I have a function that contains two for loops, and I'm using a variable called count as the counter. I've chosen to recycle the name as the the first loop will finish it's execution completely before the second one begins, so there is no chance of the counters interfering with each other. The G++ compiler has taken exception to this via the following warning:
error: name lookup of ‘count’ changed for ISO ‘for’ scoping
note: (if you use ‘-fpermissive’ G++ will accept your code)
Is variable recycling considered bad practice in professional software development, or is it a situational concern, and what other implications have I missed here?
Are you doing this?
for(int count = 0; ...)
{
...
}
for(count = 0; ...)
{
...
}
I doubt gcc would like that, as the second count isn't in scope. I think it only applies to the first for loop, but gcc has options to accept poor code. If you either make the second int count or move the first to the outer scope, gcc should be happy.
This depends on the circumstances, but I generally don't reuse variables. The name of the variable should reflect its purpose, and switching part way through a function can be confusing. Declare what you need, let the compiler take care of the optimizations.
Steve McConnell recommends not reusing local variables in functions in Code Complete.
He's not the definitive voice of practice in professional software development, but he's about as close as you're going to get to a definitive voice.
The argument is that it makes it harder to read the code.
What are you counting? Name the variables after that.
It sounds like you're defining the variable in the for? i.e. "for (int count=0; count++; count < x)"? If so, that could be problematic, as well as unclear. If you're going to use it in a second for loop define it outside both loops.
If you're using a loop counter variables like this, then it usually doesn't matter.
for (int i ...; ... ; ...) {
...
}
for (int i ...; ... ; ...) {
...
}
however, if you're intending to shadow another variable:
int i ...;
for (int i ...; ... ; ...) {
...
}
that's a red flag.
Related
I'm pretty new to programming, and I was just wondering in the following case what would be an appropriate name for the second integer I use in this piece of code
for (int i = 0; i < 10; i++)
{
for (int x = 0; x < 10; x++)
{
//stuff
}
}
I usually just name it x but I have a feeling that this could get confusing quickly. Is there a standard name for this kind of thing?
Depending upon what you're iterating over, a name might be easy or obvious by context:
for(struct mail *mail=inbox->start; mail ; mailid++) {
for (struct attachment *att=mail->attachment[0]; att; att++) {
/* work on all attachments on all mails */
}
}
For the cases where i makes the most sense for an outer loop variable, convention uses j, k, l, and so on.
But when you start nesting, look harder for meaningful names. You'll thank yourself in six months.
You could opt to reduce the nesting by making a method call. Inside of this method, you would be using a local variable also named i.
for (int i = 0; i < 10; i++)
{
methodCall(array[i], array);
}
I have assumed you need to pass the element at position i in the outer loop as well as the array to be iterated over in the inner loop - this is an assumption as you may actually require different arguments.
As always, you should measure the performance of this - there shouldn't be a massive overhead in making a method call within a loop, but this depends on the language.
Personally I feel that you should give variables meaningful names - here i and x mean nothing and will not help you understand your code in 3 months time, at which point it will appear to you as code written by a dyslexic monkey.
Name variables so that other people can understand what your code is trying to accomplish. You will save yourself time in the long run.
Since you said you are beginning, I'd say it's beneficial to experiment with multiple styles.
For the purposes of your example, my suggestion is simply replace x with j.
There's tons of real code that will use the convention of i, j, and k for single letter nested loop variables.
There's also tons that uses longer more meaningful names.
But there's much less that looks like your example.
So you can consider it a step forward because you're code looks more like real world code.
I have a simple c program for printing n Fibonacci numbers and I would like to compile it to ELF object file. Instead of setting the number of fibonacci numbers (n) directly in my c code, I would like to set them in the registers since I am simulating it for an ARM processor.How can I do that?
Here is the code snippet
#include <stdio.h>
#include <stdlib.h>
#define ITERATIONS 3
static float fib(float i) {
return (i>1) ? fib(i-1) + fib(i-2) : i;
}
int main(int argc, char **argv) {
float i;
printf("starting...\n");
for(i=0; i<ITERATIONS; i++) {
printf("fib(%f) = %f\n", i, fib(i));
}
printf("finishing...\n");
return 0;
}
I would like to set the ITERATIONS counter in my Registers rather than in the code.
Thanks in advance
The register keyword can be used to suggest to the compiler that it uses a registers for the iterator and the number of iterations:
register float i;
register int numIterations = ITERATIONS;
but that will not help much. First of all, the compiler may or may not use your suggestion. Next, values will still need to be placed on the stack for the call to fib(), and, finally, depending on what functions you call within your loop, code in the procedure are calling could save your register contents in the stack frame at procedure entry, and restore them as part of the code implementing the procedure return.
