C or C++? - Page 14

ewmayer · 2012-02-12, 20:02

Quote:

Originally Posted by CRGreathouse

What about random numbers, time/date, and other things that will change without regard to the source?

I believe the distinction y'all are circling around is "deterministic or not." This same distinction is a key parameter for RNGs (random number generators). Sometimes one needs genuinely unpredictable numbers (or initial seeds); but in many cases one needs "deterministically random" numbers for e.g. software QA purposes.

Reading from an uninitialized memory location (in the lingo, a UMR - uninitialized memory read) is clearly nondeterministic - sure, you may get the same result over multiple runs of the program, if those re-use the same memory space and nothing else writes the memloc in question in the interim. (Often when trying to track down a crash or other bug which eventually proves due to a UMR, one in fact relies on that near-term-repeatability aspect over multiple debugger runs). But there's a good reason there are dedicated code tools (e.g. Purify, valgrind) designed to ferret out nasties such as UMRs.

Ralf Recker · 2012-02-12, 20:23

Quote:

Originally Posted by ewmayer

Reading from an uninitialized memory location (in the lingo, a UMR - uninitialized memory read) is clearly nondeterministic - sure, you may get the same result over multiple runs of the program, if those re-use the same memory space and nothing else writes the memloc in question in the interim. (Often when trying to track down a crash or other bug which eventually proves due to a UMR, one in fact relies on that near-term-repeatability aspect over multiple debugger runs). But there's a good reason there are dedicated code tools (e.g. Purify, valgrind) designed to ferret out nasties such as UMRs.

Uninitialized memory reads and valgrind: That reminds me of the Debian OpenSSL incident...

Dubslow · 2012-02-12, 22:07

Code:

while((c = getc(stdin)) != EOF){
              if(cp == &in_line[LINELNG-1] || c == '\n'){
                      /*
                       stuff to end string*/
              }
              else
                      *cp++ = c;
}

Oh, after seeing that the first time, it really feels dirty, but at the same time, it's oh-such-a-beautiful line of code.

axn · 2012-02-13, 04:42

Quote:

Originally Posted by Dubslow

I guess for printing then, I just need to specify more than 6 significant digits.

Now, though we are agreed that %lf is a double for scanf, why does 3.141592653589 get truncated to 3.141593, even though the former should fit inside a double?

Nothing is getting truncated excepted for the printed output!!!! And you know why _that_ is happening.

chalsall · 2012-02-13, 04:55

Quote:

Originally Posted by axn

Nothing is getting truncated excepted for the printed output!!!! And you know why _that_ is happening.

There are reasons why teachers must have much patience....

(Or, at least, the good ones.

)

Dubslow · 2012-02-13, 06:06

Quote:

Originally Posted by axn

Nothing is getting truncated excepted for the printed output!!!! And you know why _that_ is happening.

I'm sorry, I do sometimes do smart things!!! I swear!

axn · 2012-02-13, 06:49

Quote:

Originally Posted by Dubslow

I'm sorry, I do sometimes do smart things!!! I swear!

I believe ya.

Dubslow · 2012-02-16, 03:01

As for that pointer business, I know understand why what I did was no different from just using standard array notation. As such, I rewrote the LL program, by which I mean I made better use of actual pointers.

http://dubslow.tk/random/ll.txt

On the other hand, I think I also went a bit overboard with the pointer arithmetic. One went from one extreme to the other...

Edit: I think the old one is faster...

axn · 2012-02-16, 04:51

Quote:

Originally Posted by Dubslow

On the other hand, I think I also went a bit overboard with the pointer arithmetic. One went from one extreme to the other...

Code:

/* This would definitely be better with indices than pointers.	*/

I agree. That loop is the worst loop in the whole program to be chosen for pointer arithmetic.

Dubslow · 2012-02-16, 06:14

Two typos in one quote. I am a boss.

*Went from one extreme to the other....

*with indices rather than (with) pointers.

emily · 2012-02-21, 22:00

It's also worth noting that there is the D programming language which might be of interest if you want something similar to C++ but with some features of C# and Java. http://en.wikipedia.org/wiki/D_(programming_language)

2012-02-12, 22:07	#146
Dubslow Basketry That Evening! "Bunslow the Bold" Jun 2011 40<A<43 -89<O<-88 1C35₁₆ Posts	Code: while((c = getc(stdin)) != EOF){ if(cp == &in_line[LINELNG-1] \|\| c == '\n'){ /* stuff to end string/ } else cp++ = c; } Oh, after seeing that the first time, it really feels dirty, but at the same time, it's oh-such-a-beautiful line of code. Last fiddled with by Dubslow on 2012-02-12 at 22:07

2012-02-16, 03:01	#151
Dubslow Basketry That Evening! "Bunslow the Bold" Jun 2011 40<A<43 -89<O<-88 3×29×83 Posts	As for that pointer business, I know understand why what I did was no different from just using standard array notation. As such, I rewrote the LL program, by which I mean I made better use of actual pointers. http://dubslow.tk/random/ll.txt On the other hand, I think I also went a bit overboard with the pointer arithmetic. One went from one extreme to the other... Edit: I think the old one is faster... Last fiddled with by Dubslow on 2012-02-16 at 03:28

2012-02-16, 06:14	#153
Dubslow Basketry That Evening! "Bunslow the Bold" Jun 2011 40<A<43 -89<O<-88 7221₁₀ Posts	Two typos in one quote. I am a boss. Went from one extreme to the other.... with indices rather than (with) pointers.

2012-02-21, 22:00	#154
emily Feb 2012 Athens, Greece 47 Posts	It's also worth noting that there is the D programming language which might be of interest if you want something similar to C++ but with some features of C# and Java. http://en.wikipedia.org/wiki/D_(programming_language)