I need to set the last element inside an array by multiply the last "i" with it self like this. but when i try to do i*i, i is undefined. also, when i try to print the result, cout is undefined.
void firstArray(void)
{
int MyArray[10] ;
for (unsigned int i=0; i<10; ++i)
{
MyArray[i] = i;
}
MyArray [9] = i*i;
for (unsigned int i=0; i<10; ++i)
{
cout(MyArray[i]);
}
}
I tried to put MyArray [9] = i*i inside the loop; with a condition( and it whould work), but i cant use any if for this assignment.
also, I tried to put with System.out like in java before cout, but System is undefined.
what do i need to change to make it work?
Ok, so first of all, if you want "i" outside of your loop, you need to initialize "i" outside of your loop too.
... Unsigned int i; for(i =0; i<10; i++) ...
Now, "i" will be equal the last increment outside your loop.
Also, i suggest you to read the basics for c++, System dont exist in c++, instead its with std namespace.
There is two way to do that:
Using namespace std;
Inside the function, or by write "std::" before cout and:
Std::cout << MyArray[i]
Pay attention how i wrote it, you will find
How to do in the c++ website:
https://www.cplusplus.com/reference/iostream/cout/
Related
When i try to run this code, it compiles without any error, but i wanted that it should display the string and int array that it takes as input. Instead, after giving one input, I am presented with a list of zeroes and program terminates.
however,when i removed the line containing k[j] from both for loops, it worked.
What am I doing wrong? Pardon if asked something stupid as I'm a novice, please help...
#include <iostream>
#include <string>
using namespace std;
int main ()
{
int t,count = 0;
string state[t];
int k[t];
cin>>t;
for (int j=0; j<t; j++)
{
getline(cin, state[j]);
cin>>k[j];
}
for (int j=0; j<t; j++)
{ cout<< state [j]<<'\t'<<k[j];
cout<<endl;
}
return 0
}
You declare both arrays k[] and state[] with t wich is undefined at this moment.
t should be initiated with a valid value beforehand!
I'd like to use openMP to apply multi-thread.
Here is simple code that I wrote.
vector<Vector3f> a;
int i, j;
for (i = 0; i<10; i++)
{
Vector3f b;
#pragma omp parallel for private(j)
for (j = 0; j < 3; j++)
{
b[j] = j;
}
a.push_back(b);
}
for (i = 0; i < 10; i++)
{
cout << a[i] << endl;
}
I want to change it to works lik :
parallel for1
{
for2
}
or
for1
{
parallel for2
}
Code works when #pragma line is deleted. but it does not work when I use it. What's the problem?
///////// Added
Actually I use OpenMP to more complicated example,
double for loop question.
here, also When I do not apply MP, it works well.
But When I apply it,
the error occurs at vector push_back line.
vector<Class> B;
for 1
{
#pragma omp parallel for private(j)
parallel for j
{
Class A;
B.push_back(A); // error!!!!!!!
}
}
If I erase B.push_back(A) line, it works as well when I applying MP.
I could not find exact error message, but it looks like exception error about vector I guess. Debug stops at
void _Reallocate(size_type _Count)
{ // move to array of exactly _Count elements
pointer _Ptr = this->_Getal().allocate(_Count);
_TRY_BEGIN
_Umove(this->_Myfirst, this->_Mylast, _Ptr);
std::vector::push_back is not thread safe, you cannot call that without any protection against race conditions from multiple threads.
Instead, prepare the vector such that it's size is already correct and then insert the elements via operator[].
Alternatively you can protect the insertion with a critical region:
#pragma omp critical
B.push_back(A);
This way only one thread at a time will do the insertion which will fix the error but slow down the code.
In general I think you don't approach parallelization the right way, but there is no way to give better advise without a clearer and more representative problem description.
my name is Adam, I have just begun to learn C++, I love it, but am only on pg 181 in the seventh edition of sams teach yourself C++ in one hour a day, and pg 102 in the seventh edition of C++ for dummies. I have seven multi page notes on the sams book, and twenty one multi page notes on the for dummies book. Please help me understand why I get 5 errors with my simple program which will be shown shortly. I do not want to use -fpermissive option, I need to learn how to code correctly as I am not very experienced. Thank you everyone, very very much, I absolutely love C++, and even have a very good idea on a simple program I plan to learn how to write, which could allow program development time, or writing time to be reduced by 5-20 times on average. The following program shown is not this program however, but please help me so I may one day write, and use my program idea for a college paper. Thank you again, problem program follows:
#include <iostream>
using namespace std;
int main()
{
cout<< "how many integers do you wish to enter? ";
int InputNums = 0;
cin>> InputNums;
int* pNumbers = new int [InputNums];
int* pCopy = pNumbers;
cout<< "successfully allocated memory for "<<
InputNums<< " integers"<<endl;
for(int Index = 0; Index < InputNums; ++Index)
{
cout<< "enter number "<< Index << ": ";
cin>> *(pNumbers + Index);
}
cout<< "displaying all numbers input: " <<endl;
for(int Index = 0, int* pCopy = pNumbers;
Index < InputNums; ++Index)
cout<< *(pCopy++) << " ";
cout<< endl;
delete[] pNumbers;
cout<< "press enter to continue..." << endl;
cin.ignore(10, '\n');
cin.get();
return 0;
}
The problem is indicated as being in the multiple initializations of the second for loop. Please tell me why my problem program will not compile. Thank you all. Sincerely Adam.
