Related
I saw this code:
if (cond) {
perror("an error occurred"), exit(1);
}
Why would you do that? Why not just:
if (cond) {
perror("an error occurred");
exit(1);
}
In your example it serves no reason at all. It is on occasion useful when written as
if(cond)
perror("an error occured"), exit(1) ;
-- then you don't need curly braces. But it's an invitation to disaster.
The comma operator is to put two or more expressions in a position where the reference only allows one. In your case, there is no need to use it; in other cases, such as in a while loop, it may be useful:
while (a = b, c < d)
...
where the actual "evaluation" of the while loop is governed solely on the last expression.
Legitimate cases of the comma operator are rare, but they do exist. One example is when you want to have something happen inside of a conditional evaluation. For instance:
std::wstring example;
auto it = example.begin();
while (it = std::find(it, example.end(), L'\\'), it != example.end())
{
// Do something to each backslash in `example`
}
It can also be used in places where you can only place a single expression, but want two things to happen. For instance, the following loop increments x and decrements y in the for loop's third component:
int x = 0;
int y = some_number;
for(; x < y; ++x, --y)
{
// Do something which uses a converging x and y
}
Don't go looking for uses of it, but if it is appropriate, don't be afraid to use it, and don't be thrown for a loop if you see someone else using it. If you have two things which have no reason not to be separate statements, make them separate statements instead of using the comma operator.
The main use of the comma operator is obfuscation; it permits doing two
things where the reader only expects one. One of the most frequent
uses—adding side effects to a condition, falls under this
category. There are a few cases which might be considered valid,
however:
The one which was used to present it in K&R: incrementing two
variables in a for loop. In modern code, this might occur in a
function like std::transform, or std::copy, where an output iterator
is incremented symultaneously with the input iterator. (More often, of
course, these functions will contain a while loop, with the
incrementations in separate statements at the end of the loop. In such
cases, there's no point in using a comma rather than two statements.)
Another case which comes to mind is data validation of input parameters
in an initializer list:
MyClass::MyClass( T const& param )
: member( (validate( param ), param) )
{
}
(This assumes that validate( param ) will throw an exception if
something is wrong.) This use isn't particularly attractive, especially
as it needs the extra parentheses, but there aren't many alternatives.
Finally, I've sometimes seen the convention:
ScopedLock( myMutex ), protectedFunction();
, which avoids having to invent a name for the ScopedLock. To tell
the truth, I don't like it, but I have seen it used, and the alternative
of adding extra braces to ensure that the ScopedLock is immediately
destructed isn't very pretty either.
This can be better understood by taking some examples:
First:
Consider an expression:
x = ++j;
But for time being, if we need to assign a temporarily debug value, then we can write.
x = DEBUG_VALUE, ++j;
Second:
Comma , operators are frequently used in for() -loop e.g.:
for(i = 0, j = 10; i < N; j--, i++)
// ^ ^ here we can't use ;
Third:
One more example(actually one may find doing this interesting):
if (x = 16 / 4), if remainder is zero then print x = x - 1;
if (x = 16 / 5), if remainder is zero then print x = x + 1;
It can also be done in a single step;
if(x = n / d, n % d) // == x = n / d; if(n % d)
printf("Remainder not zero, x + 1 = %d", (x + 1));
else
printf("Remainder is zero, x - 1 = %d", (x - 1));
PS: It may also be interesting to know that sometimes it is disastrous to use , operator. For example in the question Strtok usage, code not working, by mistake, OP forgot to write name of the function and instead of writing tokens = strtok(NULL, ",'");, he wrote tokens = (NULL, ",'"); and he was not getting compilation error --but its a valid expression that tokens = ",'"; caused an infinite loop in his program.
The comma operator allows grouping expression where one is expected.
For example it can be useful in some case :
// In a loop
while ( a--, a < d ) ...
But in you case there is no reason to use it. It will be confusing... that's it...
