I am trying to do a product operand on the values inside of a vector. It is a huge mess of code.. I have posted it previously but no one was able to help. I just wanna confirm which is the correct way to do a single part of it. I currently have:
vector<double> taylorNumerator;
for(a = 0; a <= (constant); a++) {
double Number = equation involving a to get numerous values;
taylorNumerator.push_back(Number);
for(b = 0; b <= (constant); b++) {
double NewNumber *= taylorNumerator[b];
}
This is what I have as a snapshot, it is very short from what I actually have. Someone told me it is better to do vector.at(index) instead. Which is the correct or best way to accomplish this? If you so desire I can paste all of the code, it works but the values I get are wrong.
When possible, you should probably avoid using indexes at all. Your options are:
A range-based for loop:
for (auto numerator : taylorNumerators) { ... }
An iterator-based loop:
for (auto it = taylorNumerators.begin(); it != taylorNuemrators.end(); ++it) { ... }
A standard algorithm, perhaps with a lambda:
#include <algorithm>
std::for_each(taylorNumerators, [&](double numerator) { ... });
In particular, note that some algorithms let you specify a number of iterations, like std::generate_n, so you can create exactly n items without counting to n yourself.
If you need the index in the calculation, then it can be appropriate to use a traditional for loop. You have to watch for a couple pitfalls: std::vector<T>::size() returns a std::vector<T>::size_type which is typically identical to std::size_type, which is (1) unsigned and (2) quite possibly larger than an int.
for (std::size_t i = 0; i != taylorNumerators.size(); ++i) { ... }
Your calculations probably deal with doubles or some numerical type other than std::size_t, so you have to consider the best way to convert it. Many programmers would rely on implicit conversions, but that can be dangerous unless you know the conversion rules very well. I'd generally start by doing a static cast of the index to the type I actually need. For example:
for (std::size_t i = 0; i != taylorNumerators.size(); ++i) {
const auto x = static_cast<double>(i);
/* calculation involving x */
}
In C++, it's probably far more common to make sure the index is in range and then use operator[] rather than to use at(). Many projects disable exceptions, so the safety guarantee of at() wouldn't really be available. And, if you can check the range once yourself, then it'll be faster to use operator[] than to rely on the range-check built into at() on each index operation.
What you have is fine. Modern compilers can optimize the heck out of the above such that the code is just as fast as the equivalent C code of accessing items direclty.
The only optimization for using vector I recommend is to invoke taylorNumerator.reserve(constant) to allocate the needed storage upfront instead of the vector resizing itself as new items are added.
About the only worthy optimization after that is to not use vector at all and just use a static array - especially if constant is small enough that it doesn't blow up the stack (or binary size if global).
double taylorNumerator[constant];
I'm trying to carry out some loop optimization as described here: Optimizing a Loop vs Code Duplication
I have the additional complication that some code inside the loop only needs to be executed depending on a combination of run-time-known variables external to the loop (which can be replaced with template parameters for optimization, as discussed in the link above) and a run-time-known variable that is only known inside the loop.
Here is the completely un-optimized version of the code:
for (int i = 0; i < 100000, i++){
if (external_condition_1 || (external_condition_2 && internal_condition[i])){
run_some_code;
}
else{
run_some_other_code;
}
run_lots_of_other_code;
}
This is my attempt at wrapping the loop in a templated function as suggested in the question linked above to optimize performance and avoid code duplication by writing multiple versions of the loop:
template<bool external_condition_1, external_condition_2>myloop(){
for (int i = 0; i < 100000, i++){
if (external_condition_1 || (external_condition_2 && internal_condition[i]){
run_some_code;
}
else{
run_some_other_code;
}
run_lots_of_other_code;
}
My question is: how can the code be written to avoid branching and code duplication?
Note that the code is sufficiently complex that the function probably can't be inlined, and compiler optimization also likely wouldn't sort this out in general.
My question is: how can the code be written to avoid branching and code duplication?
Well, you already wrote your template to avoid code duplication, right? So let's look at what branching is left. To do this, we should look at each function that is generated from your template (there are four of them). We should also apply the expected compiler optimizations based upon the template parameters.
