I've recently begun using flags to handle an input loop's validity condition so it can be checked elsewhere inside the loop rather than having to redo the same check multiple times. However, I'm unsure how best to assign the flag. Is there a generally standard practice regarding this, or just personal style? What, differences are there in the compiled code, if any?
For example, instead of the following code:
bool isValidSize;
do {
std::cout << "Enter the font size (8-12): ";
std::cin >> fontSize;
if (fontSize >= MIN_FONT_SIZE && fontSize <= MAX_FONT_SIZE) {
isValidSize = true;
} else {
isValidSize = false;
std::cout << "Invalid size. ";
}
} while (!isValidSize);
the if-statement can be changed to make it more clear what isValidSize is set to at a glance:
isValidSize = (fontSize >= MIN_FONT_SIZE && fontSize <= MAX_FONT_SIZE);
if (!isValidSize) {
std::cout << "Invalid size. ";
}
Would this be compiled as an extra if-check? Is there any portability benefit to having the assignment separate from anything else? From just looking at the code, it seems the benefit of the first way is possibly only one branch but an additional assignment per rep and also has an else?
There are no differences: proof.
Tested on GCC 6.3 with optimisations (-O3).
Go for what you think is the more readable one.
Related
I know this can look like a rookie question already asked a thousand time. But I searched for the exact answer and I haven't found one...
I'm working on a code that, to sum up, fill an XML with different data.
I'm trying to optimize a part of it. The "naïve" code is the following:
xml << "<Node>";
for(auto& input : object.m_vec)
{
if(input == "Something")
{
xml << input;
}
}
xml << "</Node>";
for(auto& input : object.m_vec)
{
if(input == "SomethingElse")
{
xml << "<OtherNode>";
xml << input;
xml << "</OtherNode>";
break;
}
}
The important thing is, while more than one input fit in <Node></Node>, only one fit in <OtherNode></OtherNode> (explaining the break;) and it may not exist either (explaining the xml << in-between the if statement).
I think I could optimize it such like:
std::vector<std::string>* VecPointer;
xml << "<Node>";
for(auto& input : object.m_vec)
{
if(input == "Something")
{
xml << input;
}
else if(input == "SomethingElse")
{
VecPointer = &input;
}
}
xml << "</Node>";
if(!VecPointer->empty())
{
xml << "<OtherNode>"
<< *VecPointer
<< "</OtherNode>";
}
The point for me here is that there is no extra memory needed and no extra loop. But the pointer to the local variable bothers me. With my beginner's eyes I can't see a case where it can lead to something wrong.
Is this okay? Why? Do you see a better way to do it?
You need to make sure your compairson also looks for an existing value within the VecPointer, since your original second loop only cares about the first value it comes across.
else if(VecPointer && input == "SomethingElse")
Don't look for ->empty(), as that's accessing the pointer and asking whether the pointed to vector is empty. If there's nothing to point to in the first place, you're going to have a bad time at the -> stage of the statement. Instead, if against it, since it's a pointer.
if(VecPointer)
Finally, you're using a Vector to save that one value from m_vec, which from other code I'm assuming is not a vector<vector<string>> but a vector<string> - in the latter case, your VecPointer should be std::string*
std::string* VecPointer = nullptr;
I'm trying to optimize a part of it.
...
Is this okay?
Maybe not! This may already be a poor use of your time. Are you sure that this is what's hurting your performance? Or that there's a performance problem at all?
Remember Don Knuth's old adage: Premature optimization is the root of all evil...
Do you see a better way to do it?
Consider profiling your program to see which parts actually take up the most time.
On an unrelated note, you could use standard library algorithms to simplify your (unoptimized) code. For example:
if (std::ranges::find(std::begin(object.m_vec) std::end(object.m_vec), "SomethingElse"s )
!= std::end(object.m_vec))
{
xml << "<OtherNode>" << whatever << "</OtherNode>";
}
if ((!m_pMediaPage->PageLayer() || !m_pMediaPage->LoadState()) &&
!m_pMediaPage->m_bRequestList)
{
GetListInfo();
m_pMediaPage->m_bRequestList = TRUE;
}
GetListInfo() does not get executed when all values are 0.
PageLayer() and LoadState() return ints and m_bRequestList is an int.
Basically rewritten as this:
if ((!0 || !0) && !0) -or- if ((1 || 1) && 1)
I can only assume that the values being evaluated by the if statement aren't really as seen by the debugger.
I am using visual studio 2005 and put breakpoints on line 1 & 4 to examine the values and see if it executes into the if statement. I am not sure how else to debug this.
Like I said, each of the 3 values are 0 as viewed by the debugger when at breakpoint 1.