In your case, it is just to avoid curly braces :
if(cond)
perror("an error occurred"), exit(1);
// =>
if (cond)
{
perror("an error occurred");
exit(1);
}
A link to a comma operator documentation.
There appear to be few practical uses of operator,().
Bjarne Stroustrup, The Design and Evolution of C++
Most of the oft usage of comma can be found out in the wikipedia article Comma_operator#Uses.
One interesting usage I have found out when using the boost::assign, where it had judiciously overloaded the operator to make it behave as a comma separated list of values which can be pushed to the end of a vector object
#include <boost/assign/std/vector.hpp> // for 'operator+=()'
using namespace std;
using namespace boost::assign; // bring 'operator+=()' into scope
{
vector<int> values;
values += 1,2,3,4,5,6,7,8,9; // insert values at the end of the container
}
Unfortunately, the above usage which was popular for prototyping would now look archaic once compilers start supporting Uniform Initialization
So that leaves us back to
There appear to be few practical uses of operator,().
Bjarne Stroustrup, The Design and Evolution of C++
In your case, the comma operator is useless since it could have been used to avoid curly braces, but it's not the case since the writer has already put them. Therefore it's useless and may be confusing.
It could be useful for the itinerary operator if you want to execute two or more instructions when the condition is true or false. but keep in mind that the return value will be the most right expression due to the comma operator left to right evalutaion rule (I mean inside the parentheses)
For instance:
a<b?(x=5,b=6,d=i):exit(1);
The boost::assign overloads the comma operator heavily to achieve this kind of syntax:
vector<int> v;
v += 1,2,3,4,5,6,7,8,9;
(If this is a duplicate please point me to an answer)
I have two scenarios where a loop is checking a complex expression over and over again (a complex expression would consist of math operations and retrieving data):
for (int i = 0; i < expression; i++) {
// stuff
}
for (int i = 0; i < someNumber; i++) {
if (i == expression) break;
}
I'm wondering if it's more efficient to pre-calculate the expression and check against a known value like so
int known = expression;
for (int i = 0; i < known; i++) {
// stuff
}
for (int i = 0; i < someNumber; i++) {
if (i == known) break;
}
or if it's done by the compiler automatically.
For reference, I'm running the loop ~700 000 000 times and the expression is something like structure->arr[j] % n or sqrt(a * n + b)
Is it even worth it?
If the compiler is able to detect that calculating expression will give the same result every time, it will only do the calculation once.
The tricky part is: "If the compiler is able to ...."
Compilers are very smart and will probably be successful in most cases. But why take the chance?
Just write that extra line to do the calculation before the loop as you did in your second example.
By doing that you send a clear message to the compiler about expression being constant within the loops. Further it may also help your co-workers to easier understand the code.
That said... you yourself must be sure that expression is in fact the same every time. Let's look at your example:
the expression is something like structure->arr[i] % n or sqrt(a * n + b)
Now the first one, i.e. structure->arr[i] % n depends on the loop variable i so it will be a big mistake to move the code outside the loop.
The second (i.e. sqrt(a * n + b)) looks better provided that a n b doesn't change inside the loop.
Is it possible to multiply using i** in C?
For example, I can increment i using i++. Why doesn't i** work in C?
#include <stdio.h>
int main(void)
{
int result;
for (int i = 2; i < 100; i**){
result = i + 1;
printf("%i\n", result);
}
return 0;
}
No, it's not possible. There is no operator like ** in C unlike unary increment (++) and decrement (--) operators. You should have try i *= i.
"i++" is shorthand for "i = i + 1".
If there were an "i**" it would, by extension, mean "i = i * 1" and be incredibly useless. So they never implemented it.
Even after editing to clarify grammar, it still isn't clear from your question that you expect "i**" to perform as "i = i * i". I'm guessing that is what you meant from the answer you accepted. If you learn to explain things clearly to others you will find that you think more clearly and can work out the answer to many questions for yourself.
