I know this could be a pipe dream, just throw it here.
So here is the code:
int f()
{
int i = 0;
int j = 0;
for (; i < 10 && j < 100; ++i, j = i * 2) {
i = f2();
}
}
So when it comes to function f(), I can run "disp i j" and then after each single line step over, the values of i and j print out. It works fine.
But is there a global setting or some trick so that, I do not need to type the "display" command, all local variables are automatically printed AND they are printed only when their values are updated.
Another words, at the line i = f2(), the value of j does not print.
You can set the watchponits on i and j. Like this
(gdb) watch i
It will automatically print the value of i whenever value of i changes
You can also print the value based upon some condition using if<condition>. Like this
(gdb) watch i if j==4
For me there is no issue in printing the value of j at
i = f2(); line.
Related
I'm trying to compare two decks of cards, yet every time I try another method of doing it, I get the same result... Everything before the code outputs, and it just freezes as soon as it hits the comparison code, as if it's stuck in an infinite loop.
I've tried for loops, static variables, do-while loops, etc. This is my first time leaving the loop at the client code.
The code that supposedly throws the program into an infinite loop.
while (repeatLoop == false)
{
deck1.shuffleDeck();
counter++;
repeatLoop = deck1.compareDecks();
}
compareDecks function.
bool deck::compareDecks()
{
int deckCount = 0;
suitType tempOriginalSuit;
suitType tempShuffleSuit;
rankType tempOriginalRank;
rankType tempShuffleRank;
while (index < 52)
{
tempOriginalSuit = originalCardDeck[index].getSuit();
tempShuffleSuit = shuffledCardDeck[index].getSuit();
if (int(tempOriginalSuit) == int(tempShuffleSuit))
{
tempOriginalRank = originalCardDeck[index].getRank();
tempShuffleRank = shuffledCardDeck[index].getRank();
if (int(tempOriginalRank) == int(tempShuffleRank))
{
deckCount++;
if (deckCount == 52)
return true;
}
}
else
{
return false;
index++;
}
}
}
The shuffleDeck function
(This function pushes back the first card from the first half of the deck and the first card from the second half of the deck towards the end until all 52 cards have been pushed in this pattern. This makes the deck have 52 x 2 cards (with the second half of the deck being the perfect shuffle), so I delete the first half of the cards using .erase as it is not needed)
void deck::shuffleDeck()
{
for (int a = 0, b = 2; a < 2 && b < 4; a++, b++)
{
for (int i = 2; i < 15; i++)
{
shuffledCardDeck.push_back(card{ static_cast<cardSpace::suitType>(a),
static_cast<cardSpace::rankType>(i) });
shuffledCardDeck.push_back(card{ static_cast<cardSpace::suitType>(b),
static_cast<cardSpace::rankType>(i) });
}
}
shuffledCardDeck.erase(shuffledCardDeck.begin(),
shuffledCardDeck.begin() + (shuffledCardDeck.size() / 2));
}
The two decks initialized by this constructor.
deck::deck()
{
for (int i = 0; i < 4; i++)
{
for (int j = 2; j < 15; j++)
{
originalCardDeck.push_back(card{ static_cast<cardSpace::suitType>(i),
static_cast<cardSpace::rankType>(j) });
shuffledCardDeck.push_back(card{ static_cast<cardSpace::suitType>(i),
static_cast<cardSpace::rankType>(j) });
}
}
}
Also note that I've done a perfect shuffle on the shuffledCardDeck vector in another function. I'm trying to repeat the perfectShuffle function until it reaches it's original state and output how many times it took to do this.
I get an infinite loop.
EDIT: I've decided to add the return false; statement in the compareDecks function into the if-else. Also, I think what's causing the problem is that my index i is reset to zero everytime it is called again. Are there any solutions you guys could propose to this? I've tried using static variables, but they just would not increment in the for loop.
EDIT 2: I enclosed my if statements within the curly braces, per users' request, as it's a flaw in my code.
EDIT 3: After commenting out
deck1.shuffleDeck()
The compareDecks function returned true, stating that the decks are equal, which isn't supposed to happen... This caused the loop to end after only one loop.
I was expecting you to actually shuffle the deck.
Your code was pushing a specific, newly synthesized card onto the end of the deck:
shuffledCardDeck.push_back(card{ static_cast<cardSpace::suitType>(a),
static_cast<cardSpace::rankType>(i) });
For example, the first card it will push is always the 2 of 0's (Whatever the 0th suit is). That's not what you want. You actually want to push a copy of the card that is at a specific position index in the deck. For example, loop index from 0 to 25 and then push shuffledCardDeck[index] and shuffledCardDeck[26 + index].
