Reading, Processing & Outputting numbers from files in C

Question

Before I go on a mission to write some code, I was hoping someone could confirm the following is possible (or at least fairly simple) to do.

I am wanting to read a file containing numbers that are in 9 columns (separated by a single space) and many rows. The second column is ordered numerically, from largest to smaller. I want to discard all rows that contain below a specified number in this second column and then generate a new file and output just the top rows that conform to this rule.

Additionally, I would like to add a 10th column that is the result of a calculation between 2 other columns.

Is this done using arrays?

Many thanks in advance.

Anything is possible in C. You should have a go at implementing it, and post back here if you come across any specific problems. As it stands, your question is too vague for anyone to be able to provide much meaningful help. — Graham Borland, Aug 28 '12 at 15:55
@GrahamBorland - this is not true, get my boyfriend to work properly then I agree. — Ed Heal, Aug 28 '12 at 16:26

William Pursell · Answer 1 · 2012-08-28T16:15:36.477

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This is trivial in awk. Suppose $N is a shell variable that contains the minimum value you want from the second column, and you want to make column 10 the sum of columns 3 and 5:

awk '$2 > '$N'{ $10 = $3 + $5 }1' input-file

This outputs all of the rows. Pipe the output to head to reduce the number of lines output, or add a counter in the awk script. If you write C code to do this, you are wasting your time unless it is an exercise to help learn C.

On the other hand, it's pretty straightforward in C. For simplicity, assume you only have 2 columns:

int c[2];
do { 
  rc = scanf( "%d %d", c, c + 1 );
  if( c[1] > N && rc == 2 )
    printf( "%d %d %d", c[0], c[1], c[0] + c[1] );
} while( rc > 0 );

edited Aug 28 '12 at 16:15

answered Aug 28 '12 at 16:00

William Pursell

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Didn't the question ask specifically in C? – rudolph9 Aug 28 '12 at 16:01
The question is tagged C, but C is not mentioned in the question. – William Pursell Aug 28 '12 at 16:02
It is doubling as an exercise in C, but I will look into AWK - thank you for the suggestion and statement on efficiency. – Protoplanet Aug 28 '12 at 16:04
William - Thank you so much! I have the C code working now, and the AWK is brilliant, hadn't heard of it before shamefully. Thanks again for your time and expertise. – Protoplanet Aug 28 '12 at 16:30
No need to feel shame. Asking questions is the best way to learn about new tools. New to you, that is. Like many of the best tools, awk is pretty old. – William Pursell Aug 28 '12 at 16:43

score 0 · Answer 2 · edited May 23 '17 at 12:04

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The most strait-forward approach is probably to convert each column within the file into an array, then manipulate it as you describe from there.

Keep in mind, the file stores characters not integers, make sure you allocate your arrays accordingly to what you desire to store.

You may also find this question discussing the conversion of characters to integers useful.

edited May 23 '17 at 12:04

Community

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answered Aug 28 '12 at 16:01

rudolph9

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Thank you rudolph, I didn't realise this was the case. Thanks for the link too. – Protoplanet Aug 28 '12 at 16:06

Reading, Processing & Outputting numbers from files in C

2 Answers2