Possible, but instead of i** which doesn't work, you need to use:
for (int i = 2; i < 100; i *= i)
Generally, multiplication operation does not use in for loop increment/decrement part because suppose our variable(i) start from 0, then every time multiplication become 0.
Multiplying by i** is it possible in C? Like i++ why i**doesn't work in C?
No,it's not possible. for your second quesition answer is explained like,
Basically , increment and decrement have exceptional usage as pre increment and post increment and a language cannot be extended just if someone needs an additional functionality as it would slow down because of extending its grammer.
So most used ++i,i++,--i,i-- are present and not others
You can use some codes like this for your task:
i*=i;
=i*i;
I am able to do this without using bitwise operators as below
int AsciiToInteger()
{
char s[] = "Stack Overflow";
int i, n = 0;
for (i = 0; s[i] !='\0'; i++)
{
n += s[i];
}
return n;
}
How can I achieve the same using bitwise operators in C without using for loop?
You can achieve the same without a for loop using recursion:
int AsciiToInteger(const char * Str)
{
if(*Str)
return (int)*Str + AsciiToInteger(Str+1);
else
return 0;
}
/* ... */
int n = AsciiToInteger("Stack Overflow");
I don't know what bitwise operators have to do with this, you surely cannot use only them without a loop and without recursion for arbitrary-length strings (for fixed length strings instead the result would probably be something like unrolling the loop).
... but now that I read the comments I'm quite sure I didn't get the sense of the question... :S
Except as an exercise in building higher level operations from bitwise operations, the task you're trying to accomplish is foolish. Don't do it.
As an exercise, the most important thing to realize is that you don't have to go back to the start every time you need to implement something new in terms of the building blocks. Instead you could write addition and subtraction functions in terms of bitwise building blocks, and put those together using the existing higher-level algorithm you've already got.
As for eliminating the loop, you could just unroll it to support a fixed max number of digits (the longest value that will fit in int, for example) unless you need to support arbitrary number of leading zeros. Recursion is a very bad approach in general and contrary to the whole "close to the metal" aspect of this exercise. Perhaps they just want you to avoid adding/incrementing a counter in the loop with "high level" addition, in which case you could use your bitwise adder function...
One of the main reasons that loops exist is so that you can do operations an unknown number of times. If you don't know how long your string is, you have no way of doing this without a loop. Even if you do know the length of the string, why would you want to do it without a loop?
What is the need for the conditional operator? Functionally it is redundant, since it implements an if-else construct. If the conditional operator is more efficient than the equivalent if-else assignment, why can't if-else be interpreted more efficiently by the compiler?
In C, the real utility of it is that it's an expression instead of a statement; that is, you can have it on the right-hand side (RHS) of a statement. So you can write certain things more concisely.
Some of the other answers given are great. But I am surprised that no one mentioned that it can be used to help enforce const correctness in a compact way.
Something like this:
const int n = (x != 0) ? 10 : 20;
so basically n is a const whose initial value is dependent on a condition statement. The easiest alternative is to make n not a const, this would allow an ordinary if to initialize it. But if you want it to be const, it cannot be done with an ordinary if. The best substitute you could make would be to use a helper function like this:
int f(int x) {
if(x != 0) { return 10; } else { return 20; }
}
const int n = f(x);
but the ternary if version is far more compact and arguably more readable.
The ternary operator is a syntactic and readability convenience, not a performance shortcut. People are split on the merits of it for conditionals of varying complexity, but for short conditions, it can be useful to have a one-line expression.
Moreover, since it's an expression, as Charlie Martin wrote, that means it can appear on the right-hand side of a statement in C. This is valuable for being concise.
It's crucial for code obfuscation, like this:
Look-> See?!
No
:(
Oh, well
);
Compactness and the ability to inline an if-then-else construct into an expression.
There are a lot of things in C that aren't technically needed because they can be more or less easily implemented in terms of other things. Here is an incomplete list:
while
for
functions
structs
Imagine what your code would look like without these and you may find your answer. The ternary operator is a form of "syntactic sugar" that if used with care and skill makes writing and understanding code easier.