First up, set condition 1 to true. This should produce two functions that are essentially (using a bit of pseudo-syntax) the following:
myloop<true, bool external_condition_2>() {
for (int i = 0; i < 100000, i++){
// if ( true || whatever ) <-- optimized out
run_some_code;
run_lots_of_other_code;
}
}
No branching there. Good. Moving on to the first condition being false and the second condition being true.
myloop<false, true>(){
for (int i = 0; i < 100000, i++){
if ( internal_condition[i] ){ // simplified from (false || (true && i_c[i]))
run_some_code;
}
else{
run_some_other_code;
}
run_lots_of_other_code;
}
}
OK, there is some branching going on here. However, each i needs to be analyzed to see which code should execute. I think there is nothing more that can be done here without more information about internal_condition. I'll give some thoughts on that later, but let's move on to the fourth function for now.
myloop<false, false>() {
for (int i = 0; i < 100000, i++){
// if ( false || (false && whatever) ) <-- optimized out
run_some_other_code;
run_lots_of_other_code;
}
}
No branching here. You already have done a good job avoiding branching and code duplication.
OK, let's go back to myloop<false,true>, where there is branching. The branching is largely unavoidable simply because of how your situation is set up. You are going to iterate many times. Some iterations you want to do one thing while other iterations should do another. To get around this, you would need to re-envision your setup so that you can do the same thing each iteration. (The optimization you are working from is based upon doing the same thing each iteration, even though it might be a different thing the next time the loop starts.)
The simplest, yet unlikely, scenario would be where internal_condition[i] is equivalent to something like i < 5000. It would also be convenient if you could do all of the "some code" before any of the "lots of other code". Then you could loop from 0 to 4999, running "some code" each iteration. Then loop from 5000 to 99999, running "other code". Then a third loop to run "lots of other code".
Any solution I can think of would involve adapting your situation to make it more like the unlikely simple scenario. Can you calculate how many times internal_condition[i] is true? Can you iterate that many times and map your (new) loop control variable to the appropriate value of i (the old loop control variable)? (Or maybe the exact value of i is not important?) Then do a second loop to cover the remaining cases? In some scenarios, this might be trivial. In others, far from it.
There might be other tricks that could be done, but they depend on more details about what you are doing, what you need to do, and what you think you need to do but don't really. (It's possible that the required level of detail would overwhelm StackOverflow.) Is the order important? Is the exact value of i important?
In the end, I would opt for profiling the code. Profile the code without code duplication but with branching. Profile the code with minimal branching but with code duplication. Is there a measurable change? If so, think about how you can re-arrange your internal condition so that i can cover large ranges without changing the value of the internal condition. Then divide your loop into smaller pieces.
In C++17, to guaranty no extra branches evaluation, you might do:
template <bool external_condition_1, bool external_condition_2>
void myloop()
{
for (int i = 0; i < 100000, i++){
if constexpr (external_condition_1) {
run_some_code;
} else if constexpr (external_condition_2){
if (internal_condition[i]) {
run_some_code;
} else {
run_some_other_code;
}
} else {
run_some_other_code;
}
run_lots_of_other_code;
}
}
I almost never see a for loop like this:
for (int i = 0; 5 != i; ++i)
{}
Is there a technical reason to use > or < instead of != when incrementing by 1 in a for loop? Or this is more of a convention?
while (time != 6:30pm) {
Work();
}
It is 6:31pm... Damn, now my next chance to go home is tomorrow! :)
This to show that the stronger restriction mitigates risks and is probably more intuitive to understand.
There is no technical reason. But there is mitigation of risk, maintainability and better understanding of code.
< or > are stronger restrictions than != and fulfill the exact same purpose in most cases (I'd even say in all practical cases).
There is duplicate question here; and one interesting answer.
Yes there is a reason. If you write a (plain old index based) for loop like this
for (int i = a; i < b; ++i){}
then it works as expected for any values of a and b (ie zero iterations when a > b instead of infinite if you had used i == b;).
On the other hand, for iterators you'd write
for (auto it = begin; it != end; ++it)
because any iterator should implement an operator!=, but not for every iterator it is possible to provide an operator<.
Also range-based for loops
for (auto e : v)
are not just fancy sugar, but they measurably reduce the chances to write wrong code.
