Finding empty line using fscanf

Question

I'm supposed to read some variables named from "A" to "Z" and then evaluate them. The values in variables are matrices. This is example input:

B=[5 2 4; 0 2 -1; 3 -5 -4]
E=[-6 -5 -8; -1 -1 -10; 10 0 -7]
R=[-1 -7 6; -2 9 -4; 6 -10 2]

R+E+B

I have written an algorithm that correctly reads all the variables. However I fail at detecting the empty line. I have written this:

// FILE* input = stdin; 
while(true) {
    char name = '#';
    // Reads the matrix, returns null on error
    Matrix* A = matrix_read_prp_2(input, &name);
    if( A==NULL ) {
        // throw error or something
    }
    // Print the matrix
    matrix_print_prp_2(A, stdout);
    // consume one new line
    char next;
    if(fscanf(input, "\n%c", &next)!=1)
        // Program returns error here
    if(next=='\n')
        break;
    // if not new line, put the char back
    // and continue
    ungetc(next, input);
}

I assumed that for empty line, fscanf(input, "\n%c", &next) would read '\n' into next, but it actually skips the second line and reads R.

How can I check if next line is empty on stream in C?

It is very, very hard to detect newlines of any sort with scanf() and friends. If you need to detect empty lines, then use fgets() or equivalent to read the material, and sscanf() to analyze the lines after you know it isn't empty. Any white space in a scanf() format string matches a sequence of zero or more white space characters; newlines, blanks, tabs are all white space. The newline in "\n%c" skips white space until the next character isn't white space. — Jonathan Leffler
– Jonathan Leffler, Commented Nov 27, 2016 at 2:00

Jonathan Leffler · Accepted Answer · 2016-11-27 03:09:06Z

1

If it is safe to assume that the matrix_read_prp_2() function leaves the newline in the input buffer, then it would be possible to revise I/O operations in the tail of the loop along these lines:

    // Read anything left over to end of line
    int c;
    while ((c = getc(input)) != EOF && c != '\n')
        ;
    // Break on EOF or second newline
    if (c == EOF || (c = getc(input)) == EOF || c == '\n')
        break;
    // if not new line, put the char back and continue
    ungetc(c, input);
}

^{Untested code.}

I'm not clear under what circumstances the nasrat(mgr, op); function call should be made; neither mgr nor op appears in the loop.

answered Nov 27, 2016 at 3:09

Jonathan Leffler

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1 Comment

Tomáš Zato Over a year ago

I used similar solution with slightly more verbose code that does the same thing. Thanks for answer. The nasrat function was used to exit() the program, the arguments passed to it were memory managers which were cleaned up before exiting. I removed it from the question. The reason it was called is that it is indeed safe to assume that when matrix_read_prp_2 ends, there should be new line in the buffer, lest the input is invalid.

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