Sometimes the ternary operator is the best way to get the job done. In particular when you want the result of the ternary to be an l-value.
This is not a good example, but I'm drawing a blank on somethign better. One thing is certian, it is not often when you really need to use the ternary, although I still use it quite a bit.
const char* appTitle = amDebugging ? "DEBUG App 1.0" : "App v 1.0";
One thing I would warn against though is stringing ternaries together. They become a real
problem at maintennance time:
int myVal = aIsTrue ? aVal : bIsTrue ? bVal : cIsTrue ? cVal : dVal;
EDIT: Here's a potentially better example. You can use the ternary operator to assign references & const values where you would otherwise need to write a function to handle it:
int getMyValue()
{
if( myCondition )
return 42;
else
return 314;
}
const int myValue = getMyValue();
...could become:
const int myValue = myCondition ? 42 : 314;
Which is better is a debatable question that I will choose not to debate.
Since no one has mentioned this yet, about the only way to get smart printf statements is to use the ternary operator:
printf("%d item%s", count, count > 1 ? "s\n" : "\n");
Caveat: There are some differences in operator precedence when you move from C to C++ and may be surprised by the subtle bug(s) that arise thereof.
The fact that the ternary operator is an expression, not a statement, allows it to be used in macro expansions for function-like macros that are used as part of an expression. Const may not have been part of original C, but the macro pre-processor goes way back.
One place where I've seen it used is in an array package that used macros for bound-checked array accesses. The syntax for a checked reference was something like aref(arrayname, type, index), where arrayname was actually a pointer to a struct that included the array bounds and an unsigned char array for the data, type was the actual type of the data, and index was the index. The expansion of this was quite hairy (and I'm not going to do it from memory), but it used some ternary operators to do the bound checking.
You can't do this as a function call in C because of the need for polymorphism of the returned object. So a macro was needed to do the type casting in the expression.
In C++ you could do this as a templated overloaded function call (probably for operator[]), but C doesn't have such features.
Edit: Here's the example I was talking about, from the Berkeley CAD array package (glu 1.4 edition). The documentation of the array_fetch usage is:
type
array_fetch(type, array, position)
typeof type;
array_t *array;
int position;
Fetch an element from an array. A
runtime error occurs on an attempt to
reference outside the bounds of the
array. There is no type-checking
that the value at the given position
is actually of the type used when
dereferencing the array.
and here is the macro defintion of array_fetch (note the use of the ternary operator and the comma sequencing operator to execute all the subexpressions with the right values in the right order as part of a single expression):
#define array_fetch(type, a, i) \
(array_global_index = (i), \
(array_global_index >= (a)->num) ? array_abort((a),1) : 0,\
*((type *) ((a)->space + array_global_index * (a)->obj_size)))
The expansion for array_insert ( which grows the array if necessary, like a C++ vector) is even hairier, involving multiple nested ternary operators.
It's syntatic sugar and a handy shorthand for brief if/else blocks that only contain one statement. Functionally, both constructs should perform identically.
like dwn said, Performance was one of its benefits during the rise of complex processors, MSDN blog Non-classical processor behavior: How doing something can be faster than not doing it gives an example which clearly says the difference between ternary (conditional) operator and if/else statement.
give the following code:
#include <windows.h>
#include <stdlib.h>
#include <stdlib.h>
#include <stdio.h>
int array[10000];
int countthem(int boundary)
{
int count = 0;
for (int i = 0; i < 10000; i++) {
if (array[i] < boundary) count++;
}
return count;
}
int __cdecl wmain(int, wchar_t **)
{
for (int i = 0; i < 10000; i++) array[i] = rand() % 10;
for (int boundary = 0; boundary <= 10; boundary++) {
LARGE_INTEGER liStart, liEnd;
QueryPerformanceCounter(&liStart);
int count = 0;
for (int iterations = 0; iterations < 100; iterations++) {
count += countthem(boundary);
}
QueryPerformanceCounter(&liEnd);
printf("count=%7d, time = %I64d\n",
count, liEnd.QuadPart - liStart.QuadPart);
}
return 0;
}
the cost for different boundary are much different and wierd (see the original material). while if change:
if (array[i] < boundary) count++;
to
count += (array[i] < boundary) ? 1 : 0;
The execution time is now independent of the boundary value, since:
the optimizer was able to remove the branch from the ternary expression.