Then you can still wrap up by using your technique of erasing the first half of the deck.
void deck::shuffleDeck()
{
for (int index = 0; index < 26; ++index) {
shuffledCardDeck.push_back(shuffledCardDeck[index]);
shuffledCardDeck.push_back(shuffledCardDeck[26 + index]);
}
shuffledCardDeck.erase(shuffledCardDeck.begin(),
shuffledCardDeck.begin() + 52);
}
You are not modifying the value in the loop, you're using a double equals sign:
repeatLoop == deck1.compareDecks();
That would explain your observed behavior.
We can change value manually by changing in variable tooltip or local/auto/watch window.
But I want to change value of variable automatically to some specific hardcoded value or based on a code snippet.
For eg.-
int main()
{
int a=0,b=1,c=2;
//bla bla
for(int i=0; i<100; ++i)
{
executeMe();
}
//bla bla
}
I want to put a breakpoint on line "executeMe()" and change value of 'b' to hardcoded value 3 or based on variable value 'c', so executing instruction 'b=c'. And continues execution without stopping everytime on breakpoint.
How to do this in VS?
Use the 'Print a message:' option instead of a macro. Values from code can be printed by placing them inside {}. The key is that VS will also evaluate the content as an expression - so {variable_name=0} should achieve the same as the macro example.
Thanks to Tom McKeown for this solution on stackoverflow.com/a/15415763/2328412
You could use #if preprocessor directives, which is similar with below code.
int a = 0, b = 1, c = 2;
for (int i = 0; i < 100; ++i)
{
#if DEBUG
b=3;
#endif
executeMe();
}
Sorry for this very simple looking problem, but I have no idea what causes it:
In a C++ project I have a loop in a loop in a loop and have to leave the inner two so I have a variable for a query. In the first iteration it works fine, but in the second the assign from dtime to abbruch does not work. In the Debugger dtime has correctly the value "1" and abbruch "0" but this stays after the assignment. Both are of type long.
for (sect = 0; sect <= sections; sect++)
{
abbruch = 0;
for(line = 0; line < maxlines ; line ++)
{
abbruch = dtime[sect][0];
if(abbruch != 0)
{
break;
}
for (index = 0; index < 30; index ++)
{
if (complicated query)
{
dtime[sect][0] = DiffTime[maxlines * sect + line];
break;
}
}
}
}
I use VS2012 Ultimate.
Has anyone an idear how this can happen ot how to solve it?
Did you maybe mean to put this?
abbruch = dtime[sect][line];
(line instead of 0)
But also what Bathseba said is true. A break will only break one for-loop.
break will only take you out of the current for loop. In your case, the loop over index will not be called following a break when abbruch != 0 as that break will take you out of the loop over line. The other break statement will take you out of the loop over index.
That's the rationale, but, by far the best thing to do is to step through with a debugger. I wouldn't use break statements in this way as it's too confusing. Consider breaking the triple loop structure into function calls using return statements in place of breaks.
Also, it's a good idea to localise the interating variables in the for loops, e.g.
for (int sect = 0; sect <= sections; sect++)
Variable outside of the loop
int number = 0;
for(int i = 0; i < 10000; i++){
number = 3 * i;
printf("%d",number);
}
or Variable inside of the loop
for(int i = 0; i < 10000; i++){
int number = 3 * i;
printf("%d",number);
}
Which one is recommended and which one is better in performance?
Edit:
This is just an example to exhibit what I mean, All I wanna know is if defining a variable inside a loop and outside a loop means the same thing , or there's a difference.
Time to learn something early: any optimization you could make on something like this will be irrelevant in the face of printf.
Printf will be very, very slow. You could quintuple the math and get no measurable speed decrease. It's just the nature of printing to the terminal.
As for your edited question, there is no difference defining it in the loop or out. Imagine that
for (i = 0; i < 500; i++) {
int a = i * 3;
}
is just the same as
int forloop::a; // This doesn't work, the idea is to show it just changes the scope
for (i = 0; i < 500; i++) {
a = i * 3;
}
They will produce identical code, unless you start needing to use that variable outside of the loop it is defined in, because it is defined in the local scope of the loop. So...more like this:
int forloop::a; // Still not valid code, just trying to show an explanation
namespace forloop {
for (i = 0; i < 500; i++) {
a = i * 3;
}
} // namespace forloop
If this is unclear please let me know I'll go into more detail or explain differently.
