Related
Given a vector v, I want to loop through each element in the vector and perform an operation that requires the current index.
I've seen a basic for loop written both of these ways:
// Using "<" in the terminating condition
for (auto i = 0; i < v.size(); ++i)
{
// Do something with v[i]
}
// Using "!=" in the terminating condition
for (auto i = 0; i != v.size(); ++i)
{
// Do something with v[i]
}
Is there any practical reason to prefer one over the other? I've seen it written using < much more often, but is there a performance benefit to using !=?
There is one, albeit kinda convoluted, reason to prefer < over !=.
If for whatever reason the body of your lop modifies i and skips over the threshold, < still terminates the loop, where != will keep iterating.
It might make no difference in the vast majority of cases, but getting used to < might prevent bugs that != won't. There's also an additional advantage, that is ridiculously minor but < takes less characters to write, so it makes the source file smaller.
Again the above argument is borderline a joke, but if you need an argument to use one over the other, there you have it.
You should probably prefer using a range-for instead.
Is there any practical reason to prefer one over the other?
There are no strong reasons to prefer one way or another given that both have equivalent behaviour.
Using the less than operator works even if the end condition is not one of the values of the accumulator which would be possible if increment was greater than 1 or the initial value was greater than the end condition. As such, it is more general, which means that using it always may be more consistent. Consistency is a weak argument, but a weak argument is better than no argument if you agree with it.
There is no difference in performance.
There is a huge difference if you decided to change ++i in such a way that the loop would step over the desired end.
For clarity for other programmers reading your code, use the convention they expect to see, namely <. When I see i != ..., I think "he is skipping over that one value".
The is an obscure bug with < -- Suppose you iterating over all 256 chars using a byte-sized i. The value will overflow and probably never show the value 256. The loop will never stop. And != won't fix the bug.
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.
Solving the following exercise:
Write three different versions of a program to print the elements of
ia. One version should use a range for to manage the iteration, the
other two should use an ordinary for loop in one case using subscripts
and in the other using pointers. In all three programs write all the
types directly. That is, do not use a type alias, auto, or decltype to
simplify the code.[C++ Primer]
a question came up: Which of these methods for accessing array is optimized in terms of speed and why?
My Solutions:
Foreach Loop:
int ia[3][4]={{1,2,3,4},{5,6,7,8},{9,10,11,12}};
for (int (&i)[4]:ia) //1st method using for each loop
for(int j:i)
cout<<j<<" ";
Nested for loops:
for (int i=0;i<3;i++) //2nd method normal for loop
for(int j=0;j<4;j++)
cout<<ia[i][j]<<" ";
Using pointers:
int (*i)[4]=ia;
for(int t=0;t<3;i++,t++){ //3rd method. using pointers.
for(int x=0;x<4;x++)
cout<<(*i)[x]<<" ";
Using auto:
for(auto &i:ia) //4th one using auto but I think it is similar to 1st.
for(auto j:i)
cout<<j<<" ";
Benchmark result using clock()
1st: 3.6 (6,4,4,3,2,3)
2nd: 3.3 (6,3,4,2,3,2)
3rd: 3.1 (4,2,4,2,3,4)
4th: 3.6 (4,2,4,5,3,4)
Simulating each method 1000 times:
1st: 2.29375 2nd: 2.17592 3rd: 2.14383 4th: 2.33333
Process returned 0 (0x0) execution time : 13.568 s
Compiler used:MingW 3.2 c++11 flag enabled. IDE:CodeBlocks
I have some observations and points to make and I hope you get your answer from this.
The fourth version, as you mention yourself, is basically the same as the first version. auto can be thought of as only a coding shortcut (this is of course not strictly true, as using auto can result in getting different types than you'd expected and therefore result in different runtime behavior. But most of the time this is true.)
Your solution using pointers is probably not what people mean when they say that they are using pointers! One solution might be something like this:
for (int i = 0, *p = &(ia[0][0]); i < 3 * 4; ++i, ++p)
cout << *p << " ";
or to use two nested loops (which is probably pointless):
for (int i = 0, *p = &(ia[0][0]); i < 3; ++i)
for (int j = 0; j < 4; ++j, ++p)
cout << *p << " ";
from now on, I'm assuming this is the pointer solution you've written.