You can have something like
for(int i = 0; i<5; ++i){
...
if(...) i++;
...
}
If your loop variable is written by the inner code, the i!=5 might not break that loop. This is safer to check for inequality.
Edit about readability.
The inequality form is way more frequently used. Therefore, this is very fast to read as there is nothing special to understand (brain load is reduced because the task is common). So it's cool for the readers to make use of these habits.
And last but not least, this is called defensive programming, meaning to always take the strongest case to avoid current and future errors influencing the program.
The only case where defensive programming is not needed is where states have been proven by pre- and post-conditions (but then, proving this is the most defensive of all programming).
I would argue that an expression like
for ( int i = 0 ; i < 100 ; ++i )
{
...
}
is more expressive of intent than is
for ( int i = 0 ; i != 100 ; ++i )
{
...
}
The former clearly calls out that the condition is a test for an exclusive upper bound on a range; the latter is a binary test of an exit condition. And if the body of the loop is non-trivial, it may not apparent that the index is only modified in the for statement itself.
Iterators are an important case when you most often use the != notation:
for(auto it = vector.begin(); it != vector.end(); ++it) {
// do stuff
}
Granted: in practice I would write the same relying on a range-for:
for(auto & item : vector) {
// do stuff
}
but the point remains: one normally compares iterators using == or !=.
The loop condition is an enforced loop invariant.
Suppose you don't look at the body of the loop:
for (int i = 0; i != 5; ++i)
{
// ?
}
in this case, you know at the start of the loop iteration that i does not equal 5.
for (int i = 0; i < 5; ++i)
{
// ?
}
in this case, you know at the start of the loop iteration that i is less than 5.
The second is much, much more information than the first, no? Now, the programmer intent is (almost certainly) the same, but if you are looking for bugs, having confidence from reading a line of code is a good thing. And the second enforces that invariant, which means some bugs that would bite you in the first case just cannot happen (or don't cause memory corruption, say) in the second case.
You know more about the state of the program, from reading less code, with < than with !=. And on modern CPUs, they take the same amount of time as no difference.
If your i was not manipulated in the loop body, and it was always increased by 1, and it started less than 5, there would be no difference. But in order to know if it was manipulated, you'd have to confirm each of these facts.
Some of these facts are relatively easy, but you can get wrong. Checking the entire body of the loop is, however, a pain.
In C++ you can write an indexes type such that:
for( const int i : indexes(0, 5) )
{
// ?
}
does the same thing as either of the two above for loops, even down to the compiler optimizing it down to the same code. Here, however, you know that i cannot be manipulated in the body of the loop, as it is declared const, without the code corrupting memory.
The more information you can get out of a line of code without having to understand the context, the easier it is to track down what is going wrong. < in the case of integer loops gives you more information about the state of the code at that line than != does.
As already said by Ian Newson, you can't reliably loop over a floating variable and exit with !=. For instance,
for (double x=0; x!=1; x+=0.1) {}
will actually loop forever, because 0.1 can't exactly be represented in floating point, hence the counter narrowly misses 1. With < it terminates.
(Note however that it's basically undefined behaviour whether you get 0.9999... as the last accepted number – which kind of violates the less-than assumption – or already exit at 1.0000000000000001.)
Yes; OpenMP doesn't parallelize loops with the != condition.
It may happen that the variable i is set to some large value and if you just use the != operator you will end up in an endless loop.
As you can see from the other numerous answers, there are reasons to use < instead of != which will help in edge cases, initial conditions, unintended loop counter modification, etc...
Honestly though, I don't think you can stress the importance of convention enough. For this example it will be easy enough for other programmers to see what you are trying to do, but it will cause a double-take. One of the jobs while programming is making it as readable and familiar to everyone as possible, so inevitably when someone has to update/change your code, it doesn't take a lot of effort to figure out what you were doing in different code blocks. If I saw someone use !=, I'd assume there was a reason they used it instead of < and if it was a large loop I'd look through the whole thing trying to figure out what you did that made that necessary... and that's wasted time.
I take the adjectival "technical" to mean language behavior/quirks and compiler side effects such as performance of generated code.
