Comments on: Efficient C Tips #6 – Don’t use the ternary operator

By: Doug

Doug — Wed, 28 Jul 2010 00:20:20 +0000

My personal favorite “do not use unless absolutely necessary” construct is modulo. Modulo looks simple and elegant when written, but try it out — write a simple modulo construct, compile it and look at the assembly output. It’s horrific. Think the problem through and simplify it instead. I haven’t yet found a modulo solution I couldn’t implement much more simply and completely by rethinking the problem, resulting in code that’s as accurate, uses less memory, and runs quicker.

By: Greg Nelson

Greg Nelson — Wed, 24 Feb 2010 19:47:03 +0000

The printf() comment reminds me of something else… Nigel, since you said you are often interested in people's weird coding conventions, this one might fit the bill. Want to guess why I write:char tmp1[10];char tmp2[10];char tmp3[10];char final[50];snprintf(tmp1, "%7.1f", f1);snprintf(tmp2, "%7.1f", f2);snprintf(tmp3, "%7.1f", f3);snprintf(final, "%s %s %s", tmp1, tmp2, tmp3);Answer:Because one of the compilers/libraries (still haven't found the culprit) used for our code can't handle multiple double arguments to a varargs call!If I do it the conventional way with "%7.1f %7.1f %7.1f" instead, I either get garbage output, or the program hangs.My guess is this has something to do with poor floating point emulation and/or different ways of stacking doubles versus ints/ptrs/etc.

By: bandit

bandit — Tue, 26 Jan 2010 01:50:50 +0000

I use them usually only for printf():printf( "%s", (bletch ? "foo" : "bar") );otherwise I use if() else for clarity. I normally dofoo = 0;if( bar ) foo = 1;because this handles the default case && the exceptional case.

By: ashleigh

ashleigh — Tue, 26 Jan 2010 01:16:26 +0000

I've seen the same as Nigel – often the ternary operator is worse.I still use it where it improves readability of the source and I don't care about performance.I guess the lesson is – think about what you write. Write with maintenance, readability AND performance in mind. Know what the compiler does (ie go in with your eyes open), and then write whats best for the situation you are handling.

By: Mike

Mike — Wed, 20 Jan 2010 16:34:16 +0000

Just ran a test with HC08 and CodeWarrior — ternary operator uses 4 more bytes than if/else in one particular test case.

By: Nigel Jones

Nigel Jones — Mon, 30 Nov 2009 12:05:02 +0000

I guess I'll just point to my previous comment. I would add, that as compilers improve (even cheap ones) this should become less of an issue. Indeed, unless I'm writing a time critical piece of code I'll use the operator that more naturally fits the problem.

By: Brian Egge

Brian Egge — Sun, 29 Nov 2009 22:04:53 +0000

I've found the ternary operator creates the same assembly as the control structure. I think with modern, optimizing compilers, it really a matter of syntaxtic preference.

By: victorh

victorh — Thu, 19 Feb 2009 21:09:00 +0000

On some platforms, this form is even more efficient:y = d;if (a > b) { y = c;}provided that y is a regular RAM-based variable. If not, e.g. it is a memory-mapped UART transmit data register, the double assignment could (and will) have disastrous side effects.Use with caution.

By: Nigel Jones

Nigel Jones — Thu, 19 Feb 2009 13:22:00 +0000

I’m not surprised. While I have seen compilers generate the same code for both the ternary operator and the if/else construct, I have never seen a case where the compiler generates better code for the ternary operator. Conversely, I have seen cases where the compiler generates better code for the if/else construct than it does for the ternary operator.

By: Uhmmmm

Uhmmmm — Wed, 18 Feb 2009 22:45:00 +0000

I just did a quick test with gcc 4.3.3, with a function that just does "return (a > b) ? c : d;" and another one with the equivalent if/else. It generates identical assembly for me on both amd64 and arm.