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Coding tips they don't teach you in school
Posted on: 2009-06-23 09:45:19

Here are some C coding tips, because I have been unable to post anything for a while.

Some of these time-saving shortcuts are intended for small projects or prototyping code.

The nested ? : trick

The switch statement is very efficient, and the compiler will often implement it as a table lookup so it doesn't have to do any comparisons. But it sure can tire your fingers in a hurry:
switch( number ) {

    case 1: str = "one"; break;
    case 2: str = "two"; break;
    case 3: str = "three"; break;
    case 4: str = "four"; break;
    case 5: str = "five"; break;
    default: str = "unknown number"; break;
}

If getting the code out is more important than its speed, you can use a nested conditional operator to save typing:

str = number == 1 ? "one" : 
      number == 2 ? "two" :
      number == 3 ? "three" :
      number == 4 || rand() == 42 ? "four" :
      number == 5 ? "five" :
      "unknown number";

You can freak out your coworkers if you do it all on one line.

DeMorgan's theorem of negativity

// if blah blah blah blah blah blah....
if ( !(A && B) )

is the same as

// if blah blah
if ( !A || !B )

Pick the one that is easier to understand and read out loud. It is usually the second.

Rounding numbers using integer math

Suppose you have 2001 boolean variables, so you want to keep them in a bitmap of bytes, 8 at a time. How do you declare this array?

#define NUMBER_OF_BITS 2001
unsigned char bitmap[ NUMBER_OF_BITS / 8 ]; // FAIL!

The computer uses integer math, so 2001 / 8 is 250 and there is one bit left over. When you store bit 2001, you will corrupt memory. You can round numbers up like this:

unsigned char bitmap[ (NUMBER_OF_BITS + 7) / 8 ];

This works because integer division always rounds down. However, if you add the divisor less one, then you will force it to always round up. It will also work for 32 bit integers:

unsigned int bitmap[ (NUMBER_OF_BITS + 31) / 32 ];

Real rounding via integer division

Adding something before dividing is a general technique. This code rounds a and b to the nearest 10.
int a = 12;
int b = 16;

printf("Round a: %d", (a + 5) / 10 * 10 );
printf("Round b: %d", (b + 5) / 10 * 10 );

Multiply by arbitrary numbers using shift

Sometimes, for good reasons, you need to hand optimize your code. But most of the time, somebody is just showing off, and you will see code like this:

b = ( a << 1 );

This is the same as multiplying by 2. Because of the way binary numbers are represented, you can multiply by any power of two by shifting by that power. You can also multiply by other numbers.

// b = 10*a, which is 8*a + 2*a
b = ( a << 3 ) + ( a << 1 );

Division isn't as nice looking.

Getting array sizes

In C, you can get the total size of an array in bytes using the sizeof operator. Dividing it by the size of each element will give you the number of elements in the array.
MyHugeStructure array[100];
int i;

for( i = 0; i < sizeof(array)/sizeof(*array); i++ ) 
{
    array[i].id = i;
}
The downside is that if you ever change the array to be dynamically allocated using new or malloc(), then sizeof() will only return the pointer size. Of course, in a large project you should have have a defined constant for the number of entries.

Else Flattening

Deeply nested else clauses are hard to understand. But if they look like this, you are in luck:

if ( A ) {

} else {
   if ( B ) {

   } else {
       if ( C ) {

       }
   }
}

Save money on wide screen monitors by flattening the elses.

if ( A ) {

} else if ( B ) {

} else if ( C ) {

}

Black is White and Up is Down if you're not using a mainstream compiler

If your code has to run on a lot of different platforms, you should know that GCC and Microsoft VC++ are very similar and this similarity will trick you into thinking that all compilers are alike. But the C standard leaves a lot of stuff out that will surprise you, if you do work on embedded systems with crappy, but standards-conforming compilers.

What is sizeof(char)? In most compilers it is 1 byte. Switch compilers, and it might be 2 or 4 bytes. The C standards only says that it cannot be larger than an int.

What happens if you add something to the maximum integer?

int a = MAX_INT;
a++;
printf("%dn", a);

In mainstream compilers you get a negative number, since the number space wraps around. But the C standard says the result is undefined. So some compilers will leave a at the maximum value, MAX_INT.

And don't even think about bit-shifting a negative number:

int a = -2;
printf("%dn", a >> 1 );

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Real Name

2009-06-24 01:09:22
THIS POST SUCKS

matt burns

2009-06-24 07:18:08
Understanding a bit of logic was essential for me recently when I worked with an in-house programming language from the 70s that didn't allow parentheses.

Modus ponendo tollens.

stevex

2009-06-24 07:47:00
Multiplying by shifting is done for you by the optimizer if it makes sense. Don't bother doing it in your own code.

jldugger

2009-06-24 14:37:39
By default, I would suggest anyone asking to do multiplication by bitshift offer concrete proof that the compiler isn't doing it already, and that it's safe.

Jakub Kaplan (jk451)

2009-07-04 14:01:29
Note that there is a rather serious nice use for bit shifting.