To this end, the answer is: no(*). The (*) is "please consult your processor manual". If you are working with some edge-case RISC or FPGA system, you may need to check what instructions are generated and what they cost. But if you're using pretty much any conventional modern architecture, then there is no significant processor level difference in cost between lt, eq, ne and gt.
If you are using an edge case you could find that != requires three operations (cmp, not, beq) vs two (cmp, blt xtr myo). Again, RTM in that case.
For the most part, the reasons are defensive/hardening, especially when working with pointers or complex loops. Consider
// highly contrived example
size_t count_chars(char c, const char* str, size_t len) {
size_t count = 0;
bool quoted = false;
const char* p = str;
while (p != str + len) {
if (*p == '"') {
quote = !quote;
++p;
}
if (*(p++) == c && !quoted)
++count;
}
return count;
}
A less contrived example would be where you are using return values to perform increments, accepting data from a user:
#include <iostream>
int main() {
size_t len = 5, step;
for (size_t i = 0; i != len; ) {
std::cout << "i = " << i << ", step? " << std::flush;
std::cin >> step;
i += step; // here for emphasis, it could go in the for(;;)
}
}
Try this and input the values 1, 2, 10, 999.
You could prevent this:
#include <iostream>
int main() {
size_t len = 5, step;
for (size_t i = 0; i != len; ) {
std::cout << "i = " << i << ", step? " << std::flush;
std::cin >> step;
if (step + i > len)
std::cout << "too much.\n";
else
i += step;
}
}
But what you probably wanted was
#include <iostream>
int main() {
size_t len = 5, step;
for (size_t i = 0; i < len; ) {
std::cout << "i = " << i << ", step? " << std::flush;
std::cin >> step;
i += step;
}
}
There is also something of a convention bias towards <, because ordering in standard containers often relies on operator<, for instance hashing in several STL containers determines equality by saying
if (lhs < rhs) // T.operator <
lessthan
else if (rhs < lhs) // T.operator < again
greaterthan
else
equal
If lhs and rhs are a user defined class writing this code as
if (lhs < rhs) // requires T.operator<
lessthan
else if (lhs > rhs) // requires T.operator>
greaterthan
else
equal
The implementor has to provide two comparison functions. So < has become the favored operator.
There are several ways to write any kind of code (usually), there just happens to be two ways in this case (three if you count <= and >=).
In this case, people prefer > and < to make sure that even if something unexpected happens in the loop (like a bug), it won't loop infinitely (BAD). Consider the following code, for example.
for (int i = 1; i != 3; i++) {
//More Code
i = 5; //OOPS! MISTAKE!
//More Code
}
If we used (i < 3), we would be safe from an infinite loop because it placed a bigger restriction.
Its really your choice whether you want a mistake in your program to shut the whole thing down or keep functioning with the bug there.
Hope this helped!
The most common reason to use < is convention. More programmers think of loops like this as "while the index is in range" rather than "until the index reaches the end." There's value is sticking to convention when you can.
On the other hand, many answers here are claiming that using the < form helps avoid bugs. I'd argue that in many cases this just helps hide bugs. If the loop index is supposed to reach the end value, and, instead, it actually goes beyond it, then there's something happening you didn't expect which may cause a malfunction (or be a side effect of another bug). The < will likely delay discovery of the bug. The != is more likely to lead to a stall, hang, or even a crash, which will help you spot the bug sooner. The sooner a bug is found, the cheaper it is to fix.
Note that this convention is peculiar to array and vector indexing. When traversing nearly any other type of data structure, you'd use an iterator (or pointer) and check directly for an end value. In those cases you have to be sure the iterator will reach and not overshoot the actual end value.
For example, if you're stepping through a plain C string, it's generally more common to write:
for (char *p = foo; *p != '\0'; ++p) {
// do something with *p
}
than
int length = strlen(foo);
for (int i = 0; i < length; ++i) {
// do something with foo[i]
}
For one thing, if the string is very long, the second form will be slower because the strlen is another pass through the string.
With a C++ std::string, you'd use a range-based for loop, a standard algorithm, or iterators, even if though the length is readily available. If you're using iterators, the convention is to use != rather than <, as in:
for (auto it = foo.begin(); it != foo.end(); ++it) { ... }
Similarly, iterating a tree or a list or a deque usually involves watching for a null pointer or other sentinel rather than checking if an index remains within a range.
