Reading from two columns into variables - c

Question

I'm writing a C program that takes an input file and stores it. The input file has two columns, with an integer in the first and a string in the second, like so:
12 apple
17 frog
20 grass
I've tried using fgets to take an entire line as a string then break it apart using scanf but I'm getting lots of issues. I have searched quite a lot but haven't found anything that answers my question, but sorry if I missed something obvious.
This is the code that I've been trying:
while(fgets(line, sizeof(line), fp))
{
scanf(line, "%d\t%s", &key, value);
insert(key, value, newdict);
}

Answer 1

Let's have a quick go at doing with strtok since someone mentioned it. Let's imagine your file is called file.txt and has the following contents:
10 aaa
20 bbb
30 ccc
This is how we can parse it:
#include <stdio.h>
#include <string.h>
#define MAX_NUMBER_OF_LINES 10 // parse a maximum of 10 lines
#define MAX_LINE_SIZE 50 // parse a maximum of 50 chars per line
int main ()
{
FILE* fh = fopen("file.txt", "r"); // open the file
char temp[MAX_LINE_SIZE]; // some buffer storage for each line
// storage for MAX_NUMBER_OF_LINES integers
int d_out[MAX_NUMBER_OF_LINES];
// storage for MAX_NUMBER_OF_LINES strings each MAX_LINE_SIZE chars long
char s_out[MAX_NUMBER_OF_LINES][MAX_LINE_SIZE];
// i is a special variable that tells us if we're parsing a number or a string (0 for num, 1 for string)
// di and si are indices to keep track of which line we're currently handling
int i = 0, di = 0, si = 0;
while (fgets(temp, MAX_LINE_SIZE, fh) && di < MAX_NUMBER_OF_LINES) // read the input file and parse the string
{
temp[strlen(temp) -1] = '\0'; // get rid of the newline in the buffer
char* c = strtok(temp, " "); // set the delimiters
while(c != NULL)
{
if (i == 0) // i equal to 0 means we're parsing a number
{
i = 1; // next we'll parse a string, let's indicate that
sscanf(c, "%d", &d_out[di++]);
}
else // i must be 1 parsing a string
{
i = 0; // next we'll parse a number
sprintf(s_out[si++], "%s", c);
}
c = strtok(NULL, " ");
}
printf("%d %s\n", d_out[di -1], s_out[si - 1]); // print what we've extracted
}
fclose(fh);
return 0;
}
This will extract the contents from the file and store them in respective arrays, we then print them and get back our original contents:
$ ./a.out
10 aaa
20 bbb
30 ccc

Answer 2

Use:
fgets (name, 100, stdin);
100 is the max length of the buffer. You should adjust it as per your need.
Use:
scanf ("%[^\n]%*c", name);
The [] is the scanset character. [^\n] tells that while the input is not a newline ('\n') take input. Then with the %*c it reads the newline character from the input buffer (which is not read), and the * indicates that this read in input is discarded (assignment suppression), as you do not need it, and this newline in the buffer does not create any problem for next inputs that you might take.

Answer 3

The problem here seems to be that you are reading from the file twice. First with fgets and then with scanf. You will probably not get an errors from the compiler in your use of scanf, but should be getting warnings as you use line for the format string and the other arguments does not match the format. It would also be pretty obvious if you checked the return value from scanf, as it returns the number of successfully scanned items. Your call would most likely return zero (or minus one when you have hit end of file).
You should be using sscanf instead to parse the line you read with fgets.
See e.g. this reference for the different scanf variants.

Answer 4

Your problem can be solved by using sscanf (with the support of getline) like below:
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
FILE *fp;
char *line = NULL;
size_t len = 0;
ssize_t read;
/* tokens bags */
char tok_str[255];
int tok_int;
fp = fopen("./file.txt", "r");
if (fp == NULL)
exit(EXIT_FAILURE);
/* Reads the line from the stream. */
while ((read = getline(&line, &len, fp)) != -1) {
/* Scans the character string pointed by line, according to given format. */
sscanf(line, "%d\t%s", &tok_int, tok_str);
printf("%d-%s\n", tok_int, tok_str);
}
if (line)
free(line);
exit(EXIT_SUCCESS);
}
Or, even simpler. You could use fscanf (with the support of feof) and replace the while loop shown above (along with some other redundant code cleanups) with the following one:
/* Tests the end-of-file indicator for the stream. */
while (!feof(fp)) {
/* Scans input from the file stream pointer. */
fscanf(fp,"%d\t%s\n",&tok_int, tok_str);
printf("%d-%s\n", tok_int, tok_str);
}
Assuming that your file contains following lines (where single line format is number[tab]string[newline]):
12 apple
17 frog
20 grass
the output will be:
12-apple
17-frog
20-grass