After watching carefully the following code I can't see why the compiler is warning me with "warning: control reaches end of non-void function".
bool Foam::solidMagnetostaticModel::read()
{
if (regIOobject::read())
{
if (permeabilityModelPtr_->read(subDict("permeability")) && magnetizationModelPtr_->read(subDict("magnetization")))
{
return true;
}
}
else
{
return false;
}
}
I can't see where is the problem, the else statement should care for returning false in every case which the first if is not true.
Trace the code path when regIOobject::read() is true, but either of permeabilityModelPtr_->read(subDict("permeability")) or magnetizationModelPtr_->read(subDict("magnetization")) is false. In that case, you enter the top if block (excluding the possibility of entering its attached else block), but then fail to enter the nested if block:
bool Foam::solidMagnetostaticModel::read()
{
if (regIOobject::read())
{
// Cool, read() was true, now check next if...
if (permeabilityModelPtr_->read(subDict("permeability")) && magnetizationModelPtr_->read(subDict("magnetization")))
{
return true;
}
// Oh no, it was false, now we're here...
}
else
{
// First if was true, so we don't go here...
return false;
}
// End of function reached, where is the return???
}
The minimalist fix is to just remove the else { } wrapping, so any fallthrough ends up at return false;:
bool Foam::solidMagnetostaticModel::read()
{
if (regIOobject::read())
{
// Cool, read() was true, now check next if...
if (permeabilityModelPtr_->read(subDict("permeability")) && magnetizationModelPtr_->read(subDict("magnetization")))
{
return true;
}
// Oh no, it was false, now we're here...
}
// Oh, but we hit return false; so we're fine
return false;
}
Alternatively, avoid specifically mentioning true or false at all, since your function is logically just a result of anding three conditions together:
bool Foam::solidMagnetostaticModel::read()
{
// No need to use ifs or explicit references to true/false at all
return regIOobject::read() &&
permeabilityModelPtr_->read(subDict("permeability")) &&
magnetizationModelPtr_->read(subDict("magnetization"));
}
The nested if is the problem.
When that branch is not taken, there is no other paths to take
the else statement should care for returning false in every case which the first if is not true.
Correct, but what if the first if condition is true, but the second if condition is not?
That is: What if regIOobject::read() returns true, but permeabilityModelPtr_->read(subDict("permeability")) returns false?
Then the flow of control enters the first if block, does not return, but does not enter the else block (because the first condition was true), so it just falls off the end of the function without hitting a return statement.
If you want the else { return false; } part to apply to either condition, you could just naively copy/paste it:
if (COND1) {
if (COND2) {
return true;
} else {
return false;
}
} else {
return false;
}
But that's quite a bit of code duplication. A better solution is to replace the nested if by a single condition:
if (COND1 && COND2) {
return true;
} else {
return false;
}
There's still some duplication: Both branches consist of a return statement followed by some expression.
We can factor out the common parts (return) and push the condition into the expression:
return COND1 && COND2 ? true : false;
But ? true : false is redundant: If the condition is true, evaluate to true, else evaluate to false? Well, that's just what the condition itself does:
return COND1 && COND2;
Or with your concrete expressions:
return regIOobject::read()
&& permeabilityModelPtr_->read(subDict("permeability"))
&& magnetizationModelPtr_->read(subDict("magnetization"));
Related
I have a doubt with the solution of this question which is stated below -
Given an arbitrary ransom note string and another string containing letters from all the magazines, write a function that will return true if the ransom note can be constructed from the magazines ; otherwise, it will return false.
Each letter in the magazine string can only be used once in your ransom note.
Strings["aa", "ab"] should return false and strings["aa", "aab"] should return true according to question.
Here is the code which I have attempted in the first place and I'm not getting a required output as mentioned above.
unordered_map<char,int>umap;
for(char m:magazine)
{
umap[m]++;
}
for(char r:ransomNote)
{
if(umap.count(r)<=1)
{
return false;
}
else{
umap[r]--;
}
}
return true;
}
In the above code, I have used umap.count(r)<=1 to return false if there is no key present.
For the strings ["aa","aab"], it is returning true but for strings ["aa","ab"], it is also returning true but it should return false.
Then I used another way to solve this problem by using just umap[r]<=0 in the place of umap.count(r)<=1 and it is working just fine and else all code is same.
bool canConstruct(string ransomNote, string magazine) {
unordered_map<char,int>umap;
for(char m:magazine)
{
umap[m]++;
}
for(char r:ransomNote)
{
if(umap[r]<=0)
{
return false;
}
else{
umap[r]--;
}
}
return true;
}
I'm not able to get what i'm missing in the if condition of first code. Can anyone help me to state what I'm doing wrong in first code. Any help is appreciated.
unordered_map::count returns the number of items with specified key.
As you don't use multi_map version, you only have 0 or 1.
Associated value doesn't change presence of key in map.
To use count, you should remove key when value reaches 0:
for (char r : ransomNote) {
if (umap.count(r) == 0) {
return false;
} else {
if (--umap[r] == 0) {
umap.erase(r);
}
}
}
return true;
I have this code. At first jobDone is false and the program does the first step, then the second and sets the jobDone variable to true stating that all tasks completed successfully.
