KalluMama
asked on
Totaling columns in a multi dim array using pointers
OK,
I am trying to study pointer and arrays and have been stuck on how to do this...can anyone help
I have an array:
int table[numOfRows][numOfCols
{1, 19, 18},
{17, 16, 15},
{14, 113, 12},
{11, 10, 9}
};
I wanna total up each column, but only using pointers. Can't seem to get it right, can anyone please help!! Thanks!
KM
I am trying to study pointer and arrays and have been stuck on how to do this...can anyone help
I have an array:
int table[numOfRows][numOfCols
{1, 19, 18},
{17, 16, 15},
{14, 113, 12},
{11, 10, 9}
};
I wanna total up each column, but only using pointers. Can't seem to get it right, can anyone please help!! Thanks!
KM
In the following, p is a pointer to an array of numOfCols of ints...
--------8<--------
#include <iostream>
const numOfCols = 3;
const numOfRows = 4;
int table[numOfRows][numOfCols
{1, 19, 18},
{17, 16, 15},
{14, 113, 12},
{11, 10, 9}
};
int main()
{
int (*p)[numOfCols] = table;
for (int row = 0;row < numOfRows;row++) {
std::cout << "First column is: " << (*p)[0] << '\n';
p++;
}
}
--------8<--------
--------8<--------
#include <iostream>
const numOfCols = 3;
const numOfRows = 4;
int table[numOfRows][numOfCols
{1, 19, 18},
{17, 16, 15},
{14, 113, 12},
{11, 10, 9}
};
int main()
{
int (*p)[numOfCols] = table;
for (int row = 0;row < numOfRows;row++) {
std::cout << "First column is: " << (*p)[0] << '\n';
p++;
}
}
--------8<--------
ASKER
...can this be done as a function call??
That should have been...
const int numOfCols = 3;
const int numOfRows = 4;
(VC 7.1 doesn't grumble when you omit the int)
const int numOfCols = 3;
const int numOfRows = 4;
(VC 7.1 doesn't grumble when you omit the int)
If you want it as a function call...
--------8<--------
#include <iostream>
const int numOfCols = 3;
const int numOfRows = 4;
int table[numOfRows][numOfCols
{1, 19, 18},
{17, 16, 15},
{14, 113, 12},
{11, 10, 9}
};
void f(int (*)[numOfCols]); // Prototype
int main()
{
f(table);
}
// Implementation
void f(int (*p)[numOfCols])
{
for (int row = 0;row < numOfRows;row++) {
std::cout << "First column is: " << (*p)[0] << '\n';
p++;
}
}
--------8<--------
--------8<--------
#include <iostream>
const int numOfCols = 3;
const int numOfRows = 4;
int table[numOfRows][numOfCols
{1, 19, 18},
{17, 16, 15},
{14, 113, 12},
{11, 10, 9}
};
void f(int (*)[numOfCols]); // Prototype
int main()
{
f(table);
}
// Implementation
void f(int (*p)[numOfCols])
{
for (int row = 0;row < numOfRows;row++) {
std::cout << "First column is: " << (*p)[0] << '\n';
p++;
}
}
--------8<--------
Clearly, it would be a smart move to pass the function the numOfRows, so it doesn't need to be global.
i.e.
--------8<--------
#include <iostream>
const int numOfCols = 3;
const int numOfRows = 4;
int table[][numOfCols] = {
{1, 19, 18},
{17, 16, 15},
{14, 113, 12},
{11, 10, 9}
};
void f(int (*)[numOfCols],int); // Prototype
int main()
{
f(table,sizeof(table)/size
}
// Implementation
void f(int (*p)[numOfCols],int numOfRows)
{
for (int row = 0;row < numOfRows;row++) {
std::cout << "First column is: " << (*p)[0] << '\n';
p++;
}
}
--------8<--------
--------8<--------
#include <iostream>
const int numOfCols = 3;
const int numOfRows = 4;
int table[][numOfCols] = {
{1, 19, 18},
{17, 16, 15},
{14, 113, 12},
{11, 10, 9}
};
void f(int (*)[numOfCols],int); // Prototype
int main()
{
f(table,sizeof(table)/size
}
// Implementation
void f(int (*p)[numOfCols],int numOfRows)
{
for (int row = 0;row < numOfRows;row++) {
std::cout << "First column is: " << (*p)[0] << '\n';
p++;
}
}
--------8<--------
As a function call? Sure, you could
void SumCols(int* pTable, int numOfCols, int numOfRows, int* sum) {
memset(sum, 0, numOfCols*sizeof(int));
for (int i = 0; i < numOfRows * numOfCols; ++i, ++pn) {
sum[i % numOfCols] + = *pn;
}
}
//...
