从标准输入读取输入直到空行

Question

I want to parse input such as:

1 2 3\n4 5 6\n7 8 9\n\n

and for each row save each value in int ant print it to stdout until I get an empty line, so for this example I would get:

1 2 3
1 2 3
4 5 6
4 5 6
7 8 9
7 8 9

I've tried something like

int n1, n2, n3;
while(scanf ("%d %d %d\n", n1, n2, n3) != EOF) {
    printf("%d %d %d\n", n1, n2, n3);
    fflush(stdout);
}

but it doesn't seem to work. Is there any simple way to do that?

Answer 1

scanf cannot achieve what you are trying to do because it keeps reading until the condition is met, and %d specifier will ignore '\\n' newline character, I propose this code

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

int  main()
{
    int n1, n2, n3;
    char line[64];

    /* read at least 63 characters or unitl newline charater is encountered with */
    /*    fgets(line, sizeof line, stdin) */
    /* if the first character is a newline, then it's an empty line of input */
    while ((fgets(line, sizeof line, stdin) != NULL) && (line[0] != '\n'))
    {
        /* parse the read line with sscanf */
        if (sscanf(line, "%d%d%d", &n1, &n2, &n3) == 3)
        {
            printf("%d %d %d\n", n1, n2, n3);
            fflush(stdout);
        }
    }
    return 0;
}

While this code works it's not robust, since it will fail in the case commented below by WhozCraig , so this is a way to do it that will keep you safe from the problem

#include <stdio.h>
#include <string.h>
#include <ctype.h> /* for isspace */

int isEmpty(const char *line)
{
    /* check if the string consists only of spaces. */
    while (*line != '\0')
    {
        if (isspace(*line) == 0)
            return 0;
        line++;
    }
    return 1;
}

int  main()
{
    int n1, n2, n3;
    char line[64];

    while ((fgets(line, sizeof line, stdin) != NULL) && (isEmpty(line) == 0))
    {
        if (sscanf(line, "%d%d%d", &n1, &n2, &n3) == 3)
        {
            printf("%d %d %d\n", n1, n2, n3);
            fflush(stdout);
        }
    }
    return 0;
}

Answer 2

I didn't really like any of the answers provided since they don't also capture the input. I needed to capture the entire read contents and process them in batch. So here's what I came up with:

char * readFromFileUntilBlankLine(FILE *in) {
    int growBy = 1000;
    int allocatedSize = growBy;
    char *buffer = malloc(allocatedSize * sizeof(char));
    strcpy(buffer, "");

    char * line = NULL;
    size_t len = 0;
    ssize_t read = 0;

    while ((read = getline(&line, &len, in)) != EOF) {
        // Are we done? Is this blank?
        if (line[0] == '\n') {
            free(line);
            return buffer;
        }

        // Ensure there is enough space in buffer for line
        int newSize = strlen(buffer) + read + 1;
        if (newSize > allocatedSize) {
            allocatedSize = newSize + growBy;
            buffer = realloc(buffer, allocatedSize * sizeof(char));
        }
        strcat(buffer, line);

        free(line);
        line = NULL;
    }

    if (line) {
        free(line);
    }

    return buffer;
}

This suffers from the same "\\n \\n" problem discussed in a previous response, but was sufficient for my needs.