But I might choose to run this bit of code again and I don't want to do the first step again since it might not be necessary. So if the job has been done at least once and if checkSomething() concludes that everything is OK it should go ahead and do only secondStep();
Otherwise, if checkSomething() finds out that something is wrong then I must repeat the first and as well the second step (like in the initial situation).
jobDone = false;
if (jobDone) {
if (checkSomething()) {
doSecondStep();
} else {
doFirstStep();
doSecondStep();
}
} else {
doFirstStep();
doSecondStep();
jobDone = true;
}
But I feel that i'm repeating myself. Is there a better way of writing this? Thank yoou.
if(jobDone && checkSomething()) {
doSecondStep();
} else {
doFirstStep();
doSecondStep();
jobDone = true;
}
The second step is always executed, I suggest you move that to the end first:
jobDone = false;
if (jobDone) {
if (!checkSomething()) {
doFirstStep();
}
} else {
doFirstStep();
jobDone = true;
}
doSecondStep();
However, there's only one condition that would cause doFirstStep() not to be executed and that's when jobDone == true and checkSomething == false, so we could just check for that.
The whole thing can be summarized to three lines:
if (!(jobDone && !checkSomething())) { doFirstStep(); }
jobDone = !jobDone;
doSecondStep();
If I write a statement:
if(!(method())
{
// do something
}
else
{
//do something
}
And method returns a boolean value, is the else - without writing else if (method() - automatically the opposite of if, i.e. does else have a relationship to the method called in the opening if statement?
Yes, if method on IF return TRUE, as it's negated, ELSE statement will execute.
It's like:
if(!myMethod()) {
//RUN SOMETHING
}
else {
//RUN OTHER THING HERE
}
If MyMethod Returns "TRUE", the if statement will se a "FALSE" because it's inverting it, so as it's not true, will run the ELSE code (//RUN OTHER THING).
Yes - implicitly, IF .. ELSE is a boolean test that essentially looks like this:
IF (condition)
{
<< condition evaluated to TRUE >>
}
ELSE
{
<< condition evaluated to FALSE >>
}
Assume I have code like:
if(condition1 || condition2 || condition 3 || condition4)
{
// this inner part will be executed if one of the conditions is true.
// Now I want to know by which condition this part is executed.
}
I'm sure there are better ways to do this, here's one:
int i = 0;
auto check = [&i](bool b)->bool
{
if (!b) ++i;
return b;
};
if (check(false) || // 0
check(false) || // 1
check(true) || // 2
check(false)) // 3
{
std::cout << i; // prints 2
}
|| is short circuit evaluation, so you can have code like this :
if(condition1 || condition2 || condition 3 || condition4)
{
if (condition1 )
{
//it must be condition1 which make the overall result true
}
else if (condition2)
{
//it must be condition2 which make the overall result true
}
else if (condition3)
{
//it must be condition3 which make the overall result true
}
else
{
//it must be condition4 which make the overall result true
}
// this inner part will executed if one of the condition true. Now I want to know by which condition this part is executed.
}
else
{
}
If the conditions are independent of each other, you need to check them separately, or, if they belong to one variable, you can use a switch statement
bool c1;
bool c2
if ( c1 || c2 )
{
// these need to be checked separately
}
int i; // i should be checked for multiple conditions. Here switch is most appropriate
switch (i)
{
case 0: // stuff
break;
case 1: // other stuff
break;
default: // default stuff if none of the conditions above is true
}
Without a switch you can use only or and if statements:
if(condition1 || condition2 || condition 3 || condition4) {
// this inner part will executed if one of the condition true.
//Now I want to know by which condition this part is executed.
if ( condition1 || condition2 ) {
if ( condition1 )
printf("Loop caused by 1");
else
printf("Loop caused by 2");
else
if ( condition3)
printf("Loop caused by 3");
else
printf("Loop caused by 4");
}
I'm not sure that this is the most efficient thing you've ever seen, but it will identify which of the four conditions caused entry into the if ... block.
If you need to know for programmatic reasons, i.e. run different code depending on which condition is true, you could do something like this
if (condition1)
{
...
}
else if (condition2)
{
...
}
else if (condition3)
{
...
}
else if (condition4)
{
...
}
else
{
...
}
If you only want to know for debugging reasons, just do a printout.
What about the comma operator?
By using that logical operators follow the short circuit evaluation method, the following works fine:
int w = 0; /* w <= 0 will mean "no one is true" */
if ( (w++, cond1) || (w++, cond2) || ... || (w++, condN) )
printf("The first condition that was true has number: %d.\n", w);
I have this C++ program with the following general structure
1st while (condition A == true)
//some code for 1st loop
2nd while (condition B == true)
//some code for 2nd loop
try
//some code for try
catch
//condition B == false (supposed to leave 2nd loop and go back to first loop)
I want it to get out of 2nd loop when there's an exception and go back to 1st loop until condition B is tue again. As described above it doesn't work as I expect. What seems to be happening is that code gets stuck in catch and never leaves it.
How can I arrange it to make it work as I need?
Note: condition A is never false.
add the break keyword to the catch
Also notice that you have b == false;
That is checking that b is equal to false, not setting b = false.
bool flag1 = true, flag2 = true;
while (flag1)
{
// some work so that flag2 == true
while (flag2)
{
try
{
}
catch (...) // any exception happens
{
break;
}
}
}
1st while (condition A == true)
//some code for 1st loop
2nd while (condition B == true)
//some code for 2nd loop
try
//some code for try
catch
{
//condition B == false (supposed to leave 2nd loop and go back to first loop)
break ;
}
Notice: Please do not use, even in examples, things like condition A == true. It is better to use while (condition A).
You can call break within the catch block to escape the second loop:
void foo(void) {
bool A(true);
while (A) {
bool B(doSomething());
while (B) {
try {
B = doSomethingElseThatMayThrow();
} catch (...) {
break;
}
}
}
}
Alternatively, you could place the second loop inside the try block:
void foo(void) {
bool A(true);
while (A) {
bool B(doSomething());
try {
while (B) {
B = doSomethingElseThatMayThrow();
}
} catch (...) {}
}
}