In such a trivial case as this, the part that will absolutely dominate your running time is the cout. The time spent in bookkeeping and checks for the loop(s) will be completely negligible comparing to doing I/O. Therefore, it won't matter which loop technique you use.
Modern compilers are great at optimizing such ubiquitous tasks and access patterns (iterating over an array.) Therefore, chances are that all these methods will generate exactly the same code (with the possible exception of the pointer version, which I will talk about later.)
The performance of most codes like this will depend more on the memory access pattern rather than how exactly the compiler generates the assembly branch instructions (and the rest of the operations.) This is because if a required memory block is not in the CPU cache, it's going to take a time roughly equivalent of several hundred CPU cycles (this is just a ballpark number) to fetch those bytes from RAM. Since all the examples access memory in exactly the same order, their behavior in respect to memory and cache will be the same and will have roughly the same running time.
As a side note, the way these examples access memory is the best way for it to be accessed! Linear, consecutive and from start to finish. Again, there are problems with the cout in there, which can be a very complicated operation and even call into the OS on every invocation, which might result, among other things, an almost complete deletion (eviction) of everything useful from the CPU cache.
On 32-bit systems and programs, the size of an int and a pointer are usually equal (both are 32 bits!) Which means that it doesn't matter much whether you pass around and use index values or pointers into arrays. On 64-bit systems however, a pointer is 64 bits but an int will still usually be 32 bits. This suggests that it is usually better to use indexes into arrays instead of pointers (or even iterators) on 64-bit systems and programs.
In this particular example, this is not significant at all though.
Your code is very specific and simple, but the general case, it is almost always better to give as much information to the compiler about your code as possible. This means that you must use the narrowest, most specific device available to you to do a job. This in turn means that a generic for loop (i.e. for (int i = 0; i < n; ++i)) is worse than a range-based for loop (i.e. for (auto i : v)) for the compiler, because in the latter case the compiler simply knows that you are going to iterate over the whole range and not go outside of it or break out of the loop or something, while in the generic for loop case, specially if your code is more complex, the compiler cannot be sure of this and has to insert extra checks and tests to make sure the code executes as the C++ standard says it should.
In many (most?) cases, although you might think performance matters, it does not. And most of the time you rewrite something to gain performance, you don't gain much. And most of the time the performance gain you get is not worth the loss in readability and maintainability that you sustain. So, design your code and data structures right (and keep performance in mind) but avoid this kind of "micro-optimization" because it's almost always not worth it and even harms the quality of the code too.
Generally, performance in terms of speed is very hard to reason about. Ideally you have to measure the time with real data on real hardware in real working conditions using sound scientific measuring and statistical methods. Even measuring the time it takes a piece of code to run is not at all trivial. Measuring performance is hard, and reasoning about it is harder, but these days it is the only way of recognizing bottlenecks and optimizing the code.
I hope I have answered your question.
EDIT: I wrote a very simple benchmark for what you are trying to do. The code is here. It's written for Windows and should be compilable on Visual Studio 2012 (because of the range-based for loops.) And here are the timing results:
Simple iteration (nested loops): min:0.002140, avg:0.002160, max:0.002739
Simple iteration (one loop): min:0.002140, avg:0.002160, max:0.002625
Pointer iteration (one loop): min:0.002140, avg:0.002160, max:0.003149
Range-based for (nested loops): min:0.002140, avg:0.002159, max:0.002862
Range(const ref)(nested loops): min:0.002140, avg:0.002155, max:0.002906
The relevant numbers are the "min" times (over 2000 runs of each test, for 1000x1000 arrays.) As you see, there is absolutely no difference between the tests. Note that you should turn on compiler optimizations or test 2 will be a disaster and cases 4 and 5 will be a little worse than 1 and 3.