Assuming you are in an environment where saving memory is more precious than saving time, you can do so by "packing" information into casual numbers.

E.g. you've got 32 boolean flags determining settings of your program. Then you can save 1 to 5-th flag by doing:

settingsSoFar+=1<<(5-1)

Extracting is analogous.

David Kerkeslager

2009-07-18 00:23:05
Adding to the else-if flattening tip: you can make your indentation more consistent by using a if-elif*-else? structure (* and ? used in their regular expression sense). This can be accomplished in C/C++ using a simple macro:

#define elif else if

OK Programmer

2009-07-18 01:14:26
The nested conditional type is a terrible idea in this circumstance. Instead of letting the compiler generate a O(0) table, you let it generate a O(n) set of if statements. For the non-algorithmic people out there, this means the table lookup always take the same amount of time (and will be much faster than the ifs here), whereas the nested conditionals will keeping checking deeper executed until a match is made.

David Kerkeslager

2009-07-18 01:16:16
Jakub, this will be faster and will work on more compilers. flags should be an unsigned int to avoid twos-complement storage (let's assume 32 bits, so bits will be numbered 0-31).

// Set the xth bit (to 1).

flags |= 1 << x;

// Clear the xth bit (set it to 0).

flags &= ~(1 << x);

// Flip the xth bit (if it's set, clear it; if it's clear, set it).

flags ^= 1 << x;

// Extract the xth bit as an integer and store it in test.

test = flags >> x & 1;

Dan D.

2009-07-30 17:35:16
This list is... just not very good. Parts of it are plain wrong, and others are not as advertised.

On the ternary conditional operator ?: : yes it saves on typing but it's dog-slow compared to a switch statement. Save it for when a switch statement won't work (for instance, when an expression must be reevaluated if it doesn't match). The example you gave is the perfect time to use a switch statement, although your default case should be "str = rand() == 42?"four":"unknown number";break; to make the nested ?: code.

On DeMorgan's: This is CompSci 101, or 201 at the latest. They teach you precisely this in school.

On rounding: this code is not portable to platforms where a sizeof(char) is not 8. For chars the code should be unsigned char bitmap [NUMBER_OF_BITS + sizeof(char)-1/sizeof(char)]; You should be able to figure out how to do that with ints as well.

On real rounding: The best one of the list.

On bit-shifting: if they don't teach you this in school, you're at the wrong school. Same if they don't tell you that if you want multiplication, unless you have hardware reasons to bit-shift (side-effects in a MIPS processor, for instance), you should simply multiply. Let the complier sort it out. The compiler knows better than you, for instance, that multiplication always works, but weird things can happen on certain models of 64-bit processors running in 32-bit mode if you shift left by 32.

Else flattening: seriously dude, what crappy school did you go to that they didn't tell you about this?

On non-mainstream compilers: indeed, so apply that to your own examples! And bit-shifting a negative number works exactly as well as bit shifting a positive number, which is to say, again, if you want multiplication (or division), JUST DO IT!

Sachin Pujari

2009-09-13 07:28:56
You are no big programmer than me, Steve u r just a theif stealing some other authors tips and tricks...

cool

2009-10-21 19:22:48
I'd like to see another one of these what you didnt learn in school articles. hint

guy

2010-01-10 00:28:03
> What is sizeof(char)? In most compilers it is 1 byte. Switch compilers, and it might be 2 or 4 bytes. The C standards only says that it cannot be larger than an int.

No, sorry, that is wrong. sizeof(char) is always 1, and the sizeof all other types is measured in chars. Even if a character is stored as 2 or 4 bytes on some platform, sizeof(char) will equal 1. And don't forget that a "byte" is not 8 bits on every architecture.

John From

2010-01-10 02:24:23
FAIL.

WoW

2010-04-10 09:57:43
That is very interrsting in embedded systems

settingsComingSoon

2011-08-08 10:55:22
@Jakub Kaplan (jk451)

rather serious not nice Danger: your flags carry into the next flag if it is set twice. |= would probably be better, (or perhaps a language that doesn't use smileys <= ?:) (or perhaps less than a million settings ;)

settingsSoFar+=1<<(5-1)

bumper

2011-08-08 11:01:58
Ok, OK programmer,

branch prediction: but the lookup table is then mispredicted all the time, whereas the nested ifs are correctly branch predicted ~always and therefore free.

this blog is awesome :) all mediocre programmers should advertise themsleves widely so they, and their followers can be screened.

Chris

2012-02-03 08:02:29
"What is sizeof(char)? In most compilers it is 1 byte. Switch compilers, and it might be 2 or 4 bytes. The C standards only says that it cannot be larger than an int."

This is wrong. On all compilers sizeof(char) is 1. The C standard defines sizeof in terms of numbers of chars, since a char is one byte. If sizeof(char) is not 1, you're not using a conforming C compiler.

Alex Ratushnyak

2012-05-17 01:17:44
Hi Steve!

If you are a C expert, please try this challenge:

http //www.linkedin.com/groups?gid=4264441

Email
steve.hanov@gmail.com

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