Functions in .h:
int PageLayer() {return m_iCurrentLayer;} - protected
BOOL LoadState() {return m_bLoadDone;} - protected
BOOL:
typedef int BOOL;
This conditional statement looks as if it would be executed if all values return from the different functions return zero. If the body of the function isn't executed, I would debug the problem as follows:
Log the values of all functions prior to the if-statement:
std::cout << "page-layer=" << !m_pMediaPage->PageLayer() << ' '
<< "load-state=" << !m_pMediaPage->LoadState() << ' '
<< "request-list=" << !m_pMediaPage->m_bRequestList << '\n';
Yes, the debugger should show these values as well but I have great faith in the values being printed to be the values actually evaluated.
If that doesn't give the necessary insight into what goes wrong, I would start breaking down the condition into separate parts and verify success at each level, e.g.:
if (!m_pMediaPage->PageLAyer()) {
std::cout << "page-layer is not set\n";\
}
if (!m_pMediaPAge->LoadState()) {
std::cout << "load-state is not set\n";
...
If this still doesn't give any insight, I'd start suspecting that the functions return funny values and I would verify that the different results are funny values and I would start looking at the output after preprocesing using the -E option.
You tagged the question as VS2005; do you have all relevant service packs installed to ensure you aren't running into some long-fixed compiler issue?
Secondly, the functions you've listed appear to be very simple setters (you might want to make them const, although that is unrelated to your problem).
You're stepping thru with the debugger, it might therefore be valuable to check your assertion that they are all zero:
bool plCond = (m_pMediaPage->PageLayer());
bool lsCond = (m_pMediaPage->LoadState());
bool rlCond = (m_pMediaPage->m_bRequestList);
bool getListInfoCond = ((!cond1 || !cond2) && !cond3);
if (getListInfoCond)
{
GetListInfo();
m_pMediaPage->m_bRequestList = TRUE;
}
If this fixes the problem, you either have a heisenbug or a stack/memory stomp.
If this doesn't fix the problem, it may home in on the cause.
If this DOES fix the problem, you may want to consult the assembly for the code and see if you have somehow tripped a compiler bug.
I have an assignment where I must read from a file and perform various calculations on it and write the answer to an output file. Everything was going great until I came to this step:
"Reread the file and compute the sum of the integers in the file as long as the sum does not exceed 1000. Use a flag controlled loop structure."
My code snippet is as follows:
dataFile2.close();
dataFile2.clear();
dataFile2.open("J:\\datafile2.txt");
sum = 0;
while(sum < 1000)
{
dataFile2 >> num;
sum = sum + num;
if(sum > 1000)
sum = sum - num;
}
answers << "The sum of the integers not exceeding 1000 is " << sum << endl;
cout << "The sum of the integers not exceeding 1000 is " << sum << endl;
return 0;
My variables have already been declared. when I take out the if statement the sum adds the last number and the sum then exceeds 1000. When the If statement is left in, the answers and cout statements are not executed and there are no compiler warnings or errors.
Any help on this would be greatly appreciated.
-ThePoloHobo
Since no one seems to want to give you a correct answer... (and
to be fair, it's hard to give a correct answer without actually
doing your work for you).
There are two issues in you code. The first is the requirement
that you use a flag. As I said in my comment, the idiomatic
solution would not use a flag, but there's no problem using one.
A flag is a boolean variable which will be tested in the
while, and will be set in a conditional in the loop, when you
find something that makes you want to leave the loop.
The second issue is that you are using num without checking
that the input has succeeded. You must check after the >>
operator. The idiomatic way of checking (and the only thing
that should ever be used by someone not experienced in the
language) is to treat the stream as if it were a boolean:
dataFile2 >> num;
if ( dataFile2 ) {
// Input succeeded...
} else {
// Input failed for some reason, maybe end of file
}
Since all operations on a stream return a reference to the
stream, it is usual to merge the test and the input:
if ( dataFile2 >> num ) {
// succeeded
} else {
// failed
}
(Personally, I find the idea of modifying state in the condition
of an if or a while horrible. But this idiom is so
ubiquitous that you should probably use it, for the simple
reason that that's what everyone expects.)
In pedagogical environments, it's probably acceptable to
consider any failure to be end of file, and just move the test
up into the while (except, of course, that you've been asked
to use a flag). In other contexts, you'll want to take into
account the fact that the failure could be due to a syntax error
in the input—someone inserted "abc" into the file where
you were expecting a number. There are a number of ways of
handling this, all of which are beyond the scope of what you are
trying to do, but be aware that after you've detected failure,
you can interogate the stream to know why. In particular, if
dataFile2.eof() is true, then the failure was (probably) due
to you having read all of the data, and everything is fine. (In
other words, failure to read a data is not necessarily an error.
It can be simply end of file.)
You don't seem to be using a flag variable, which could help in this case. Something like this should fix it:
sum = 0;
bool sumUnder1000 = true; //Or the C++ equivalent, I'm a bit rusty
while(sumUnder1000)
{
if(!dataFile2.good()){
sumUnder1000 = false; //We've reached end of file or an error has occurred
return;
}
dataFile2 >> num;
sum = sum + num;
else if(sum > 1000){
sum = sum - num;
sumUnder1000 = false;
}
}
I have the following C++ code:
if(x==y||m==n){
cout<<"Your message"<<endl;
}
If x is equal to y or m is equal to n, the program prints "Your message". But if both conditions are true,the program tests only one of them and eventually prints one "Your Message".
