Does msvc have analog of gcc's ({ }).
I assume the answer is no.
Plase note that this is question of compiler capabilities, not question of taste or style.
Not that I recommend anybody to start using the ({}) construct by ths question.
The reference to ({}) construct is: http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC62 officially called "Statements and Declarations in Expressions". It allows to embed statements (like for, goto) and declarations into expressions.
In some way, yes. This is a compound statement expression, which one could consider like a lambda function that is immediately called, and only called once.
Recent versions of MSVC should support lambda functions, so that would be something like:
[](){ /* your compound statement expression here */ }();
EDIT: removed a surplus parenthesis
EDIT 2: For your amusement, here is an example of how to use either variation with some (admittedly totally silly) real code. Don't mind too much the actual usefulness of the code, but how expressive it is and how nicely the compiler even optimizes it:
#include <string.h>
#include <stdio.h>
int main()
{
unsigned int a =
({
unsigned int count = 0;
const char* str = "a silly thing";
for(unsigned int i = 0; i < strlen(str); ++i)
count += str[i] == 'i' ? 1 : 0;
count;
});
unsigned int b =
[](){
unsigned int count = 0;
const char* str = "a silly thing";
for(unsigned int i = 0; i < strlen(str); ++i)
count += str[i] == 'i' ? 1 : 0;
return count;
}();
printf("Number of 'i' : %u\t%u\n", a, b);
return 0;
}
... which gcc 4.5 compiles to:
movl $2, 8(%esp)
movl $2, 4(%esp)
movl $LC0, (%esp)
call _printf
No, it does not contain an equivalent form.
Related
I'm learning to programming using pthread for a adder program, after reference several codes still don't get how to pass multiple arguments into a thread using a struct, here is my buggy program:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <pthread.h>
typedef struct s_addition {
int num1;
int num2;
int sum;
} addition;
void *thread_add_function (void *ad)
{
printf ("ad.num1:%d, ad.num2:%d\n",ad.num1, ad.num2);
ad.sum = ad.num1 + ad.num2;
pthread_exit(0);
}
int main()
{
int N = 5;
int a[N], b[N], c[N];
srand (time(NULL));
// fill them with random numbers
for ( int j = 0; j < N; j++ ) {
a[j] = rand() % 392;
b[j] = rand() % 321;
}
addition ad1;
pthread_t thread[N];
for (int i = 0; i < N; i++) {
ad1.num1 = a[i];
ad1.num2 = b[i];
printf ("ad1.num1:%d, ad1.num2:%d\n",ad1.num1, ad1.num2);
pthread_create (&thread[i], NULL, thread_add_function, &ad1);
pthread_join(thread[i], NULL);
c[i] = ad.sum;
}
printf( "This is the result of using pthread.\n");
for ( int i = 0; i < N; i++) {
printf( "%d + %d = %d\n", a[i], b[i], c[i]);
}
}
But when compiling I got the following error:
vecadd_parallel.c:15:39: error: member reference base type 'void *' is not a
structure or union
printf ("ad.num1:%d, ad.num2:%d\n",ad.num1, ad.num2);
I tried but still cannot get a clue, what I am doing wrong with it?
Seems like you have a problem with trying to access the members of a void datatype.
You will need to add a line to cast your parameter to thread_add_function to the correct datatype similar to addition* add = (addition*)ad;, and then use this variable in your function (note that you also have to change you r .'s to -> because it's a pointer)
You also should only pass data to threads that was malloc()'d, as stack allocated data may not be permanent. It should be fine for the current implementation, but changes later could easily give strange, unpredictable behaviour.
This question already has answers here:
How can I convert an int to a string in C?
(10 answers)
Closed 8 years ago.
I want to convert an int to a string without printing anything on my screen. For now I used sprintf, but this also printed the int to my screen.
Also itoa is not supported by my compiler so I can't use that either.
I assume You use ANSI C.
