extracting columns from arrays

[kamasin]

Hi there, new to C here. I can't find information anywhere on how I could extract say a 2nd column from a 2D array and save it as a 1D array. Many thanks if you could help.

[tabstop]

It's not built-in, like you would get from an array language. You'll have to do it yourself. I would suggest a for loop.

[kamasin]

if I have a 2D array x[m][n], then why doesn't this work:

for (m=0; m <= NR-1; m++);
x_values[m] = x[m][0]; (storing column 1 in a separate array)

it compiles just fine, but printed elements of an array don't make sense.

[quzah]

if I have a 2D array x[m][n], then why doesn't this work:

for (m=0; m <= NR-1; m++);
x_values[m] = x[m][0]; (storing column 1 in a separate array)

it compiles just fine, but printed elements of an array don't make sense.

Also, why do you have <= SIZE-1 when < SIZE would work?


Quzah.

[kamasin]

all array elements start with 0. So if you declare # of rows as 50, you would need to do the iteration only 49 times.

[tabstop]

all array elements start with 0. So if you declare # of rows as 50, you would need to do the iteration only 49 times.

This is of course false. 0 through 49 is still 50 loops.

[kamasin]

my bad, if you declare # of rows as 50, and begin with row = 0, you would need to do the for loop until row = 49, right?

[quzah]

my bad, if you declare # of rows as 50, and begin with row = 0, you would need to do the for loop until row = 49, right?

No. You would loop until it reached 50, at which point the test would fail, so it wouldn't execute the body of the loop for x == 50.

for( x = 0; x < 50; x++ )
    printf( "x is %d\n", x ) /*Would print 0 ... 49*/
printf( "x is %d\n", x ); /*Would print x is 50*/

Quzah.

[tabstop]

You would do [49] but not [50], yes.

[kamasin]

what about the original question

[CommonTater]

Hi there, new to C here. I can't find information anywhere on how I could extract say a 2nd column from a 2D array and save it as a 1D array. Many thanks if you could help.

Set the colum number and Use a for() loop... Like they told you.

[tabstop]

what about the original question

Did you fix your for loop yet?

[kamasin]

for (m=0; m <= 999; m++);
n=2;
B[m] = A[m][n];
printf("new array %f \n", B[m]);
can't figure out why this doesn't work

[tabstop]

If you ever see a semi-colon at the end of a line with a for-statement on it, then you know you have screwed up.

[quzah]

If you ever see a semi-colon at the end of a line with a for-statement on it, then you know you have screwed up.

I guess making it big and bright red wasn't enough to clue him in.


Quzah.