn't immediately apparent, this answer was mainly written for the lols. I take no responsibility for better or worse golfing or adverse psychological outcomes from reading this.

Answer 7

Certain compilers, such as GCC, allow you to omit basic #includes, param, and return types for main.

The following is a valid C89 and C99 program that compiles (with warnings) with GCC:

main(i) { printf("%d", i); }

Notice that the #include for stdio.h is missing, the return type for main is missing, and the type declaration for i is missing.

Answer 8

The ternary conditional operator ?: can often be used as a stand in for simple if--else statements at considerable savings.

Unlike the c++ equivalent the operator does not formally yield an lvalue, but some compilers (notably gcc) will let you get away with it, which is a nice bonus.

Answer 9

http://graphics.stanford.edu/~seander/bithacks.html

Bits are nice.

~-x = x - 1
-~x = x + 1

But with different precedences, and don't change x like ++ and --. Also you can use this in really specific cases: ~9 is shorter than -10.

if(!(x&y)) x | y == x ^ y == x + y
if(!(~x&y)) x ^ y == x - y

That's more esoteric, but I've had occassion to use it. If you don't care about short-circuiting

x*y == x && y
if(x!=-y) x+y == x || y

Also:

if(x>0 && y>0) x/y == x>=y   

Answer 10

Define parameters instead of variables.

f(x){int y=x+1;...}

f(x,y){y=x+1;...}

You don't need to actually pass the second parameter.

Also, you can use operator precedence to save parenthesis.
For example, (x+y)*2 can become x+y<<1.

Answer 11

Use cursors instead of pointers. Snag the brk() at the beginning and use it as a base-pointer.

char*m=brk();

Then make a #define for memory access.

#define M [m]

M becomes a postfix * applied to integers. (The old a[x] == x[a] trick.)

But, there's more! Then you can have pointer args and returns in functions that are shorter than macros (especially if you abbreviate 'return'):

f(x){return x M;} //implicit ints, but they work like pointers
#define f(x) (x M)

To make a cursor from a pointer, you subtract the base-pointer, yielding a ptrdiff_t, which truncates into an int, losses is yer biz.

int p = sbrk(sizeof(whatever)) - m;
strcpy(m+p, "hello world");

This technique is used in my answer to Write an interpreter for the untyped lambda calculus.

Answer 12

The ternary operator ?: is unusual in that it has two separate pieces. Because of this, it provides a bit of a loophole to standard operator precedence rules. This can be useful for avoiding parentheses.

Take the following example:

if (t()) a = b, b = 0;  /* 15 chars */

The usual golfing approach is to replace the if with &&, but because of the low precedence of the comma operator, you need an extra pair of parentheses:

t() && (a = b, b = 0);  /* still 15 chars */

The middle section of the ternary operator doesn't need parentheses, though:

t() ? a = b, b = 0 : 0;  /* 14 chars */

Similar comments apply to array subscripts.

Answer 13

Any part of your code that repeats several times is a candidate for replacement with the pre-processor.

#define R return

is a very common use case if you code involves more than a couple of functions. Other longish keywords like while, double, switch, and case are also candidates; as well as anything that is idomatic in your code.

I generally reserve uppercase character for this purpose.

Answer 14

Since usually EOF == -1, use the bitwise NOT operator to check for EOF: while(~(c=getchar())) or while(c=getchar()+1) and modify value of c at every place

Answer 15

If you ever need to output a single newline character (\n), don't use putchar(10), use puts("").

Answer 16

Reverse Loops

If you can, try to replace

for(int i=0;i<n;i++){...}

with

for(int i=n;i--;){...}

Answer 17

Use for rather than while

Any while can be changed into a for of the same length:

while(*p++)
for(;*p++;)

On its own, that's not golf. But we now have an opportunity to move an immediately-preceding statement into the parens, saving its terminating semicolon. We might also be able to hoist an expression statement from the end of the loop; if the loop contained two statements, we could also save the braces:

a=5;while(*p++){if(p[a])--a;++b;}

for(a=5;*p++;++b)if(p[a])--a;

Even do...while loops should be replaced with for loops. for(;foo,bar,baz;); is shorter than do foo,bar;while(baz);.

Answer 18

If your program is reading or writing on one in each step basis always try to use read and write function instead of getchar() and putchar().

Example (Reverse stdin and place on stdout)

main(_){write(read(0,&_,1)&&main());}

Exercise:Use this technique to get a good score here.

