User input for array size.

coolmoniker · 08-24-2006, 02:38 PM

Hello.

I haven't done any programming in since I took CS1100 last year. Now I am taking CS1110 and I don't want to get owned, so I am trying to practice some exercises from the book before my professor assigns anything.

So I am currently stuck with this part of a program--I want to be able to creat an array, the size of which is based upon the input of the user.

This is what I have right now. It doesn't work for obvious reasons but I can't figure anything out to work around it. (I don't know why the indentation is all weird)

Code:

		

cout << "Please enter the number of salespeople./n";
                int n;
	cin >> n;
	const int * const sPtr = &n;
	int salaryarray[ *sPtr ];

I have never used pointers until today, and I am very rusty with arrays. I understand that arrays have to have a constant value as their size. But there has to be a way to make that size be specified by the user. Right?

Desolation · 08-24-2006, 02:41 PM

Yes there is and it is called dynamic memory allocation.

Code:

std::cin >> n;
int* salaryarray = new int[n];

The last line creates a pointer to hold the address of memory allocated by the 'new' operator and then you need to specify the type (so that the compiler knows the size of the type) and in the brackets the number of elements of that type to allocate. When allocating with new[], deallocate using delete[] and the same thing if you allocate with new, you'll want to use delete.

dwks · 08-24-2006, 02:43 PM

Yes, use dynamic memory allocation.

Code:

int size, *array;

cout << "How many? ";
cin >> size;

array = new int[size];

for(int x = 0; x < size; x ++) {
    cout << "Enter number " << x << ": ";
    cin >> array[x];
}

for(int x = 0; x < size; x ++) {
    cout << array[x] << endl;
}

delete [] array;  // Don't forget to free the memory

[edit] Two minutes too late . . . [/edit]

Ancient Dragon · 08-24-2006, 03:12 PM

I think C99 standard allows this

Code:

	cin >> n;
	int salaryarray[ n ];

or, since this is a c++ program, you might use a vector

Code:

	cin >> n;
	vector<int> salaryarray(n);

dwks · 08-24-2006, 03:17 PM

Yes, the C99 standard allows variable-sized arrays.

[edit]

Code:

cout << "Please enter the number of salespeople./n";

BTW, I think you meant "\n". [/edit]

Daved · 08-24-2006, 03:19 PM

In C++ you should really use the standard vector class for these types of things, although if your professor teaches C style arrays you should learn them as well.

Desolation · 08-24-2006, 04:23 PM

For such a small need I think using a vector would be like shooting a fly with a canon.

dwks · 08-24-2006, 04:26 PM

Not really. A vector doesn't store any pointers for each element (the memory is just a contiguous chunk, like you would get with a dynamically allocated array). And I can't imagine it would impact execution speed very much.

Daved · 08-24-2006, 04:29 PM

>> For such a small need I think using a vector would be like shooting a fly with a canon.
It's your opinion, but why are you even using C++ then?

Mario F. · 08-24-2006, 04:41 PM

>> For such a small need I think using a vector would be like shooting a fly with a canon.

Or using an array would be like trimming your nails with a butchers knife. And I do echo Daved's question.

siavoshkc · 08-24-2006, 05:09 PM

Quote:

For such a small need I think using a vector would be like shooting a fly with a canon.

Why you think so?

dwks · 08-24-2006, 05:12 PM

Because a vector is slower and uses more memory than an array. But its advantages far outweigh its disadvantages. (We're using C++, right?)

Cat · 08-24-2006, 05:15 PM

Quote:

Originally Posted by Desolation

For such a small need I think using a vector would be like shooting a fly with a canon.

I tend to disagree. Using pointers in general requires careful planning to avoid memory leaks. Even a simple piece of code like this:

Code:

int * a = new int[10];
int * b = new int[10];
delete[] a;
delete[] b;

has one possible scenario where it leaks memory.

Daved · 08-24-2006, 05:16 PM

>> Because a vector is slower and uses more memory than an array.
That's not a sufficient reason (even if it were true), unless you would say using C++ instead of C, or C instead of assembly is like shooting a fly with a cannon.

siavoshkc · 08-24-2006, 05:16 PM

Vector is slower than new/delete? Why?