If you really need to make every instruction count, then you will need to write machine code (using an assembly language). That way, you have direct control over your register usage. Assembly language programming is not for the faint of heart. Assembly language development is several times slower than using higher level languages, your risk of inserting bugs is greater, and they are much more difficult to track down. High level languages were developed for a reason, and the C language was developed to help write Unix. The minicomputers that ran the first Unix systems were extremely slow, but the reason C was used instead of assembly was that even then, it was more important to have code that took less time to code, had fewer bugs, and was easier to debug than assembler.
If you want to try this, here are the answers to a previous question on stackoverflow about resources for ARM programming that might be helpful.
One tactic you might take is to isolate your performance-critical code into a procedure, write the procedure in C, the capture the generated assembly language representation. Then rewrite the assembler to be more efficient. Test thoroughly, and get at least one other set of eyeballs to look the resulting code over.
Good Luck!
Make ITERATIONS a variable rather than a literal constant, then you can set its value directly in your debugger/simulator's watch or locals window just before the loop executes.
Alternatively as it appears you have stdio support, why not just accept the value via console input?
Why do some people use while(true){} blocks in their code? How does it work?
It's an infinite loop. At each iteration, the condition will be evaluated. Since the condition is true, which is always... true... the loop will run forever. Exiting the loop is done by checking something inside the loop, and then breaking if necessary.
By placing the break check inside the loop, instead of using it as the condition, this can make it more clear that you're expecting this to run until some event occurs.
A common scenario where this is used is in games; you want to keep processing the action and rendering frames until the game is quit.
It's just a loop that never ends on its own, known as an infinite-loop. (Often times, that's a bad thing.)
When it's empty, it serves to halt the program indefinitely*; otherwise there's typically some condition in the loop that, when true, breaks the loop:
while (true)
{
// ...
if (stopLoop)
break;
// ...
}
This is often cleaner than an auxiliary flag:
bool run = true;
while (run)
{
// ...
if (stopLoop)
{
run = false;
continue; // jump to top
}
// ...
}
Also note some will recommend for (;;) instead, for various reasons. (Namely, it might get rid of a warning akin to "conditional expression is always true".)
*In most languages.
Rather than stuff all possible conditions in the while statement,
// Always tests all conditions in loop header:
while( (condition1 && condition2) || condition3 || conditionN_etc ) {
// logic...
if (notable_condition)
continue; // skip remainder, go direct to evaluation portion of loop
// more logic
// maybe more notable conditions use keyword: continue
}
Some programmers might argue it's better to put the conditions throughough the logic, (i.e. not just inside the loop header) and to employ break statements to get out at appropriate places. This approach will usually negate the otherwise original conditions to determine when to leave the loop (i.e. instead of when to keep looping).
// Always tests all conditions in body of loop logic:
while(true) {
//logic...
if (!condition1 || !condition2)
break; // Break out for good.
// more logic...
if (!condition3)
break;
// even more logic ...
}
In real life it's often a more gray mixture, a combination of all these things, instead of a polarized decision to go one way or another.
Usage will depend on the complexity of the logic and the preferences of the programmer .. and maybe on the accepted answer of this thread :)
Also don't forget about do..while. The ultimate solution may use that version of the while construct to twist conditional logic to their liking.
do {
//logic with possible conditional tests and break or continue
} while (true); /* or many conditional tests */
In summary it's just nice to have options as a programmer. So don't forget to thank your compiler authors.
When Edsger W. Dijkstra was young, this was equivalent to:
Do loop initialization
label a:
Do some code
If (Loop is stoppable and End condition is met) goto label b
/* nowadays replaced by some kind of break() */
Do some more code, probably incrementing counters
go to label a
label b:
Be happy and continue
After Dijkstra decided to become Antigotoist, and convinced hordes of programmers to do so, a religious faith came upon earth and the truthiness of code was evident.
So the
Do loop initialization
While (true){
some code
If (Loop is stoppable and End condition is met) break();
Do some more code, probably incrementing counters
}
Be happy and continue
Replaced the abomination.
Not happy with that, fanatics went above and beyond. Once proved that recursion was better, clearer and more general that looping, and that variables are just a diabolic incarnation, Functional Programming, as a dream, came true:
Nest[f[.],x, forever[May God help you break]]
And so, loops recursion became really unstoppable, or at least undemonstratively stoppable.
while (the condition){do the function}
when the condition is true.. it will do the function.
so while(true)
the condition is always true
it will continue looping.
the coding will never proceed.
It's a loop that runs forever, unless there's a break statement somewhere inside the body.
The real point to have while (true) {..} is when semantics of exit conditions have no strong single preference, so its nice way to say to reader, that "well, there are actually break conditions A, B, C .., but calculations of conditions are too lengthy, so they were put into inner blocks independently in order of expected probability of appearance".