Is there a way to print each "Your message" independently based on each condition using a single if statement?
The output would be identical to the below using multiple if statements.
if(x==y){
cout<<"Your message"<<endl;
}
if (m==n){
cout<<"Your message"<<endl;
}
Not that I'd ever do it this way, but ...
for(int i = 0; i < (x==y)+(m==n); ++i) {
std::cout << "Your message\n";
}
Let me expand on this. I'd never do it this way because it violates two principles:
1) Code for maintainability. This loop is going to cause the maintainer to stop, think, and try to recover your original intent. A pair of if statements won't.
2) Distinct input should produce distinct output. This principle benefits the user and the programmer. Few things are more frustrating than running a test, getting valid output, and still not knowing which path the program took.
Given these two principles, here is how I would actually code it:
if(x==y) {
std::cout << "Your x-y message\n";
}
if(m==n) {
std::cout << "Your m-n message\n";
}
Aside: Never use endl when you mean \n. They produce semantically identical code, but endl can accidentally make your program go slower.
I don't think that's possible. What you have inside your bracket is a statement which is either true or false, there's no such thing like a true/true or true/false statement. What you could do is a do/while loop with a break statement. But I don't think that's the way to go. Why do you want to avoid two if statements?
single "|" or "&" gaurantees both side evaluation even if the result can be determined by left side operator alone.
You could do something like this, to build up the "message":
string msg = "Your Message\n";
string buildSt = x == y ? m == n ? msg + msg : msg : m == n ? msg : "";
Compiler checks only one condition when both are true because you've connected your conditions with OR.
If even one condition in ORs chain is true there is no need to check others as a result already true and will be false if one of them is false. So if you think that your logic is right then there is no need to do multiple checks. Your code is asking that you will print a message if one of the conditions is true and program doing it. If you want something special for a case when both conditions are true then add it separately. Shortly you should never expect from the compiler to do all checks in the expressions connected by OR.
Regards,
Davit
Tested code:
#include <iostream>
#include <string>
using namespace std;
void main() {
int x=1;
int y=1;
int m=1;
int n=1;
string mess1="Your message 1\n";
string mess2="Your message 2\n";
cout<<((x==y)?mess1:"")+((m==n)?mess2:"");
getchar();
}
If you are trying to see if both statements are true an && is what you will want to use.
Take a look at Boolean Operators to see all of the possible options when comparing boolean (true/false) values.
To answer your question:
if ((x==y) && (m==n))
{
cout<<"Your Message"<<endl<<"Your Message"<<endl;
}
else if((x==y) || (m==n))
{
cout<<"Your Message"<<endl;
}
I am very curious to learn why the below code does not run in a continuous loop. And I'm also looking for some ways to achieve what I want to achieve--which is resetting a loop inside of the loop. I need to do this because I need to account for each element in a container. The reason why this is because I might start off in the middle, and need to loop back around to check the others / and need to recheck other information too. So on with my little test example:
for ( int i = 0; i != 10; i++ ) {
std::cout << std::endl << "TEST: " << i << std::endl;
if ( i++ == 10 ) {
i = 0;
} else {
i--;
}
}
Is there any particular reason why the above does not work? I am very interested in knowing why, so I can learn how everything works. This also leads into a much bigger problem I am facing. Which is the below code. I am using MSVC++ 2010 Express. Also, this is one thread, so other data is not accessing it. It is an unordered_map using STL. its size if 2 (i checked).
for (game_player_client_map::const_iterator it = gpc_map_ptr->begin(); it != gpc_map_ptr->end(); ++it) {
if ( it++ == gpc_map_ptr->end() ) {
cout << endl << "IT == gpc_map_ptr->end()" << endl;
it = gpc_map_ptr->begin();
} else {
it--;
}
}
I appreciate any feedback SO has to offer, and any new things to learn :-) If further information is needed I will provide. Thank you for your time.
Because the condition is checked before the body of the loop is entered. When i == 10, the loop is broken, before your code can execute at the time that i++ would evaluate to 10.
Remember that postincrement increments the variable and returns the old value. So if i is 9, i++ evaluates to 9 also, but the next time you use i, it will be 10.
If you want the variable to be incremented and use the new value in an expression, use preincrement:
if (++i == 10) // changes i to i + 1 and checks if the new value of i is 10
You could completely ditch the increment however, and just use i + 1. That way you don't have to de-increment i in the else block.
Your misunderstanding of postincrement is probably also the source of the bug in the second block of code you posted. You can change it to preincrement, or if it is a random-access iterator, you can do the same thing as mentioned above and check if it + 1 == gpc_map_ptr->end() and not have to de-increment it in the else block.