Why is this output happening? - c

Question

This is an assignment for one of my classes, that requires you to convert from one base to another while ignoring all the comment lines (starting with '#') from an input file by command line.
I wrote a C program but when I ran it with the input file, it seems to covert the first number correctly but would just print the first result with increased line number for almost indefintely. I just have no clue on how to fix the code any suggestions would help.
My code:
#include<stdio.h>
#include<math.h>
#include<string.h>
#include<ctype.h>
void data_line(char line[])
{
int input_base;
char value[35];
int output_base;
sscanf(line, "%d %s %d", & input_base, value, & output_base);
int n = strlen(value);
int decimal = 0;
for (int i = n - 1; i >= 0; i--)
{
if (value[i] >= '0' && value[i] <= '9')
{
decimal += pow(input_base, n - i - 1) * (value[i] - '0');
}
else
{
value[i] = tolower(value[i]);
decimal += pow(input_base, n - i - 1) * (value[i] - 'a' + 10);
}
}
char output[35];
int k = 0;
while (decimal > 0)
{
int rem = decimal % output_base;
if (rem >= 10)
output[k] = ('A' + rem - 10);
else
output[k] = ('0' + rem);
k++;
decimal = decimal / output_base;
}
if (k == 0)
{
printf("0\n");
} else
{
for (int i = k - 1; i >= 0; i--)
printf("%c", output[i]);
printf("\n");
}
}
int main(int argc, char * argv[])
{
FILE * fp = fopen(argv[1], "r");
char line[100];
int line_no = 1;
while (fscanf(fp, "%[^\n]%*c", line) != EOF)
{
if (line[0] >= '0' && line[0] <= '9')
{
printf("%d: ", line_no);
data_line(line);
}
line_no++;
}
fclose(fp);
}
my input.txt samples:
# a first example
8 70 4
2 011100 16
7 0 21
11 3A 10
my output:
2: 320
3: 320
4: 320
5: 320
6: 320
7: 320
8: 320
9: 320
10: 320
11: 320
12: 320
13: 320
14: 320
15: 320
16: 320
17: 320
18: 320
19: 320
20: 320
...
...
...
expected output:
2: 320
4: 1C
5: 0
6: 43

Answer 1

As told in the comments always test the return codes from scanf() family functions. It is not an error for these functions to read nothing. They are scanners for formatted input, as says their names.
Also you may find easier to consume the file lines using fgets()
Example
The loop below reads the file, prints out the comment lines for testing and maybe easier to read.
while (NULL != (p = fgets(line, sizeof(line), fp)))
{
switch (line[0])
{
case '#':
printf("%s\n", line);
break;
case '\n': // blank
break;
default:
data_line(line);
break;
}
}
A complete test with a few changes
input.txt
# a first example
8 70 4
2 011100 16
7 0 21
11 3A 10
# end of input
output
# a first example
input base: 8 output base: 4
value is "70"
output value is 320
input base: 2 output base: 16
value is "011100"
output value is 1C
input base: 7 output base: 21
value is "0"
output value is 0
input base: 11 output base: 10
value is "3a"
output value is 43
# end of input
Using your code
I changed the main loop to use fgets()
added input.txt as the default file name
changed data_line() to return int so you can test for lines that are not comments but are ill-formed
grouped the output of data_line() to make it easier to read and change
added some casts to avoid the many compiler warnings
complete code
#include <ctype.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
int data_line(const char* line)
{
int input_base;
char value[35];
int output_base;
int res = sscanf(
line, "%d %s %d", &input_base, value, &output_base);
if (res != 3) return -1;
size_t n = strlen(value);
int decimal = 0;
for (int i = (int)n - 1; i >= 0; i--)
{
if (value[i] >= '0' && value[i] <= '9')
{
decimal += (int)pow(
(double)input_base,
(double)(n - i - 1)) *
(value[i] - '0');
}
else
{
value[i] = tolower(value[i]);
decimal += (int)pow(
(double)input_base,
(double)(n - i - 1)) *
(value[i] - 'a' + 10);
}
}
char output[35] = {0};
int k = 0;
while (decimal > 0)
{
int rem = decimal % output_base;
if (rem >= 10)
output[k] = ('A' + rem - 10);
else
output[k] = ('0' + rem);
k++;
decimal = decimal / output_base;
}
// all output here
printf(
" input base: %d output base: %d\n value is "
"\"%s\"\n output value is ",
input_base, output_base, value);
if (k == 0)
printf("0");
else
for (int i = k - 1; i >= 0; i--)
printf("%c", output[i]);
printf("\n\n");
return 0;
}
int main(int argc, char* argv[])
{
const char* dflt = "input.txt";
FILE* fp = NULL;
if (argc < 2)
fp = fopen(dflt, "r");
else
fp = fopen(argv[1], "r");
if (fp == NULL) return -1;
char line[100];
int line_no = 0;
char* p = NULL;
while (NULL != (p = fgets(line, sizeof(line), fp)))
{
switch (line[0])
{
case '#':
printf("%s\n", line);
break;
case '\n': // blank
break;
default:
data_line(line);
break;
}
}
fclose(fp);
}