int sum[numOfCols];
int* pn = (int*)table;
SumCols( pn, numOfCols,numOfRows, sum);
void SumCols(int* pTable, int numOfCols, int numOfRows, int* sum) {
memset(sum, 0, numOfCols*sizeof(int));
for (int i = 0; i < numOfRows * numOfCols; ++i, ++pn) {
sum[i % numOfCols] + = *pn;
}
}
//...
int sum[numOfCols];
int* pn = (int*)table;
SumCols( pn, numOfCols,numOfRows, sum);
ASKER
oops i forgot to mention...what i was trying to do was set a pointer to the first element and one to the last...and then find the sum without using number of rows or columns...is it possible?
Yes..
// Implementation
void f(int (*itr)[numOfCols],int (*end)[numOfCols])
{
while (itr <= end) {
std::cout << "First column is: " << (*itr)[0] << '\n';
itr++;
}
}
Call the function with (say)
f(&table[0],&table[2]);
// Implementation
void f(int (*itr)[numOfCols],int (*end)[numOfCols])
{
while (itr <= end) {
std::cout << "First column is: " << (*itr)[0] << '\n';
itr++;
}
}
Call the function with (say)
f(&table[0],&table[2]);
ASKER
rstaveley,
the code you gave only list the first element in the first 3 rows of the array
what i wanted to get was:
Sum of row 1: XXX
Sum of row 2: XXX
Sum of row 3: XXX
Sum of row 4: XXX
the code you gave only list the first element in the first 3 rows of the array
what i wanted to get was:
Sum of row 1: XXX
Sum of row 2: XXX
Sum of row 3: XXX
Sum of row 4: XXX
>>what i was trying to do was set a pointer to the first element and one to the last.
That would work also:
void SumCols(int* pStart, int* pStart,int numOfCols, int* sum) {
int i = 0;
int* pn = pStart;
memset(sum, 0, numOfCols*sizeof(int));
for (; pn != pEnd; ++i, ++pn) {
sum[i % numOfCols] + = *pn;
}
}
That would work also:
void SumCols(int* pStart, int* pStart,int numOfCols, int* sum) {
int i = 0;
int* pn = pStart;
memset(sum, 0, numOfCols*sizeof(int));
for (; pn != pEnd; ++i, ++pn) {
sum[i % numOfCols] + = *pn;
}
}
Ooops, that should be
void SumCols(int* pStart, int* pEnd,int numOfCols, int* sum) {
int i = 0;
int* pn = pStart;
memset(sum, 0, numOfCols*sizeof(int));
for (; pn != pEnd; ++i, ++pn) {
sum[i % numOfCols] + = *pn;
}
}
//...
int sum[numOfCols];
int* p1 = &table[0][0];
int* p1 = &table[numOfRows][numOfCol
SumCols( p1, p2, numOfCols, sum);
void SumCols(int* pStart, int* pEnd,int numOfCols, int* sum) {
int i = 0;
int* pn = pStart;
memset(sum, 0, numOfCols*sizeof(int));
for (; pn != pEnd; ++i, ++pn) {
sum[i % numOfCols] + = *pn;
}
}
//...
int sum[numOfCols];
int* p1 = &table[0][0];
int* p1 = &table[numOfRows][numOfCol
SumCols( p1, p2, numOfCols, sum);
ASKER
where is pEnd defined here?
ASKER
what is memset??
ASKER
...but jkr, how do i display the totals as i had listed it above...sorry i'm stil very new to C++...thanks
SOLUTION
membership
This solution is only available to members.
To access this solution, you must be a member of Experts Exchange.
ASKER CERTIFIED SOLUTION
membership
This solution is only available to members.
To access this solution, you must be a member of Experts Exchange.
int sum[numOfCols];
int* pn = (int*)table;
memset(sum, 0, numOfCols*sizeof(int));
for (int i = 0; i < numOfRows * numOfCols; ++i, ++pn) {
sum[i % numOfCols] + = *pn;
}