You cannot use itoa because it's not a standard function.
sprintf or snprintf is dedicated to that.
Since You do not want to use sprintf, make your own itoa instead:
#include <stdio.h>
char* itoa(int i, char b[]){
char const digit[] = "0123456789";
char* p = b;
if(i<0){
*p++ = '-';
i *= -1;
}
int shifter = i;
do{ //Move to where representation ends
++p;
shifter = shifter/10;
}while(shifter);
*p = '\0';
do{ //Move back, inserting digits as u go
*--p = digit[i%10];
i = i/10;
}while(i);
return b;
}
original answer: here
i am trying to use dgels function of lapacke:
when i use it with malloc fucntion. it doesnot give correct value.
can anybody tell me please what is the mistake when i use malloc and create a matrix?
thankyou
/* Calling DGELS using row-major order */
#include <stdio.h>
#include <lapacke.h>
#include <conio.h>
#include <malloc.h>
int main ()
{
double a[3][2] = {{1,0},{1,1},{1,2}};
double **outputArray;
int designs=3;
int i,j,d,i_mal;
lapack_int info,m,n,lda,ldb,nrhs;
double outputArray[3][1] = {{6},{0},{0}};*/
outputArray = (double**) malloc(3* sizeof(double*));
for(i_mal=0;i_mal<3;i_mal++)
{
outputArray[i_mal] = (double*) malloc(1* sizeof(double));
}
for (i=0;i<designs;i++)
{
printf("put first value");
scanf("%lf",&outputArray[i][0]);
}
m = 3;
n = 2;
nrhs = 1;
lda = 2;
ldb = 1;
info = LAPACKE_dgels(LAPACK_ROW_MAJOR,'N',m,n,nrhs,*a,lda,*outputArray,ldb);
for(i=0;i<m;i++)
{
for(j=0;j<nrhs;j++)
{
printf("%lf ",outputArray[i][j]);
}
printf("\n");
}
getch();
return (info);
}
The problem may come from outputArray not being contiguous in memory. You may use something like this instead :
outputArray = (double**) malloc(3* sizeof(double*));
outputArray[0]=(double*) malloc(3* sizeof(double));
for (i=0;i<designs;i++){
outputArray[i]=&outputArray[0][i];
}
Don't forget to free the memory !
free(outputArray[0]);
free(outputArray);
Edit : Contiguous means that you have to allocate the memory for all values at once. See http://www.fftw.org/doc/Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way : some packages, like fftw or lapack require this feature for optimization. As you were calling malloc three times, you created three parts and things went wrong.
If you have a single right hand side, there is no need for a 2D array (double**). outputArray[i] is a double*, that is, the start of the i-th row ( row major). The right line may be outputArray[i]=&outputArray[0][i*nrhs]; if you have many RHS.
By doing this in your code, you are building a 3 rows, one column, that is one RHS. The solution, is of size n=2. It should be outputArray[0][0] , outputArray[1][0]. I hope i am not too wrong, check this on simple cases !
Bye,
I am experimenting a bit with std::thread and C++11, and I am encountering strange behaviour.
Please have a look at the following code:
#include <cstdlib>
#include <thread>
#include <vector>
#include <iostream>
void thread_sum_up(const size_t n, size_t& count) {
size_t i;
for (i = 0; i < n; ++i);
count = i;
}
class A {
public:
A(const size_t x) : x_(x) {}
size_t sum_up(const size_t num_threads) const {
size_t i;
std::vector<std::thread> threads;
std::vector<size_t> data_vector;
for (i = 0; i < num_threads; ++i) {
data_vector.push_back(0);
threads.push_back(std::thread(thread_sum_up, x_, std::ref(data_vector[i])));
}
std::cout << "Threads started ...\n";
for (i = 0; i < num_threads; ++i)
threads[i].join();
size_t sum = 0;
for (i = 0; i < num_threads; ++i)
sum += data_vector[i];
return sum;
}
private:
const size_t x_;
};
int main(int argc, char* argv[]) {
const size_t x = atoi(argv[1]);
const size_t num_threads = atoi(argv[2]);
A a(x);
std::cout << a.sum_up(num_threads) << std::endl;
return 0;
}
The main idea here is that I want to specify a number of threads which do independent computations (in this case, simple increments).