One reason not to use this construct is floating point numbers. != is a very dangerous comparison to use with floats as it'll rarely evaluate to true even if the numbers look the same. < or > removes this risk.
There are two related reasons for following this practice that both have to do with the fact that a programming language is, after all, a language that will be read by humans (among others).
(1) A bit of redundancy. In natural language we usually provide more information than is strictly necessary, much like an error correcting code. Here the extra information is that the loop variable i (see how I used redundancy here? If you didn't know what 'loop variable' means, or if you forgot the name of the variable, after reading "loop variable i" you have the full information) is less than 5 during the loop, not just different from 5. Redundancy enhances readability.
(2) Convention. Languages have specific standard ways of expressing certain situations. If you don't follow the established way of saying something, you will still be understood, but the effort for the recipient of your message is greater because certain optimisations won't work. Example:
Don't talk around the hot mash. Just illuminate the difficulty!
The first sentence is a literal translation of a German idiom. The second is a common English idiom with the main words replaced by synonyms. The result is comprehensible but takes a lot longer to understand than this:
Don't beat around the bush. Just explain the problem!
This is true even in case the synonyms used in the first version happen to fit the situation better than the conventional words in the English idiom. Similar forces are in effect when programmers read code. This is also why 5 != i and 5 > i are weird ways of putting it unless you are working in an environment in which it is standard to swap the more normal i != 5 and i < 5 in this way. Such dialect communities do exist, probably because consistency makes it easier to remember to write 5 == i instead of the natural but error prone i == 5.
Using relational comparisons in such cases is more of a popular habit than anything else. It gained its popularity back in the times when such conceptual considerations as iterator categories and their comparability were not considered high priority.
I'd say that one should prefer to use equality comparisons instead of relational comparisons whenever possible, since equality comparisons impose less requirements on the values being compared. Being EqualityComparable is a lesser requirement than being LessThanComparable.
Another example that demonstrates the wider applicability of equality comparison in such contexts is the popular conundrum with implementing unsigned iteration down to 0. It can be done as
for (unsigned i = 42; i != -1; --i)
...
Note that the above is equally applicable to both signed and unsigned iteration, while the relational version breaks down with unsigned types.
Besides the examples, where the loop variable will (unintentional) change inside the body, there are other reasions to use the smaller-than or greater-than operators:
Negations make code harder to understand
< or > is only one char, but != two
In addition to the various people who have mentioned that it mitigates risk, it also reduces the number of function overloads necessary to interact with various standard library components. As an example, if you want your type to be storable in a std::set, or used as a key for std::map, or used with some of the searching and sorting algorithms, the standard library usually uses std::less to compare objects as most algorithms only need a strict weak ordering. Thus it becomes a good habit to use the < comparisons instead of != comparisons (where it makes sense, of course).
There is no problem from a syntax perspective, but the logic behind that expression 5!=i is not sound.
In my opinion, using != to set the bounds of a for loop is not logically sound because a for loop either increments or decrements the iteration index, so setting the loop to iterate until the iteration index becomes out of bounds (!= to something) is not a proper implementation.
It will work, but it is prone to misbehavior since the boundary data handling is lost when using != for an incremental problem (meaning that you know from the start if it increments or decrements), that's why instead of != the <>>==> are used.
Which of the following is better and why? (Particular to c++)
a.
int i(0), iMax(vec.length());//vec is a container, say std::vector
for(;i < iMax; ++i)
{
//loop body
}
b.
for( int i(0);i < vec.length(); ++i)
{
//loop body
}
I have seen advice for (a) because of the call to length function. This is bothering me. Doesn't any modern compiler do the optimization of (b) to be similar to (a)?
Example (b) has a different meaning to example (a), and the compiler must interpret it as you write it.
If, (for some made-up reason that I can't think of), I wrote code to do this:
for( int i(0);i < vec.length(); ++i)
{
if(i%4 == 0)
vec.push_back(Widget());
}
I really would not have wanted the compiler to optimise out each call to vec.length(), because I would get different results.
I like:
for (int i = 0, e = vec.length(); i != e; ++i)
Of course, this would also work for iterators:
for (vector<int>::const_iterator i = v.begin(), e = v.end(); i != e; ++i)
I like this because it's both efficient (calling end() just once), and also relatively succinct (only having to type vector<int>::const_iterator once).
