In C++, what is the point of flags/bools?

Question

I'm doing a C++ programming assignment and one of the requirements is to use flags. However, I've written the entire program which is about 600 lines long, and it executes exactly the way it should. But I never used flags. Can someone point me in the right direction by telling me when to use flags and when to use only if/else statements without flags? Thanks.

PS will using flags/bools make my program shorter? The professor says the sample program is at 500 lines with comments, but mine is 600 lines WITHOUT comments. eek!

Thanks.

Answer 1

A flag is an on/off switch. It is a place where 1/0, y/n, yes/no or x/y can be stored.

It is boolean because this is a storage location that only holds 1 bit and can hold the 1/0 value.

But it can also be a single char or an int or a string.



Note: whilst a boolean will only take up a bit of storage the compiler will probably align all storage outside of a structure to a word boundary. So a boolean will actually take up at least a byte of storage. This is very much dependant on the compiler though.



If you want something in a program to be done only on the first execution of a loop then a flag is ideal. If you want something done first one way then another and then the first way etc. then a flag is ideal.



It is all about program design.

The key thing to remember about program design is that there is no right way to write or code a program. As long as the program does what the user requests and what the user needs then the program is correct. It is important to note that what the user wants may not be what they need. However what the user needs is always what the program should do.

You can only find out what the user needs by talking to the user. Years of experience can also point you towards how to approach what the user needs.



Go back to your program and draw a flowchart.

This should point you at where you may have gone wrong.



Have fun.

Answer 2

Well, flags and bools are pretty amazing overall, let me show you why.



Let's say you want a program to run unless one certain thing happens, under normal circumstances, that would be pretty tricky... example:



while (statement x!= variable or constant y){

if statement (x == something else){

//do stuff

}

//do stuff

//do stuff

//do stuff

}



Whereas the alternative allows you a little more control



bool isDone = false;

while (isDone==false){

//do stuff

//do stuff

//do stuff

if (certainCondition == true){

isDone=true;

}

}

As you can see it makes ending a loop a LOT easier and more precise.



Bools can only have 2 states, 'true' and 'false' and they must be set as such. If I recall correctly bools start off as true unless you explicitly state otherwise (Don't quote me on that, since I always initialize them anyways). In this case, a bool is known as a 'flag' since a flag only has 2 states "up" or "Down" (hence the nickname).



Just as a sidenote about syntax: you can do while(boolname) for while(true) or while(!boolname) for while(false).



If you posted your code I could give it a quick skim and tell you where a bool might help things out. Typically a program can be shortened a bit by using case over complex if/else, and bool over uncontrolled while.

Answer 3

Well, a simple flag/bool is no different than performing any other check.

Now, the thing is that using flags/bools, often will increase the number of lines of code.



Here's an example where the flag/bool makes the program longer:



int main()

{

  bool myflag = false;

  int iSomeVal = 10;



  if (iSomeVal == 10)

    myflag = true;



  if (myflag)

    cout << "iSomeVal = 10" << endl;

  else

    cout << "iSomeVal != 10" << endl;



  return 0;

}



Here's the same thing done without using a flag/bool:

int main()

{

  int iSomeVal = 10;



  if (iSomeVal == 10)

    cout << "iSomeVal = 10" << endl;

  else

    cout << "iSomeVal != 10" << endl;



  return 0;

}



From the sounds of it, you need to simplify some things. So, if you're code is repeating the same mini-process more than one time that takes several lines of code or more, you can make it simpler by moving those lines into a function/method of their own.



An example of that would be like this:

int main()

{

  int iSomeVal = 10;



  if (iSomeVal == 10)

    cout << "iSomeVal = 10" << endl;

  else

    cout << "iSomeVal != 10" << endl;



  iSomeVal = 50;

  if (iSomeVal == 10)

    cout << "iSomeVal = 10" << endl;

  else

    cout << "iSomeVal != 10" << endl;



  return 0;

}



You'll notice that the same 4 lines are repeated. For our example, this can be a function call:

void showResult(int val)

{

  if (val == 10)

    cout << "val = 10" << endl;

  else

    cout << "val != 10" << endl;

}



int main()

{

  int iSomeVal = 10;



  showResult(iSomeVal);



  iSomeVal = 50;

  showResult(iSomeVal);



  iSomeVal = 65;

  showResult(iSomeVal);



  return 0;

}

Answer 4

bools and flags help your program run faster. They take up less space in memory and especially in larger programs you'll need stuff to take up less memory so that your program will run more effeciently.



Now I can't say whether bools and flags would make your code short because I cannot see your code because you haven't posted it. Sometimes adding bools and flags can make your program shorter and easier and sometimes it can't. If your professor said that it will make it easier and shorter then it most likely will make the code shorter and since his/her code is 500 lines with comments and yours is 600 without comments then your code is definately too long



So use flags and bools



Hope This Helped :D

Answer 5

Flags are used to check for error cases.. If a flag is raised then you just have to exit the program with and errror message (or do something else). It is a good style to program beacuse you may have less bugs in the end.

For example:



...

bool check;

check = my_func();

// my_func may do something useful and would return false upon failure

if (check == false)

// cout error message

return -1;

else

// Continue with the program

Answer 6

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