08-24-2006, 02:38 PM	#1
coolmoniker Registered User Join Date: Aug 2006 Posts: 4	User input for array size. Hello. I haven't done any programming in since I took CS1100 last year. Now I am taking CS1110 and I don't want to get owned, so I am trying to practice some exercises from the book before my professor assigns anything. So I am currently stuck with this part of a program--I want to be able to creat an array, the size of which is based upon the input of the user. This is what I have right now. It doesn't work for obvious reasons but I can't figure anything out to work around it. (I don't know why the indentation is all weird) Code: cout << "Please enter the number of salespeople./n"; int n; cin >> n; const int * const sPtr = &n; int salaryarray[ sPtr ]; I have never used pointers until today, and I am very rusty with arrays. I understand that arrays have to have a constant value as their size. But there has* to be a way to make that size be specified by the user. Right?
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08-24-2006, 02:41 PM	#2
Desolation Registered User Join Date: May 2006 Posts: 894	Yes there is and it is called dynamic memory allocation. Code: std::cin >> n; int* salaryarray = new int[n]; The last line creates a pointer to hold the address of memory allocated by the 'new' operator and then you need to specify the type (so that the compiler knows the size of the type) and in the brackets the number of elements of that type to allocate. When allocating with new[], deallocate using delete[] and the same thing if you allocate with new, you'll want to use delete. __________________ www.atempsperdu.net/forum/ www.atempsperdu.net/site/en/
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08-24-2006, 02:43 PM	#3
dwks Frequently Quite Prolix Join Date: Apr 2005 Location: Canada Posts: 7,698	Yes, use dynamic memory allocation. Code: int size, array; cout << "How many? "; cin >> size; array = new int[size]; for(int x = 0; x < size; x ++) { cout << "Enter number " << x << ": "; cin >> array[x]; } for(int x = 0; x < size; x ++) { cout << array[x] << endl; } delete [] array; // Don't forget to free the memory [edit] Two minutes too late . . . [/edit] __________________ dwk Seek and ye shall find. quaere et invenies.* "Simplicity does not precede complexity, but follows it." -- Alan Perlis "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra "The only real mistake is the one from which we learn nothing." -- John Powell Other boards: DaniWeb, TPS Unofficial Wiki FAQ: cpwiki.sf.net My website: http://dwks.theprogrammingsite.com/ Projects: codeform, xuni, atlantis, nort, etc.
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08-24-2006, 03:12 PM	#4
Ancient Dragon Registered User Join Date: Aug 2005 Posts: 1,265	I think C99 standard allows this Code: cin >> n; int salaryarray[ n ]; or, since this is a c++ program, you might use a vector Code: cin >> n; vector<int> salaryarray(n);
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08-24-2006, 03:17 PM	#5
dwks Frequently Quite Prolix Join Date: Apr 2005 Location: Canada Posts: 7,698	Yes, the C99 standard allows variable-sized arrays. [edit] Code: cout << "Please enter the number of salespeople./n"; BTW, I think you meant "\n". [/edit] __________________ dwk Seek and ye shall find. quaere et invenies. "Simplicity does not precede complexity, but follows it." -- Alan Perlis "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra "The only real mistake is the one from which we learn nothing." -- John Powell Other boards: DaniWeb, TPS Unofficial Wiki FAQ: cpwiki.sf.net My website: http://dwks.theprogrammingsite.com/ Projects: codeform, xuni, atlantis, nort, etc. Last edited by dwks; 08-24-2006 at 03:39 PM.
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08-24-2006, 03:19 PM	#6
Daved Registered User Join Date: Jan 2005 Posts: 7,252	In C++ you should really use the standard vector class for these types of things, although if your professor teaches C style arrays you should learn them as well.
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08-24-2006, 04:23 PM	#7
Desolation Registered User Join Date: May 2006 Posts: 894	For such a small need I think using a vector would be like shooting a fly with a canon. __________________ www.atempsperdu.net/forum/ www.atempsperdu.net/site/en/
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08-24-2006, 04:26 PM	#8
dwks Frequently Quite Prolix Join Date: Apr 2005 Location: Canada Posts: 7,698	Not really. A vector doesn't store any pointers for each element (the memory is just a contiguous chunk, like you would get with a dynamically allocated array). And I can't imagine it would impact execution speed very much. __________________ dwk Seek and ye shall find. quaere et invenies. "Simplicity does not precede complexity, but follows it." -- Alan Perlis "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra "The only real mistake is the one from which we learn nothing." -- John Powell Other boards: DaniWeb, TPS Unofficial Wiki FAQ: cpwiki.sf.net My website: http://dwks.theprogrammingsite.com/ Projects: codeform, xuni, atlantis, nort, etc.
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08-24-2006, 04:29 PM	#9
Daved Registered User Join Date: Jan 2005 Posts: 7,252	>> For such a small need I think using a vector would be like shooting a fly with a canon. It's your opinion, but why are you even using C++ then?
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08-24-2006, 04:41 PM	#10
Mario F. (?<!re)tired Join Date: May 2006 Location: Portugal Posts: 5,656	>> For such a small need I think using a vector would be like shooting a fly with a canon. Or using an array would be like trimming your nails with a butchers knife. And I do echo Daved's question. __________________ Originally Posted by brewbuck: Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.
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08-24-2006, 05:12 PM	#12
dwks Frequently Quite Prolix Join Date: Apr 2005 Location: Canada Posts: 7,698	Because a vector is slower and uses more memory than an array. But its advantages far outweigh its disadvantages. (We're using C++, right?) __________________ dwk Seek and ye shall find. quaere et invenies. "Simplicity does not precede complexity, but follows it." -- Alan Perlis "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra "The only real mistake is the one from which we learn nothing." -- John Powell Other boards: DaniWeb, TPS Unofficial Wiki FAQ: cpwiki.sf.net My website: http://dwks.theprogrammingsite.com/ Projects: codeform, xuni, atlantis, nort, etc.
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08-24-2006, 05:16 PM	#14
Daved Registered User Join Date: Jan 2005 Posts: 7,252	>> Because a vector is slower and uses more memory than an array. That's not a sufficient reason (even if it were true), unless you would say using C++ instead of C, or C instead of assembly is like shooting a fly with a cannon.
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08-24-2006, 05:16 PM	#15
siavoshkc System Novice Join Date: Jan 2006 Location: Tehran Posts: 1,075	Vector is slower than new/delete? Why? __________________ Microsoft Visual Studio 2008 Professional (On Microsoft Windows XP SP2) Learn the language before using it. (C++ Books and C Books) Read the FAQ before making a problem. Then make a Google and Forum search. My code painter new version Version 0.97 DOWNLOAD NOW! (Let the pop up, pop!) SiavoshKC
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