I'm surprised nobody has said the obvious:
In 99.99% of cases, it doesn't matter.
Unless you are using some container where calculating size() is an expensive operation, it is unfathomable that your program will go even a few nanoseconds slower. I would say stick with the more readable until you profile your code and find that size() is a bottleneck.
There are two issues to debate here:
The variable scope
The end condition re-evaluation
Variable scope
Normally, you wouldn't need the loop variable to be visible outside of the loop. That's why you can declare it inside the for construct.
End condition re-evaluation
Andrew Shepherd stated it nicely: it means something different to put a function call inside the end condition:
for( vector<...>::size_type i = 0; i < v.size(); ++i ) { // vector size may grow.
if( ... ) v.push_back( i ); // contrived, but possible
}
// note: this code may be replaced by a std::for_each construct, the previous can't.
for( vector<...>::size_type i = 0, elements = v.size(); i != elements; ++i ) {
}
Why is it bodering you?
Those two alternatives dont see to be doing the same. One is doing a fixed number of iterations, while the other is dependant on the loops body.
Another alternative colud be
for (vector<T>::iterator it=vec.begin();it!=vec.end();it++){
//loop body
}
Unless you need the loop variable outside the loop, the second approach is preferable.
Iterators will actually give you as good or better performance. (There was a big comparison thread on comp.lang.c++.moderated a few years back).
Also, I would use
int i = 0;
Rather than the constructor like syntax you're using. While valid, it's not idiomatic.
Somewhat unrelated:
Warning: Comparison between signed and unsigned integer.
The correct type for array and vector indices is size_t.
Strictly speaking, in C++ it is even std::vector<>::size_type.
Amazing how many C/C++ developers still get this one wrong.
Let's see on the generated code (I use MSVS 2008 with full optimization).
a.
int i(0), iMax(vec.size());//vec is a container, say std::vector
for(;i < iMax; ++i)
{
//loop body
}
The for loop produces 2 assembler instructions.
b.
for( int i(0);i < vec.size(); ++i)
{
//loop body
}
The for loop produces 8 assembler instructions. vec.size() is successfully inlined.
c.
for (std::vector<int>::const_iterator i = vec.begin(), e = vec.end(); i != e; ++i)
{
//loop body
}
The for loop produces 15 assembler instructions (everything is inlined, but the code has a lot of jumps)
So, if your application is performance critical use a). Otherwise b) or c).
It should be noted that the iterator examples:
for (vector<T>::iterator it=vec.begin();it!=vec.end();it++){
//loop body
}
could invalidate the loop iterator 'it' should the loop body cause the vector to reallocate. Thus it is not equivalent to
for (int i=0;i<vec.size();++i){
//loop body
}
where loop body adds elements to vec.
Simple question: are you modifying vec in the loop?
answer to this question will lead to your answer too.
jrh
It's very hard for a compiler to hoist the vec.length() call in the safe knowledge that it's constant, unless it gets inlined (which hopefully it often will!). But at least i should definitely be declared in the second style "b", even if the length call needs to be "manually" hoisted out of the loop!
This one is preferable:
typedef vector<int> container; // not really required,
// you could just use vector<int> in for loop
for (container::const_iterator i = v.begin(); i != v.end(); ++i)
{
// do something with (*i)
}
I can tell right away that the vector
is not being updated
anyone can tell what is happening
here
I know how many loops
v.end() returns pointer one past the
last element so there's no overhead
of checking size
easy to update for different
containers or value types
(b) won't calculate/call the function each time.
-- begin excerpt ----
Loop Invariant Code Motion:
GCC includes loop invariant code motion as part of its loop optimizer as well as in its partial redundancy elimination pass. This optimization removes instructions from loops, which compute a value which does not change throughout the lifetime of a loop.
--- end excerpt --
More optimizations for gcc:
https://www.in.redhat.com/software/gnupro/technical/gnupro_gcc.php3
Why not sidestep the issue entirely with BOOST_FOREACH
#include <boost/foreach.hpp>
std::vector<double> vec;
//...
BOOST_FOREACH( double &d, vec)
{
std::cout << d;
}