Answer 19

Make use of return values to zero stuff. If you call some function, and that function returns zero under normal conditions, then you can place it in a location where zero is expected. Likewise if you know the function will return non-zero, with the addition of a bang. After all, you don't do proper error handling in a code golf in any case, right?

Examples:

close(fd);foo=0;   →  foo=close(fd);    /* saves two bytes */
putchar(c);bar=0;  →  bar=!putchar(c);  /* saves one byte  */

Answer 20

Abuse dark corners of array indexing

i[array] desugars into *(i+array), and since + is commutative for pointer+integer too, it is equivalent to *(array+i) and therefore array[i].

It's not very common to see an array indexing expression (whatever)[x] where whatever requires wrapping in parens and x doesn't, but when you see one, you can swap the two positions and write x[whatever] to save two bytes.

A real golfing example.

Answer 21

1. Use *a instead of a[0] for accessing the first element of an array.

2. Relational operators (!=, >, etc.) give 0 or 1. Use this with arithmetic operators to give different offsets depending on whether the condition is true or false: a[1+2*(i<3)] would access a[1] if i >= 3 and a[3] otherwise.

Answer 22

Assign instead of return.

This is not really standard C, but works with every compiler and CPU that I know of:

int sqr(int a){return a*a;}

has the same effect as:

int sqr(int a){a*=a;}

Because the first argument is stored into the same CPU register as the return value.

Note: As noted in one comment, this is undefined behaviour and not guaranteed to work for every operation. And any compiler optimization will just skip over it.

X-Macros

Another useful feature: X-Macros can help you when you have a list of variables and you need to do some operation which involve all of them:

https://en.wikipedia.org/wiki/X_Macro

Answer 23

You may look into the IOCCC archives (international obfuscated C code contest).

One notable trick is to #define macros whose expansion has unbalanced braces/parentheses, like

#define P printf(

Answer 24

dprintf for conditional printing

As you may know, printing something by a condition can be done with ?:, && or ||:

cond&&printf("%d\n",n);
cond||printf("%d\n",n);
cond?:printf("%d\n",n);

Another interesting idea is to use dprintf, which is the same as printf except that it takes an extra argument, specifying the output file descriptor. It will only output to STDOUT if said argument is equal to 1. This can be abused to potentially save a few bytes over the previously mentioned methods:

x-1||printf("%d\n",n);
dprintf(x,"%d\n",n);

Answer 25

Swap variables

If you ever need to swap variables, don't use the pattern with an extra variable or that addition-subtraction-method, just do some chained XORing:

a^=b^=a^=b;

Answer 26

Using asprintf() saves you the explicit allocating and also measuring the length of a string aka char*! This isn't maybe too useful for code golfing, but eases the everyday work with a char arrays. There are some more good advises in 21st Century C.

Usage example:

#define _GNU_SOURCE
#include <stdio.h>

int main(int argc, char** argv) {
  char* foo;
  asprintf(&foo, "%s", argv[1]);
  printf("%s",foo);
}

Answer 27

for(int i=0;i<n;i++){a(i);b(i);} can be made shorter a few ways:

for(int i=0;i<n;){a(i);b(i++);} -1 for moving the ++ to the last i in the loop

for(int i=0;i<n;b(i++))a(i); -3 more for moving all but one statement into the top and out of the main loop, removing the braces

Answer 28

Go functional!

If you can reduce your problem to simple functions with the same signature and defined as single expressions, then you can do better than #define r return and factor-out almost all of the boilerplate for defining a function.

#define D(f,...)f(x){return __VA_ARGS__;}
D(f,x+2)
D(g,4*x-4)
D(main,g(4))

Program result is its status value returned to the OS or controlling shell or IDE.

Using __VA_ARGS__ allows you to use the comma operator to introduce sequence points in these function-expressions. If this is not needed, the macro can be shorter.

#define D(f,b)f(x){return b;}

Answer 29

1. use scanf("%*d "); to read the dummy input. (in case that input is meaningless in further program) it is shorter than scanf("%d",&t); where you also need to declare the variable t.

2. storing characters in int array is much better than character array. example.

   s[],t;main(c){for(scanf("%*d ");~(c=getchar());s[t++]=c)putchar(s[t]);}

Answer 30

Print a character then carriage return, instead of:

printf("%c\n",c);

or

putchar(c);putchar('\n'); // or its ascii value, whatever!

simply, declare c as an int and:

puts(&c);