And here are the code for the tests:
// 1. Simple iteration (nested loops)
unsigned sum = 0;
for (unsigned i = 0; i < gc_Rows; ++i)
for (unsigned j = 0; j < gc_Cols; ++j)
sum += g_Data[i][j];
// 2. Simple iteration (one loop)
unsigned sum = 0;
for (unsigned i = 0; i < gc_Rows * gc_Cols; ++i)
sum += g_Data[i / gc_Cols][i % gc_Cols];
// 3. Pointer iteration (one loop)
unsigned sum = 0;
unsigned * p = &(g_Data[0][0]);
for (unsigned i = 0; i < gc_Rows * gc_Cols; ++i)
sum += *p++;
// 4. Range-based for (nested loops)
unsigned sum = 0;
for (auto & i : g_Data)
for (auto j : i)
sum += j;
// 5. Range(const ref)(nested loops)
unsigned sum = 0;
for (auto const & i : g_Data)
for (auto const & j : i)
sum += j;
It has many factors affecting it:
It depends on the compiler
It depends on the compiler flags used
It depends on the computer used
There is only one way to know the exact answer: measuring the time used when dealing with huge arrays (maybe from a random number generator) which is the same method you have already done except that the array size should be at least 1000x1000.
We can use for loop and while loop for same purpose.
in what means they effect our code if I use for instead of while? same question arises between if-else and switch-case? how to decide what to use?
for example which one you would prefer?
This code:
int main()
{
int n = 10;
for(int i=0;i<n;i++)
{
do_something();
}
return 0;
}
Or this code:
int main()
{
int n=10,i=0;
while(i<n)
{
do_something();
i++;
}
return 0;
}
if using for or while loop does not effect the code by any means then may I know What was the need to make 2 solution for same problem?
Use whichever one makes the intention of your code clearest.
If you know the number of iterations the loop should run beforehand, I would recommend the for construct. While loops are good for when the loop's terminating condition happens at some yet-to-be determined time.
I try to prefer the for loop. Why? Because when I see a for loop, I can expect all of the loop bookeeping is kept in a single statement. I can insert break or continue statements without worrying about breaking how the loop operates. And most importantly, the body of the loop focuses on what you actually want the loop to be doing, rather than maintaining the loop itself. If I see a while, then I have to look at and understand the entire loop body before I can understand what iteration pattern the loop uses.
The only place I end up using while is for those few cases where the control of the loop is provided by some outside routine (i.e. FindFirstFileW)
It's all a matter of personal opinion though. Lots of people don't like what I end up doing with for loops because the loop statement often ends up spanning multiple lines.
There are some very subtle differences..
scope of loop variable(s), for example, with the for loop i has local scope, with a while this has to be defined before (which means it is available after, of course you can do that with for as well..)
continue, with a for loop, variable will be increment/decremented, with a while, you'd have to insert the operation before continue
Frankly, if you need to increment/decrement, a for loop makes sense, if you don't know the bounds, and there is no real increment/decrement, a while loop makes more sense, e.g.
while(some_stream >> input)
{
// do stuff...
}
In general, a for loop might be preferable for simple loops, since the logic of the loop is contained in a single line:
for (int i = 0; i < 10; ++i) {...}
However, sometimes we need more complex logic or flow control. A while loop allows us to implement more complicated loops. For example, suppose we only want to increment the counter variable under certain conditions:
int i = 0;
while (i < 10)
{
if (some_condition) ++i;
else if (some_other_condition) { ... }
else break;
}
Just use the one that makes the code readable and logical.
In some cases the compiler (gcc at least) will be able to optimize a very slightly better than a for loop doing the same thing. If I remember correctly that optimization is only about few clock cycles so it probably never will have any noticeable affect on the performance.
You cannot write while(int i=0, i < n); that is, you've to define i before the while loop; means i exists inside as well as outside the loop.
However, in case of for loop, you can define i right in the for loop itself; and so i doesn't exist outside the loop. That is one difference. Just because of this difference, I like for more than while. And use while rarely, when for makes thing more cumbersome!
By no means they affect your program the way it works ! Its the matter of ease to understand better.
switch(i) // Once finding your case, you can easily know where the switch ends
// and thus the next statement of execution
{
case 1: break ;
case 2: break ;
// .....
case 10: break ;
default:break ;
}
if( i==1 ) // Here you have the pain of finding where the last else if ends !
{}
else if( i==2)
{}
// ...
else if( i==10)
{}
However, it is a matter of taste. I prefer switch.