How to go to the next line while reading a file in C++

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Hi, I was working on solving the 13th problem of project euler. the problem is "Work out the first ten digits of the sum of the following one-hundred 50-digit numbers.", I copied the number into a text file and read each digit then add it to an element in an array and I have solved it, But to read only the first ten digits, I had to skip the next 40 digits by reading them without adding.

C++

for (i = 0; i < 100; i++)
{
        for (j = 0; j < 50; j++)
        {
            if (file >> digit)
            {
                if (j > 9)
                    continue;
                numbers[i] *= 10;
                numbers[i] += (digit - '0');
            }
}

But, of course, doing such thing with a big file will be a big problem so any suggestions for a better solution such as going to the next line or something else ?

Thanks, Samuel.

Posted 3-Jul-15 14:07pm

Samuel Shokry

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[no name] 3-Jul-15 20:23pm

I guess I don't understand what your question is. Why don't you read a line, substring the first 10 characters which are the digits you are looking for and use that? You don't need these for loops at all.

PIEBALDconsult 5-Jul-15 15:02pm

The "lean and mean" technique is to hold only one character from the file at a time -- holding a string is a waste.

PIEBALDconsult 3-Jul-15 20:30pm

printf ( "0000000000" )
done.

[no name] 3-Jul-15 21:02pm

I looked at the actual question. For C++ you need to find yourself a Big Integer class to use, read the file, for each line convert the text to your big integer then substring the first 10 digits from the total. All of that should be fewer lines than you posted here.

PIEBALDconsult 3-Jul-15 22:40pm

I wouldn't bother with a big integer class.

[no name] 4-Jul-15 8:02am

He would have to to get the correct answer.

PIEBALDconsult 4-Jul-15 13:31pm

No, the answer can be calculated without a big integer type.

Samuel Shokry 3-Jul-15 22:36pm

I'm sorry for lack of clarity, my problem is I have such a number
37107287533902102798797998220837590246510135740250 in a file, I want to read just the first ten digits, so I used a char to read each digit, but I had to read also the next 40 digits without adding them which waste a lot of time in the case of more data to read, how to skip that or to go to the next line for example. I wish that my question is clear enough this time.
Thanks, Samuel.

PIEBALDconsult 3-Jul-15 22:37pm

That won't give you the correct answer.

PIEBALDconsult 3-Jul-15 23:43pm

I would also point out that if the file is _known_ to be exactly fifty digits (one-byte characters) per line (with LF or CRLF line breaks (not both)), then you need only move the position ahead to get to the next value.

PIEBALDconsult 4-Jul-15 2:17am

The entire sum, in some base n, is 0000000010.
Done.

PIEBALDconsult 4-Jul-15 2:25am

The problem isn't well-defined; by "first ten digits" do they mean the most-significant digits? Or the least-significant digits? When summing, you get the least-significant digits before the most-significant digits.

Richard MacCutchan 4-Jul-15 3:57am

Why not read the entire line into a string variable and extract the first 10 as a substring? It is then a trivial matter to convert those digits into an integer value.

PIEBALDconsult 4-Jul-15 14:15pm

That won't give the correct answer. His tactic is flawed; he must not ignore any digits.

Richard MacCutchan 5-Jul-15 2:52am

OP mentioned using only the first 10 digits of each line and ignoring the remaining 40.

PIEBALDconsult 5-Jul-15 3:14am

Yes; which will give an incorrect answer.

Richard MacCutchan 5-Jul-15 3:26am

Oh, you mean he wants the correct answer?

PIEBALDconsult 5-Jul-15 3:31am

I feel he would be best served by the correct answer, but yet he may feel otherwise.

Richard MacCutchan 5-Jul-15 3:40am

I agree entirely.

PIEBALDconsult 5-Jul-15 14:42pm

"doing such thing with a big file will be a big problem"
Poor assumption. I'm seeing sub-second times for reading the whole file and calculating the full sum.

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Frankie-C · Answer 1 · 2015-07-04T04:51:00

Use fseek() to for random access to each line, but consider that it works well only when opening the file as binary (remove text \n translation).
In binary your file layout is as follows:

37107287533902102798797998220837590246510135740250\r\n
46376937677490009712648124896970078050417018260538\r\n
74324986199524741059474233309513058123726617309629\r\n
91942213363574161572522430563301811072406154908250\r\n
23067588207539346171171980310421047513778063246676\r\n
....
53503534226472524250874054075591789781264330331690\r\n

Means that each number starts at

C++

(n-1)*52

Where n is the line number the last two bytes are \r & \n.
If d is the digit you want read on line n then:

digit is at ((n-1)*52)+(50-1-d)

This sample shows as to access file:

C++

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
	FILE *fp;

	fp = fopen("BigNums.txt", "rb");

	if (!fp)
	{
		printf("File not found!\n");
		return -1;
	}

	for(;;)
	{
		char str[128];
		char line[52];

		printf("Insert number line [1-10]: ");
		scanf ("%s", str);
		int n = atoi(str);
		if (n < 0)
			break;
		if (n<1 || n>100)
			continue;

		printf("Insert digit [1-50]: ");
		scanf ("%s", str);
		int digit = atoi(str);
		if (digit<1 || digit>50)
			continue;

		fseek(fp, (n-1)*52, SEEK_SET);
		fread(line, sizeof(line), 1, fp);

		printf("Line [%50.50s] digit [%c], first 10 chars [%10.10s]\n", line, line[49-(digit-1)], line);

	}

	return 0;
}

I choosed to read a whole number each time and access single digits in memory using fread() because fseek() is an heavy function and is not a good idea to use it very often.

If you want read full lines sequentially you can use fgets(). Using it you cannot move back.

P.S. The sample miss checks on file and data format assuming that the file has the specified format and number of entries.