Do not bother you with performance at first: make it safe before everything.
I would just quote Scott Meyers (Effective C++) for your concern:
"Postpone declarations as far as you can".
Thus, the second pattern is safer.
Example:
int j = 0;
for(int i = 0; i < 10000; i++){
j = 3 * i;
printf("%d",j);
}
...
// Use of j out of control!!!
int k = j * 5;
Now with the second pattern:
for(int i = 0; i < 10000; i++){
int j = 3 * i;
printf("%d",j);
}
...
// j not declared at this point.
// You get informed of the mistake at compile time, which is far much better.
int k = j * 5;
You do have a C++ tag, and you mention "declaring a string" in the question. Therefore there might be a performance difference (and yes, the printf could swamp it). Declaring a non-simple variable means calling a constructor, which might mean a non-trivial amount of work. In that case, declaring it inside of the loop could be hiding significant work in what appears to be an innocent declaration.
In general, the answer is that if you really care about performance - and treating the sample code as only an example of the difference between two places to declare a variable - then for non-simple variables, it is better to declare it outside the loop, unless the semantics require a fresh version of a temporary at each iteration.
There are likely many other places first to look at if performance is an issue, but one consideration is always moving loop invariants out of loops, especially if it is much easier for you to tell that it is invariant than for the compiler. And what looks like a declaration, can, in C++, fall into that category.
If, for (silly) example, you have
int k = 43;
for ( int i = 0; i < N; ++i )
{
int j = 17 + k; // k was previously defined outside the loop, but doesn't change in it
l = j * j; // l was also declared outside the loop
}
any good optimizing compiler can recognize that k is constant, and that j is always assigned 60, and l is assigned 3600 N times, and the loop can simply be removed and replaced with a single assignment to l. Here k and j are both loop invariants.
But a not-quite-so-good compiler might miss even one link in that chain, and wind up creating the loop.
It gets harder for the compiler to figure things out when you have
Foo k( 43 ); // a class that takes an int argument to its constructor
for( int i = 0; i < N; ++i )
{
Bar j( k ); // a Bar takes an int argument, adds 17 and stores it.
l = j.squared();
}
Same invariants. Not as easy to detect without looking inside the workings of bar; and if the constructor and squared method aren't inline, we've just made it slower.
In this case, printf("%d", i * 3) would be better than defining the variable at all.
To answer your question and not nitpicking:
The difference between the 2 variants is, that you are declaring your variable number in different "variable environments" - by which I mean that the scope changes. A variable environment is given by your curly braces { ... }. Everytime you open a new curly brace like this { ... { ... } ... }, you declare a new variable environment inside the old one, which means, that if you declare numbers like so:
{ ... { int numbers; ... } ... }
this variable will only be visible or existent in the innermost environment. So
{ ... { int numbers; ... } ... do_something(numbers); ... }
will give a compiler error.
And to your concerns about performance: Neither variant is better performing. Most, if not all compilers will give the same assembly.
I'm working in a real time ploting appication with MSChart...I need to set some y values to NaN but I'm getting an overflow exception. Here is the part of the code where it happen:
if (j_ecg < 2569)
{
for (int i = 0; i < 32; i++)
{
this.Invoke((MethodInvoker)delegate
{
ECG.Points.AddXY(puntos_ecg[j_ecg].X,puntos_ecg[j_ecg].Y);
});
j_ecg++;
}
}
else
{
for (int i = 0; i < 32; i++)
{
this.Invoke((MethodInvoker)delegate
{
ECG.Points[ecg_s].SetValueY(puntos_ecg[j_ecg].Y);
for (int j = 1; j < 10; j++){ ECG.Points[ecg_s + j].SetValueY(double.NaN); }
});
j_ecg++;
ecg_s++;
if (ecg_s == 2560) { ecg_s = 0; }
}
}
The Invokes are there to avoid cross threads issues.
Any idea of how can I do it for not getting the exception? I've try using unchecked keyword just before the SetValueY call but nothing changes.
First thing to try is disabling autoscaling so that min/max don't need to be calculated:
chart1.ChartAreas["Default"].AxisY.Minimum = <your min>;
chart1.ChartAreas["Default"].AxisY.Maximum = <your max>;
However you still need to have at least one real value in one of your series.
I recommend you keep track of your displayed values. If everything is NaN, stop plotting! When the next valid value comes in resume plotting.
Note: using zeroes instead of NaN is another solution.