[C] printing integers from file

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November 22nd, 2012, 01:30 PM

Luko

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[C] printing integers from file

I want to print integers from file. If I know how many integers will be in a file, let's say 10, i would write a simple loop, like this:

Code:

  for(i = 0;  i < 10;  i++){
        scanf("%d",&i);
        printf("%d ",i);
    }

but what to do if i don't know how many integers will be in a file?

November 22nd, 2012, 02:07 PM

bdb

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Read a number.
If that succeeds, print it and loop back
Otherwise you can assume the data has ended and proceed with the rest of the program.

November 25th, 2012, 04:18 AM

Luko

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Thanks. But how do i check is it succeeded to read a number?

Code:

    for (i = 1; i < 2; i++){
        scanf("%d",&n);
        if (/* number found */){
            printf("%d ",n);
            i--;
        }
    }

November 25th, 2012, 04:32 AM

salem

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if ( scanf("%d", &n ) == 1 )

But beware that typing in say 123456789012345678901234567890 will generate numeric overflow, but scanf will still claim success and give you some modulo-n result.

If this is a problem, use fgets() to read a line of input, then strtol() to convert from a string to a long integer. This also sets errno in case of overflow (of a long), then you can range-check it for an integer (or whatever the valid range is).

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November 25th, 2012, 05:45 AM

Luko

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Thanks, works like a charm.

But now i'm a bit confused how comparison in C works. If a number is not 1, but say 3, then '3 == 1' should be false, because 3 is not equal 1. So 'if' sentece shouldn't work. But it works

November 25th, 2012, 06:47 AM

salem

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I think you mis-understand what scanf returns.

if ( scanf("%d %d", &x, &y ) == 2 )
The 2 refers to the fact that there are two conversions in the format string, and that both of them were successful - from say typing in 123 456.

But if you typed in 123 abc, then scanf() would return 1 (assigning 123 to x, and leaving y undefined).

November 25th, 2012, 07:38 AM

Luko

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Ohh, i got it now.

[C] printing integers from file

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