This code refers to that inside of it will run indefinitely.
i = 0
while(true)
{
i++;
}
echo i; //this code will never be reached
Unless inside of curly brackets is something like:
if (i > 100) {
break; //this will break the while loop
}
or this is another possibility how to stop while loop:
if (i > 100) {
return i;
}
It is useful to use during some testing. Or during casual coding. Or, like another answer is pointing out, in videogames.
But what I consider as bad practice is using it in production code.
For example, during debugging I want to know immediately what needs to be done in order to stop while. I don't want to search in the function for some hidden break or return.
Or the programmer can easily forget to add it there and data in a database can be affected before the code is stopped by other manners.
So ideal would be something like this:
i = 0
while(i < 100)
{
i++;
}
echo i; //this code will be reached in this scenario
Does it make any difference if I use e.g. short or char type of variable instead of int as a for-loop initializer?
for (int i = 0; i < 10; ++i) {}
for (short i = 0; i < 10; ++i) {}
for (char i = 0; i < 10; ++i) {}
Or maybe there is no difference? Maybe I make the things even worse and efficiency decreases? Does using different type saves memory and increases speed? I am not sure, but I suppose that ++ operator may need to widen the type, and as a result: slow down the execution.
It will not make any difference you should be caring about, provided the range you iterate over fits into the type you choose. Performance-wise, you'll probably get the best results when the size of the iteration variable is the same as the platform's native integer size, but any decent compiler will optimize it to use that anyway. On a managed platform (e.g. C# or Java), you don't know the target platform at compile time, and the JIT compiler is basically free to optimize for whatever platform it is running on.
The only thing you might want to watch out for is when you use the loop counter for other things inside the loop; changing the type may change the way these things get executed, up to the point (in C++ at least) that a different overload for a function or method may get called because the loop variable has a different type. An example would be when you output the loop variable through a C++ stream, like so: cout << i << endl;. Similarly, the type of the loop variable can infest the implicit types of (sub-)expressions that contain it, and lead to hidden overflows in numeric calculations, e.g.: int j = i * i;.
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As far as variable naming conventions go, should iterators be named i or something more semantic like count? If you don't use i, why not? If you feel that i is acceptable, are there cases of iteration where it shouldn't be used?
Depends on the context I suppose. If you where looping through a set of Objects in some
collection then it should be fairly obvious from the context what you are doing.
for(int i = 0; i < 10; i++)
{
// i is well known here to be the index
objectCollection[i].SomeProperty = someValue;
}
However if it is not immediately clear from the context what it is you are doing, or if you are making modifications to the index you should use a variable name that is more indicative of the usage.
for(int currentRow = 0; currentRow < numRows; currentRow++)
{
for(int currentCol = 0; currentCol < numCols; currentCol++)
{
someTable[currentRow][currentCol] = someValue;
}
}
"i" means "loop counter" to a programmer. There's nothing wrong with it.
Here's another example of something that's perfectly okay:
foreach (Product p in ProductList)
{
// Do something with p
}
I tend to use i, j, k for very localized loops (only exist for a short period in terms of number of source lines). For variables that exist over a larger source area, I tend to use more detailed names so I can see what they're for without searching back in the code.
By the way, I think that the naming convention for these came from the early Fortran language where I was the first integer variable (A - H were floats)?
i is acceptable, for certain. However, I learned a tremendous amount one semester from a C++ teacher I had who refused code that did not have a descriptive name for every single variable. The simple act of naming everything descriptively forced me to think harder about my code, and I wrote better programs after that course, not from learning C++, but from learning to name everything. Code Complete has some good words on this same topic.
i is fine, but something like this is not:
for (int i = 0; i < 10; i++)
{
for (int j = 0; j < 10; j++)
{
string s = datarow[i][j].ToString(); // or worse
}
}
Very common for programmers to inadvertently swap the i and the j in the code, especially if they have bad eyesight or their Windows theme is "hotdog". This is always a "code smell" for me - it's kind of rare when this doesn't get screwed up.
i is so common that it is acceptable, even for people that love descriptive variable names.
What is absolutely unacceptable (and a sin in my book) is using i,j, or k in any other context than as an integer index in a loop.... e.g.
foreach(Input i in inputs)
{
Process(i);
}
i is definitely acceptable. Not sure what kind of justification I need to make -- but I do use it all of the time, and other very respected programmers do as well.
Social validation, I guess :)
Yes, in fact it's preferred since any programmer reading your code will understand that it's simply an iterator.
What is the value of using i instead of a more specific variable name? To save 1 second or 10 seconds or maybe, maybe, even 30 seconds of thinking and typing?
What is the cost of using i? Maybe nothing. Maybe the code is so simple that using i is fine. But maybe, maybe, using i will force developers who come to this code in the future to have to think for a moment "what does i mean here?" They will have to think: "is it an index, a count, an offset, a flag?" They will have to think: "is this change safe, is it correct, will I be off by 1?"