My first advice would be to find a better book.
Once you've done that, forget everything you think you know about using new to allocate an array (e.g., int* pNumbers = new int [InputNums];). It's an obsolete construct that you shouldn't use (ever).
If I had to write a program doing what you've outlined above, the core of it would look something like this:
cout<< "how many integers do you wish to enter? ";
int InputNums;
cin>> InputNums;
std::vector<int> numbers;
int temp;
for (int i=0; i<InputNums; i++) {
cin >> temp;
numbers.push_back(temp);
}
cout<< "displaying all numbers input:\n";
for (auto i : numbers)
cout << i << " ";
Directly answer your question: you cannot initialize variables of different types in the same for loop declaration.
In your example:
for(int Index = 0, int* pCopy = pNumbers;
int and int * are different types. Even if you use auto to let the compiler automatically deduct the types, both variables cannot have different deducted types.
The solution:
int Index = 0;
for(int *pCopy=pNumbers; ...
Having this one single secondary effect: Index is now not only confined to the scope of the for. Should this be a problem, you may do:
{
int Index = 0;
for(int *pCopy=pNumbers; ...
...
}
And now the scope of Index is limited to the surrounding curly braces.
I would like to know if these 2 codes are the same for performance respecting the variable declarations:
int Value;
for(int i=0; i<1000; i++)
{ Value = i;
}
or
for(int i=0; i<1000; i++)
{ int Value = i;
}
Basically I need to know if the process time to create the variable Value and allocate it in Ram is just once in the first case and if it is, or not, repeated 1000 times in the second.
If you are Programming in c++ or c#, there will be no runtime difference since no implicite initialization will be done for simple int type.
Here is my question about openacc.
I read the APIs (v1 and v2), and the behavior of nested data environment with different subparts of the same array is unclear to me.
Code example:
#pragma acc data pcopyin(a[0:20])
{
#pragma acc data pcopyin(a[100:20])
{
#pragma acc parallel loop
for(i=0; i<20; i++)
a[i] = i;
a[i+100] = i;
}
}
My understanding is that this should work (or at leaste the two acc data parts):
The first pragma checks if a[0,20] is on the accelerator
NO -> data are allocated on the device and transferred
The second pragma checks if a[100,120] is on the accelerator
The pointer a is on the accelerator, but not the data from a[100,120]
The data are allocated on the device and transferred
I tried this kind of thing with CAPS compiler (v3.3.0 which is the only available right now on my test machine), and the second pragma acc data returns me an error (my second subarray don't have the correct shape).
So what happens with my test (I suppose) is that the pointer "a" was found on the accelerator, but the shape associated with it ([0:20]) is not the same in my second pragma ([100:20]).
Is this the normal behavior planned in the API, or should my example work?
Moreover, if this is supposed to work, is there some sort of coherence between the subparts of the same array (somehow, they will be positionned like on the host and I will be able to put a[i] += a[100+i] in my kernel)?
The present test will be looking if "a" is on the device. Hence, when the second data region is encountered, "a" is already on the device but only partially. Instead, a better method would be to add a pointer to point into "a" and reference this pointer on the device. Something like:
#include <stdio.h>
int main () {
int a[200];
int *b;
int i;
for(i=0; i<200; i++) a[i] = 0;
b=a+100;
#pragma acc data pcopy(a[0:20])
{
#pragma acc data pcopy(b[0:20])
{
#pragma acc parallel loop
for(i=0; i<20; i++) {
a[i] = i;
b[i] = i;
}
}
}
for(i=0; i<22; i++) printf("%d = %d \n", i, a[i]);
for(i=100; i<122; i++) printf("%d = %d \n", i, a[i]);
return 0;
}
If you had just copied "a[100:20]", then accessing outside this range would be considered a programmer error.
Hope this helps,
Mat