When can I omit curly braces in C?

Question

When can I omit curly braces in C? I've seen brace-less return statements before, such as

if (condition)
  return 5;

But this doesn't always seem to work for all statements, i.e., when declaring a method.

Are the rules for brace omission the same as in Java?

Answer 1

The only places you can omit brackets are for the bodies of if-else, for, while, do-while, or switch statements if the body consists of a single statement:

if (cond)
  do_something();

for (;;)
  do_something();

while(condition)
  do_something();

do 
  do_something();
while(condition);

However, note that each of the above examples counts as single statement according to the grammar; that means you can write something like

if (cond1)
  if (cond2)
    do_something();

This is perfectly legal; if (cond2) do_something(); reduces to a single statement. So, for that matter, does if (cond1) if (cond2) do_something();, so you could descend further into madness with something like

for (i=0; i < N; i++)
  if (cond1)
    if (cond2)
      while (cond3)
        for (j=0; j < M; j++)
          do_something(); 

Don't do that.

Answer 2

If you look at the C syntax, there are a number of contexts that require a statement, a term that's defined by the grammar itself.

In any of those contexts, one of the forms of statement you can use is a compound-statement, which consists of an opening brace {, a sequence of zero or more declarations and/or statements, and a closing brace }. (In C89, all declarations in a compound-statement must precede all statements; C99 removed that restriction.)

A function-definition specifically requires a compound-statement, not just any kind of statement. (I'm fairly sure that's the only case where a compound-statement is the only kind of statement you can use). If not for that restriction, you'd be able to write:

void say_hello(void) printf("Hello, World!\n");

But since most function definitions contain multiple declarations and/or statements, there wouldn't be much advantage in permitting that.

There's a separate question: when should you omit braces. In my personal opinion, the answer is "hardly ever". This:

if (condition)
     statement;

is perfectly legal, but this:

if (condition) {
    statement;
}

IMHO reads better and is easier to maintain (if I want to add a second statement, the braces are already there). It's a habit I picked up from Perl, which requires braces in all such cases.

The only time I'll omit the braces is when an entire if statement or something similar fits on a single line, and doing so makes the code easier to read, and I'm unlikely to want to add more statements to each if:

if (cond1) puts("cond1");
if (cond2) puts("cond2");
if (cond3) puts("cond3");
/* ... */

I find such cases are fairly rare. And even then, I'd still consider adding the braces anyway:

if (cond1) { puts("cond1"); }
if (cond2) { puts("cond2"); }
if (cond3) { puts("cond3"); }
/* ... */

Answer 3

But this doesn't always seem to work for all statements.

Specifically? When a single statement is expected, then it's perfectly valid. Loops, the if statement, etc. all expect a statement and that's either a block, or, well, a single statement without being enclosed in a block.

Answer 4

An example:

int a[2][2] = {{1, 2}, {3, 4}};

you can use the valid equivalent form:

int a[2][2] = {1, 2, 3, 4};

Some verbose compilers may warn, but it is valid.

Note that for the if statement (same for the while, the for statement, ...) the {} are not omitted. They are just not requested. The if statement has this syntax:

if (expression) statement

If you want a multiple statement instead of a single statement you can use a compound statement which is surrounded {}.

Answer 5

You mainly need curly braces when you want to combine multiple statements or expressions into one, e.g.:

{
  x = 1;
  y = 2;
}

So if you put the above under if or else the whole thing in the braces will be executed as a whole, whereas if you omit the braces, only the first one (x = 1; in this case) will be used as part of if or else.

You also typically use them with switch():

switch (x)
{
case 1:
  // Do something
  break;
case 2:
  // Do something else
  break;
}

You typically need them with the do-while statement:

do
{
  printf("%d\n", x);
  x++;
} while (x < 10);

You need them with C89 compilers when you want to define and use a temporary variable in the middle of the code:

int main(void)
{
  puts("Hello, World!");
  {
    int x;
    for (x = 0; x < 10; x++)
      printf("%d\n", x);
  }
  return 0;
}

You use them to begin and end the body of a function, a structure/union definition, an enumeration definition, initialization of a structure/union/array, e.g.:

void blah(void)
{
}

enum e
{
  e1 = 1,
  e2 = 2
};

struct s
{
  int foo;
  int bar;
} s = { 1, 2 };

int a[3] = { 1, 2, 3 };

You may omit them sometimes in initializations, e.g.:

char str2[] = { "abc" };
char str1[] = "abc";

Answer 6

You can omit them when there's a single statement you're executing:

for(...)
  printf("brackets can be omitted here");

if(...)
  printf("brackets can be omitted here");
else
  printf("brackets can be omitted here too");

etc.. You can always add them in. It never hurts and help to clarify the scope, but you're pretty safe. The only "catch" I can think of is if you attempt to make a declaration in that scope (which is useless by the way):

if(...)
  int a = 5;

This will cause an error, but that's because you can perform a single statement without curly brackets, not a declaration. It is an important distinction.

Technically speaking, you can even do more than one operation without brackets if they're concatenated with the , operator... not that I advice doing it, but it is just worthy of note:

if(...)
    value++, printf("updated value with brackets");

And to your second part, you can see in the Java specification that the rules are the same. I linked specifically to the if section, but you can see after the if it's expected a statement, thus you can skip the curly brackets.

Answer 7

Anytime there is a block with more than one line (counting lines as a single statement with a ; )

Examples:

for (i = 0; i < j; i++ ) 
   foo(i);

while (1) 
   foo();

if( bool )
   foo();

void cheese()
   printf("CHEESE");

Everything after the ; in the above lines does not count as 'inside' the block as if there were { }.

Answer 8

If in doubt, use braces. They don't "cost" any extra (unless you absolutely use them incorrectly).

A fairly basic rule is "if you have more than one semicolon at the same indentation, you should have braces". It's not entirely that simple, but as a "one sentence with no ifs, buts or other complications" it will have to do.

And yes, Java being based on C-syntax has the same fundamental rules - there are possibly some weirdness where Java is different from C or C++, but that's unusual/strange things, not something you'd normally hit when writing typical code (I haven't done much Java, so I can't say I've found any of them, but I'm pretty sure there are some differences - they are different languages, after all).

Answer 9

The following seems a little tricky:

if (...)

        printf("I'm conditional\n");

I guess the C preprocessor takes care of empty lines, so this statement is fine. Very bad practice of course.

Answer 10

One could obviously use a for loop without curly braces, and without actually leaving the parentheses!

#include <stdio.h>

int main(){
    for(int i=0; i<10;i++, printf("%d ", i), printf("%d\n", i));
    return 0;
}

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_{I copied my own answer from another post I answered.}