Using i saves time and intellectual effort when writing code but may end up costing more intellectual effort in the future, or perhaps even result in the inadvertent introduction of defects due to misunderstanding the code.
Generally speaking, most software development is maintenance and extension, so the amount of time spent reading your code will vastly exceed the amount of time spent writing it.
It's very easy to develop the habit of using meaningful names everywhere, and once you have that habit it takes only a few seconds more to write code with meaningful names, but then you have code which is easier to read, easier to understand, and more obviously correct.
I use i for short loops.
The reason it's OK is that I find it utterly implausible that someone could see a declaration of iterator type, with initializer, and then three lines later claim that it's not clear what the variable represents. They're just pretending, because they've decided that "meaningful variable names" must mean "long variable names".
The reason I actually do it, is that I find that using something unrelated to the specific task at hand, and that I would only ever use in a small scope, saves me worrying that I might use a name that's misleading, or ambiguous, or will some day be useful for something else in the larger scope. The reason it's "i" rather than "q" or "count" is just convention borrowed from mathematics.
I don't use i if:
The loop body is not small, or
the iterator does anything other than advance (or retreat) from the start of a range to the finish of the loop:
i doesn't necessarily have to go in increments of 1 so long as the increment is consistent and clear, and of course might stop before the end of the iterand, but if it ever changes direction, or is unmodified by an iteration of the loop (including the devilish use of iterator.insertAfter() in a forward loop), I try to remember to use something different. This signals "this is not just a trivial loop variable, hence this may not be a trivial loop".
If the "something more semantic" is "iterator" then there is no reason not to use i; it is a well understood idiom.
i think i is completely acceptable in for-loop situations. i have always found this to be pretty standard and never really run into interpretation issues when i is used in this instance. foreach-loops get a little trickier and i think really depends on your situation. i rarely if ever use i in foreach, only in for loops, as i find i to be too un-descriptive in these cases. for foreach i try to use an abbreviation of the object type being looped. e.g:
foreach(DataRow dr in datatable.Rows)
{
//do stuff to/with datarow dr here
}
anyways, just my $0.02.
It helps if you name it something that describes what it is looping through. But I usually just use i.
As long as you are either using i to count loops, or part of an index that goes from 0 (or 1 depending on PL) to n, then I would say i is fine.
Otherwise its probably easy to name i something meaningful it its more than just an index.
I should point out that i and j are also mathematical notation for matrix indices. And usually, you're looping over an array. So it makes sense.
As long as you're using it temporarily inside a simple loop and it's obvious what you're doing, sure. That said, is there no other short word you can use instead?
i is widely known as a loop iterator, so you're actually more likely to confuse maintenance programmers if you use it outside of a loop, but if you use something more descriptive (like filecounter), it makes code nicer.
It depends.
If you're iterating over some particular set of data then I think it makes more sense to use a descriptive name. (eg. filecounter as Dan suggested).
However, if you're performing an arbitrary loop then i is acceptable. As one work mate described it to me - i is a convention that means "this variable is only ever modified by the for loop construct. If that's not true, don't use i"
The use of i, j, k for INTEGER loop counters goes back to the early days of FORTRAN.
Personally I don't have a problem with them so long as they are INTEGER counts.
But then I grew up on FORTRAN!
my feeling is that the concept of using a single letter is fine for "simple" loops, however, i learned to use double-letters a long time ago and it has worked out great.
i asked a similar question last week and the following is part of my own answer:// recommended style ● // "typical" single-letter style
●
for (ii=0; ii<10; ++ii) { ● for (i=0; i<10; ++i) {
for (jj=0; jj<10; ++jj) { ● for (j=0; j<10; ++j) {
mm[ii][jj] = ii * jj; ● m[i][j] = i * j;
} ● }
} ● }
in case the benefit isn't immediately obvious: searching through code for any single letter will find many things that aren't what you're looking for. the letter i occurs quite often in code where it isn't the variable you're looking for.
i've been doing it this way for at least 10 years.
note that plenty of people commented that either/both of the above are "ugly"...
I am going to go against the grain and say no.
For the crowd that says "i is understood as an iterator", that may be true, but to me that is the equivalent of comments like 'Assign the value 5 to variable Y. Variable names like comment should explain the why/what not the how.
To use an example from a previous answer:
for(int i = 0; i < 10; i++)
{
// i is well known here to be the index
objectCollection[i].SomeProperty = someValue;
}
Is it that much harder to just use a meaningful name like so?
for(int objectCollectionIndex = 0; objectCollectionIndex < 10; objectCollectionIndex ++)
{
objectCollection[objectCollectionIndex].SomeProperty = someValue;
}
Granted the (borrowed) variable name objectCollection is pretty badly named too.