C++ Programming :: Need help in my array


	Need help in my array This is part of my program but for some reason it is not taken the value of c in the array P[n]. Can some one help me please. int main() { int x, c, p; cout<<"Enter the value for x: "; cin>>x; cout<<"How many coefficients? "; cin>>c; const int n = c-1; int P[n]; // here are the errors: expected constant expression, //cannot allocate an array of constant size 0, 'P': unknown size. } Thanks <sdl@gmail.com> wrote in message news:1181092218.606289.19590@q75g2000hsh.googlegroups.com... - Hide quoted text - > This is part of my program but for some reason it is not taken the > value of c in the array P[n]. Can some one help me please. > int main() > { > int x, c, p; > cout<<"Enter the value for x: "; > cin>>x; > cout<<"How many coefficients? "; > cin>>c; > const int n = c-1; > int P[n]; // here are the errors: expected constant > expression, > //cannot allocate an > array of constant size 0, 'P': unknown size. > } You cannot allocate a static array with a dynamic size. The compiler doesn't know what n is until after you run the program... so it cannot allocate the space before hand... it doesn't know thatyou want a dynamic array. you need something more special: int P[] = new int[c-1]; Then it should work. Better yet you might want to look at STL vector or list... they are similar but implemented in classes and have some other functionality you can do that by #include <vector> vector<int> P; and you don't have to worry about the size as it will deal with it automatically. to add elements though you need to use something like P.push_back(342); Look up details on it for more info. sdl@gmail.com wrote in news:1181092218.606289.19590@q75g2000hsh.googlegroups.com: - Hide quoted text - > This is part of my program but for some reason it is not taken the > value of c in the array P[n]. Can some one help me please. > int main() > { > int x, c, p; > cout<<"Enter the value for x: "; > cin>>x; > cout<<"How many coefficients? "; > cin>>c; > const int n = c-1; > int P[n]; // here are the errors: expected > constant > expression, Yep. Arrays must have a compile-time constant size. Yours does not. c isn't known at compile time, thus c-1 isn't known, thus n isn't known. You may wish to consider using std::vector<int>: std::vector<int> P(n); - Hide quoted text - > //cannot allocate an > array of constant size 0, 'P': unknown size. > } > Thanks yes, Array is nothing but a pointer.. that means: int p[4]; can be represented as (p+i) so another way of declaring it is int p=new int[n]; //and n doesnot need to be a constant.. thanks, Sumedh "sumedh....." <sumedhsak@gmail.com> wrote in news:1181110045.600222.47530@i13g2000prf.googlegroups.com: > yes, > Array is nothing but a pointer.. that means: int p[4]; can be > represented as (p+i) > so another way of declaring it is > int p=new int[n]; //and n doesnot need to be a constant.. Not the same thing. The two allocations aren't necessarily from the same location. In some implementations "int p[4]" only requires a stack pointer to be moved. new int[4] requires a visit to a dynamic memory allocator (generally slower). On 2007-06-06 09:11, Andre Kostur wrote: > "sumedh....." <sumedhsak@gmail.com> wrote in > news:1181110045.600222.47530@i13g2000prf.googlegroups.com: >> yes, >> Array is nothing but a pointer.. that means: int p[4]; can be >> represented as (p+i) >> so another way of declaring it is >> int p=new int[n]; //and n doesnot need to be a constant.. > Not the same thing. The two allocations aren't necessarily from the same > location. In some implementations "int p[4]" only requires a stack pointer > to be moved. new int[4] requires a visit to a dynamic memory allocator > (generally slower). And it requires a manual deallocation, which makes it a bit more error prone. -- Erik Wikstrm On Jun 6, 8:07 am, "sumedh....." <sumedhsak@gmail.com> wrote: > Array is nothing but a pointer.. Nonsense. Arrays and pointers are two completely different things. > that means: int p[4]; can be > represented as (p+i) I think you mean that the expression p[i] is the same as (p+i). This is a misfeature of C, and doesn't hold if you use C++ style arrays (i.e. std::vector). And it only holds in expressions; it has nothing to do with the type of any objects. > so another way of declaring it is > int p=new int[n]; //and n doesnot need to be a constant.. There is never any reason to use array new in C++. The correct way to declare an array is: std::vector< int > array( n ) ; About the only exception is when you need static initialization, to avoid order of initialization issues. -- James Kanze (GABI Software) email:james.ka@gmail.com Conseils en informatique oriente objet/ Beratung in objektorientierter Datenverarbeitung 9 place Smard, 78210 St.-Cyr-l'cole, France, +33 (0)1 30 23 00 34 On Jun 6, 5:07 am, James Kanze <james.ka@gmail.com> wrote: - Hide quoted text - > On Jun 6, 8:07 am, "sumedh....." <sumedhsak@gmail.com> wrote: > > Array is nothing but a pointer.. > Nonsense. Arrays and pointers are two completely different > things. > > that means: int p[4]; can be > > represented as (p+i) > I think you mean that the expression p[i] is the same as (p+i). > This is a misfeature of C, and doesn't hold if you use C++ style > arrays (i.e. std::vector). And it only holds in expressions; it > has nothing to do with the type of any objects. > > so another way of declaring it is > > int p=new int[n]; //and n doesnot need to be a constant.. > There is never any reason to use array new in C++. The correct > way to declare an array is: > std::vector< int > array( n ) ; > About the only exception is when you need static initialization, > to avoid order of initialization issues. > -- > James Kanze (GABI Software) email:james.ka@gmail.com > Conseils en informatique oriente objet/ > Beratung in objektorientierter Datenverarbeitung > 9 place Smard, 78210 St.-Cyr-l'cole, France, +33 (0)1 30 23 00 34 Alright thanks ^_^