After all threads are finished, the results should be merged in order to obtain an overall result.
Just to clarify: This is only for testing purposes, in order to get me understand how
C++11 threads work.
However, when compiling this code using the command
g++ -o threads threads.cpp -pthread -O0 -std=c++0x
on a Ubuntu box, I get very strange behaviour, when I execute the resulting binary.
For example:
$ ./threads 1000 4
Threads started ...
Segmentation fault (core dumped)
(should yield the output: 4000)
$ ./threads 100000 4
Threads started ...
200000
(should yield the output: 400000)
Does anybody has an idea what is going on here?
Thank you in advance!
Your code has many problems (see even thread_sum_up for about 2-3 bugs) but the main bug I found by glancing your code is here:
data_vector.push_back(0);
threads.push_back(std::thread(thread_sum_up, x_, std::ref(data_vector[i])));
See, when you push_back into a vector (I'm talking about data_vector), it can move all previous data around in memory. But then you take the address of (reference to) a cell for your thread, and then push back again (making the previous reference invalid)
This will cause you to crash.
For an easy fix - add data_vector.reserve(num_threads); just after creating it.
Edit at your request - some bugs in thread_sum_up
void thread_sum_up(const size_t n, size_t& count) {
size_t i;
for (i = 0; i < n; ++i); // see that last ';' there? means this loop is empty. it shouldn't be there
count = i; // You're just setting count to be i. why do that in a loop? Did you mean +=?
}
The cause of your crash might be that std::ref(data_vector[i]) being invalidated by the next push_back in data_vector. Since you know the number of threads, do a data_vector.reserve(num_threads) before you start spawning off the threads to keep the references from being invalidated.
As you resize the vector with the calls to push_back, it is likely to have to reallocate the storage space, causing the references to the contained values to be invalidated. This causes the thread to write to non-allocated memory, which is undefined behavior.
Your options are to pre-allocate the size you need (vector::reserve is one option), or choose a different container.
Evening everyone hope on of you gurus can help. I am trying to find the answer to this issue I need to read the data out of the string below by searching the tags. i.e IZTAG UKPART etc however the code I am using is no good as it only stores the 1st part of it for example UKPART = 12999 and misses out the -0112. Is there a better way to search strings ?
UPDATE SO FAR.
#include <stdio.h>
#include <string.h>
#include <windows.h>
int main ()
{
// in my application this comes from the handle and readfile
char buffer[255]="TEST999.UKPART=12999-0112...ISUE-125" ;
//
int i;
int codes[256];
char *pos = buffer;
size_t current = 0;
//
char buffer2[255];
if ((pos=strstr(pos, "UKPART")) != NULL) {
strcpy (buffer2, pos); // buffer2 <= "UKPART=12999-0112...ISUE-125"
}
printf("%s\n", buffer2);
system("pause");
return 0;
}
NOW WORKS BUT RETURN WHOLE STRING AS OUTPUT I NEED TO JUST RETURN UKPART FOR EXAMPLE THANKS SO FAR :-)
strstr() is absolutely the right way to search for the substring. Cool :)
It sounds like you want something different from "sscanf()" to copy the substring.
Q: Why not just use "strcpy ()" instead?
EXAMPLE:
char buffer[255]="IZTAG-12345...UKPART=12999-0112...ISUE-125" ;
char buffer2[255];
if ((pos=strstr(pos, "UKPART")) != NULL) {
strcpy (buffer2, pos); // buffer2 <= "UKPART=12999-0112...ISUE-125"