Greetings all i am having a horrible time trying to pass a 2
dimensional array of doubles to a function... basically a watered down
version of my code looks like:
void t1(double a[][]);
int main() {
double d[5][5];
d[2][2] = 2.3;
t1(d);
}
void t1(double a[][]) {
for(int i=0;i<5;i++)
for (int j=0;j<5;j++)
cout << "a["<<i<<"]["<<j<<"]: "<<a[i][j]<<endl;
}
it keeps giving me all kinds of errors, how exactly do i get a filled 2
dimensional array of doubles to a function? thanks.
Cheers,
Adam. 7 1709 ff**@hotmail.co m wrote: Greetings all i am having a horrible time trying to pass a 2 dimensional array of doubles to a function... basically a watered down version of my code looks like:
void t1(double a[][]);
int main() {
double d[5][5]; d[2][2] = 2.3; t1(d); }
void t1(double a[][]) { for(int i=0;i<5;i++) for (int j=0;j<5;j++) cout << "a["<<i<<"]["<<j<<"]: "<<a[i][j]<<endl; }
it keeps giving me all kinds of errors, how exactly do i get a filled 2 dimensional array of doubles to a function? thanks.
You have two options:
const int DIM = 5;
1) void t1(double a[DIM][DIM])
2) void t1(double a[][DIM]) or the equal
void t1(double (*a)[DIM])
--
Regards,
Karsten
If you change your functions to look like
---- CODE ----
void t1(double **a, int y, int x);
int main() {
double d[5][5];
d[2][2] = 2.3;
t1(d, 5, 5);
}
void t1(double **a, int y, int x) {
for(int i=0;i<y;i++)
for (int j=0;j<x;j++)
cout << "a["<<i<<"]["<<j<<"]: "<<a[i][j]<<endl;
}
---- /CODE ----
it would work better.
Adam wrote: I am having a horrible time trying to pass a 2-dimensional array of doubles to a function. Basically, a watered down version of my code looks like:
Eventually, we expect C++ to adopt C99 style variable size arrays.
Until then, you can work around this deficiency in C++
by implementing t1 in C and linking it into your program:
cat t1.c
#include <stdio.h>
void t1(size_t m, size_t n, double a[m][n]) {
for(size_t i = 0; i < m; ++i) {
for (size_t j = 0; j < n; ++j) {
if (0 < j)
fprintf(stdout, "\t");
fprintf(stdout, "a[%u][%u]: %f", i, j, a[i][j]);
}
fprintf(stdout, "\n");
}
}
gcc -Wall -std=c99 -pedantic -c t1.c cat main.cc
#include <cstdlib>
extern "C" {
void t1(size_t m, size_t n, double a[]);
}
int main(int argc, char* argv[]) {
const
size_t m = 5;
const
size_t n = 3;
double d[m][n];
for(size_t i = 0; i < m; ++i)
for (size_t j = 0; j < n; ++j)
d[i][j] = 10.0*i + j;
t1(m, n, d[0]);
return 0;
}
g++ -Wall -ansi -pedantic -o main main.cc t1.o ./main
a[0][0]: 0.000000 a[0][1]: 1.000000 a[0][2]: 2.000000
a[1][0]: 10.000000 a[1][1]: 11.000000 a[1][2]: 12.000000
a[2][0]: 20.000000 a[2][1]: 21.000000 a[2][2]: 22.000000
a[3][0]: 30.000000 a[3][1]: 31.000000 a[3][2]: 32.000000
a[4][0]: 40.000000 a[4][1]: 41.000000 a[4][2]: 42.000000
* James Aguilar: If you change your functions to look like
---- CODE ----
void t1(double **a, int y, int x);
int main() {
double d[5][5]; d[2][2] = 2.3; t1(d, 5, 5); }
void t1(double **a, int y, int x) { for(int i=0;i<y;i++) for (int j=0;j<x;j++) cout << "a["<<i<<"]["<<j<<"]: "<<a[i][j]<<endl; }
The code above is incorrect: an array of arrays is not convertible
to an array of pointers (which would be convertible to a pointer
to an array of pointers, the required function argument).
Don't use raw pointers: for newbies & experienced alike that's an
invitation for disaster.
Use e.g. std::vector instead.
Off the cuff:
typedef std::vector<dou ble> DoubleVector;
typedef std::vector<Dou bleVec> DoubleMatrix;
void display( DoubleMatrix const& m )
{
for( std::size_t row = 0; row <= m.size(); ++row )
{
for( std::size_t col = 0; col < m[row].size(); ++col )
{
double const valueAtRowCol = m[row][col];
}
}
}
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
James Aguilar wrote: If you change your functions to look like
---- CODE ----
void t1(double **a, int y, int x);
int main() {
double d[5][5]; d[2][2] = 2.3; t1(d, 5, 5); }
void t1(double **a, int y, int x) { for(int i=0;i<y;i++) for (int j=0;j<x;j++) cout << "a["<<i<<"]["<<j<<"]: "<<a[i][j]<<endl; }
---- /CODE ----
it would work better.
Not really :-)
The reason is that the memory layouts of [][] and ** are different.
if you declare
double d[2][3];
then 'd' looks in memory like this:
d
+---+---+---+---+---+---+
| | | | | | |
+---+---+---+---+---+---+
| | |
+---- 3 ----+---- 3 ----+
| |
+---- 2 ----+
That's: a piece of memory which gets divided by the compiler into 2 subarrays
of size 3 each. The 2D structure exists only for the compiler and is used for
address calculation of a single element only.
while in t1 you did:
double ** a
which has a memory layout of (again with numbers 2 and 3)
a
+----+ +---+---+---+
| o------->| | | |
+----+ +---+---+---+ +---+---+---+
| o----------------------->| | | |
+----+ +---+---+---+
That is: an array of 2 pointers pointing to arrays of size 3.
As you can see, those structures are completely different.
*But*: The syntax to access a single element is identical in both
cases. Exercise for the reader: Why is this so? Why can
d[1][1] be written as well as a[1][1] and do both expressions
produce the same machine code?
--
Karl Heinz Buchegger kb******@gascad .at
Yeah, thank you for correcting me on that. I'm also a relatively young C++
programmer, and I almost never use arrays directly after one bad
experience -- I find vectors much nicer to debug and to write code for. I
just hope the OP didn't follow my advice =/.
- JFA1
thanks for all the help! alomst immediatly after posting the i realized
a solution (isnt that how it always works?) i just ended up using the
STL vector library similarly as someone noted above, but hopfully this
can shed light for everyone.
cheers & thanks,
Adam. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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