How can I stop before reading the next line? - c

Question

I have to read a file like this:
0 -> 1:50 2:30 3:10
1 ->
2 -> 0:10 3:20
3 -> 1:20 2:10 3:30
Here's my code:
graphs = fopen(argv[2],"r");
if(graphs==NULL){
printf("File hostgraphs not found\n");
exit(EXIT_FAILURE);
}
while((err=fscanf(graphs,"%d ->",&num))==1){
row=num;
while((err1=fscanf(graphs," %d:%d ",&column,&visits))==2){
hostgraphs[row*n+column]=visits;
}
if(err1!=2)
break;
if(err==0){
printf("Hostgraph out of bounds\n");
exit(EXIT_FAILURE);
}
}
for(i=0;i<n;i++){
for(j=0;j<n;j++){
printf("%d ", hostgraphs[i*n+j]);
}
printf("\n");
}
It gives me the following result:
0 50 30 10
0 0 0 0
0 0 0 0
0 0 0 0
The expected output must be:
0 50 30 10
0 0 0 0
10 0 0 20
0 20 10 30
Can anyone help me please?

Answer 1

You can do this by reading the file, one line at a time, using getline, and then applying the same logic, as you are doing, on each line separately.
In the following code, I first get a single line from the file in buffer using getline method. Then I process that line as a FILE pointer fbuffer using fmemopen method, which is used to read a string like a file:
graphs = fopen(argv[2],"r");
if(graphs==NULL)
{
printf("File hostgraphs not found\n");
exit(EXIT_FAILURE);
}
char *buffer = NULL;
size_t len = 0;
while(getline(&buffer, &len, graphs) != -1)
{
FILE * fbuffer = fmemopen(buffer, len, "r");
while((err=fscanf(fbuffer,"%d -> ",&num))==1)
{
row=num;
while((err1=fscanf(fbuffer," %d:%d ",&column,&visits))==2)
{
hostgraphs[row*n+column]=visits;
}
if(err1!=2)
{
break;
}
if(err==0)
{
printf("Hostgraph out of bounds\n");
exit(EXIT_FAILURE);
}
}
free(buffer);
buffer = NULL;
len = 0;
}
for(i=0;i<n;i++)
{
for(j=0;j<n;j++)
{
printf("%d ", hostgraphs[i*n+j]);
}
printf("\n");
}
Note: I have just added code to make your existing code work, without making any changes to your code.

Answer 2

The fix is simple: remove the trailing space in the fscanf format string in
fscanf(graphs," %d:%d ",&column,&visits)
It should be simplified as
fscanf(graphs,"%d:%d", &column, &visits)

Answer 3

fscanf ignores the newline then after read first row, it moves to next row and read 5 and results 1 in err1. That is the root-cause. You can add fseek to move back and start new sequence as bellow code-snip or simply change the algorithm to another effective one.
graphs = fopen(argv[2],"r");
if (!graphs) {
printf("File hostgraphs not found<%s>\n", argv[1]);
exit(EXIT_FAILURE);
}
while((err = fscanf(graphs, "%d ->", &num)) == 1) {
pos = ftell(graphs); // save position
while((err1 = fscanf(graphs, " %d:%d", &column, &visits)) == 2 ) {
hostgraphs[num * n + column] = visits;
pos = ftell(graphs); // save position
}
// seek back to previous one and start new sequence
fseek ( graphs , pos, SEEK_SET );
}
for(i = 0; i< n; i++){
for(j = 0; j < n; j++){
printf("%d ", hostgraphs[i * n + j]);
}
printf("\n");
}

Answer 4

You were close, but you needed a way of locating each '\n' without reading the next row value from the file. That is very difficult to do with fscanf where you have varying number of elements in each row in your input file.
Another approach is to use fgets to read the entire line, and then separate the initial "row -> " prefix from the data values. If you move the values into a separate values buffer (say vbuf), you can then repeatedly loop through the buffer by finding the space, advancing to the next digit and then using sscanf to split the column and visits.
(you actually don't even need to split the values in vbuf, you can simply use a pointer to advance past the "row -> " text working with the entire line.)
The following example puts those pieces together and parses the values into the correct positions in hostgraphs. Look it over and let me know if you have questions:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
enum { COLS = 4, MAXH = 16, MAXC = 64 };
int main (int argc, char **argv) {
char buf[MAXC] = "", vbuf[MAXC] = "";
char *p = NULL, *rfmt = "%d -> %[^\n]";
int row, column, visits, hostgraphs[MAXH] = {0};
size_t i, j, n = 0;
FILE *graphs = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!graphs) {
fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
return 1;
}
while (fgets (buf, MAXC, graphs))
{
*vbuf = 0; /* initialize vbuf, split %d:%d pars into vbuf */
if (sscanf (buf, rfmt, &row, vbuf) < 1)
break;
if (!*vbuf) { /* check vbuf contians values */
n++;
continue;
}
/* read 1st pair */
if (sscanf (vbuf, "%d:%d", &column, &visits) != 2) {
fprintf (stderr, "error: invalid line format\n");
exit (EXIT_FAILURE);
}
hostgraphs[row*COLS+column] = visits;
p = vbuf; /* assign p, parse remaining pairs */
while ((p = strchr (p, ' '))) /* find space */
{
while (*p < '0' || '9' < *p) /* find digit */
p++;
if (sscanf (p, "%d:%d", &column, &visits) == 2)
hostgraphs[row*COLS+column] = visits;
}
n++; /* advance row count */
}
for (i = 0; i < n; i++) { /* output values */
printf (" row[%2zu] : ", i);
for(j = 0; j < COLS; j++) {
printf (" %2d", hostgraphs[i * COLS + j]);
}
printf ("\n");
}
if (graphs != stdin)
fclose (graphs);
return 0;
}
Input File
$ cat ../dat/hostgraph.txt
0 -> 1:50 2:30 3:10
1 ->
2 -> 0:10 3:20
3 -> 1:20 2:10 3:30
Output
$ ./bin/hostgraph < ../dat/hostgraph.txt
row[ 0] : 0 50 30 10
row[ 1] : 0 0 0 0
row[ 2] : 10 0 0 20
row[ 3] : 0 20 10 30