but on my desktop intel i5 cpu/windows 10/vs2015, my test result is quite different with msdn blog.
when using debug mode, if/else cost:
count= 0, time = 6434
count= 100000, time = 7652
count= 200800, time = 10124
count= 300200, time = 12820
count= 403100, time = 15566
count= 497400, time = 16911
count= 602900, time = 15999
count= 700700, time = 12997
count= 797500, time = 11465
count= 902500, time = 7619
count=1000000, time = 6429
and ternary operator cost:
count= 0, time = 7045
count= 100000, time = 10194
count= 200800, time = 12080
count= 300200, time = 15007
count= 403100, time = 18519
count= 497400, time = 20957
count= 602900, time = 17851
count= 700700, time = 14593
count= 797500, time = 12390
count= 902500, time = 9283
count=1000000, time = 7020
when using release mode, if/else cost:
count= 0, time = 7
count= 100000, time = 9
count= 200800, time = 9
count= 300200, time = 9
count= 403100, time = 9
count= 497400, time = 8
count= 602900, time = 7
count= 700700, time = 7
count= 797500, time = 10
count= 902500, time = 7
count=1000000, time = 7
and ternary operator cost:
count= 0, time = 16
count= 100000, time = 17
count= 200800, time = 18
count= 300200, time = 16
count= 403100, time = 22
count= 497400, time = 16
count= 602900, time = 16
count= 700700, time = 15
count= 797500, time = 15
count= 902500, time = 16
count=1000000, time = 16
the ternary operator is slower than if/else statement on my machine!
so according to different compiler optimization techniques, ternal operator and if/else may behaves much different.
Some of the more obscure operators in C exist solely because they allow implementation of various function-like macros as a single expression that returns a result. I would say that this is the main purpose why the ?: and , operators are allowed to exist, even though their functionality is otherwise redundant.
Lets say we wish to implement a function-like macro that returns the largest of two parameters. It would then be called as for example:
int x = LARGEST(1,2);
The only way to implement this as a function-like macro would be
#define LARGEST(x,y) ((x) > (y) ? (x) : (y))
It wouldn't be possible with an if ... else statement, since it does not return a result value. Note)
The other purpose of ?: is that it in some cases actually increases readability. Most often if...else is more readable, but not always. Take for example long, repetitive switch statements:
switch(something)
{
case A:
if(x == A)
{
array[i] = x;
}
else
{
array[i] = y;
}
break;
case B:
if(x == B)
{
array[i] = x;
}
else
{
array[i] = y;
}
break;
...
}
This can be replaced with the far more readable
switch(something)
{
case A: array[i] = (x == A) ? x : y; break;
case B: array[i] = (x == B) ? x : y; break;
...
}
Please note that ?: does never result in faster code than if-else. That's some strange myth created by confused beginners. In case of optimized code, ?: gives identical performance as if-else in the vast majority of the cases.
If anything, ?: can be slower than if-else, because it comes with mandatory implicit type promotions, even of the operand which is not going to be used. But ?: can never be faster than if-else.
Note) Now of course someone will argue and wonder why not use a function. Indeed if you can use a function, it is always preferable over a function-like macro. But sometimes you can't use functions. Suppose for example that x in the example above is declared at file scope. The initializer must then be a constant expression, so it cannot contain a function call. Other practical examples of where you have to use function-like macros involve type safe programming with _Generic or "X macros".
ternary = simple form of if-else. It is available mostly for readability.
The same as
if(0)
do();
if(0)
{
do();
}