Get the text before and after strstr in C - c

Question

I need to be able to extract the characters before and after a substring, currently I have the following code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[]){
char *text = (char *) malloc (10000000);
char *word = argv[1];
int rep;
FILE *f;
if(argc < 2)
{
printf("Usage: GET <website> | ./word_counter <word>\n");
exit(1);
}
fread(text, 100, 10000000, stdin);
const char *tmp = text;
f = fopen("output.txt", "w");
fprintf(f, "%s\n", "REPS");
while(tmp = strstr(tmp, word)){
printf("%.50s\n", tmp);
rep++;
tmp++;
}
printf("Word count: %d\n", rep);
fclose(f);
system("gedit output.txt");
return 0;
}
I made a copy of the original input so I could leave it untouched and get the "before" characters from it.
Using strstr() on tmp (the original input copy) I can find the instances of the word I'm looking for and print the first 50 characters. But knowing this, how can I access the 50 characters BEFORE this instance?
Any help will be appreciated. Thanks!

Answer 1

Apart from the printing question itself, there are a couple of errors in your code. I have corrected most of them; a short list is:
Always test if malloc succeeded.
fread(text, 100, 10000000, ..) reads way too many text. 100 * 10000000 = 1000000000, almost a full gigabyte. You only allocated enough memory for 10 Mb.
You read from a text file and treat this data as a string. Therefore, you must make sure the data ends with a 0, else functions such as printf and strstr will try to continue reading after the end.
Your rep variable starts out uninitialized and therefore you will always see a random number.
Always free memory you allocated.
That said, it is slightly more efficient to use a dedicated function to print out text – if only to not put too much in your main. And since it's a function, you can add as many useful parameters into it as you want; I added before and after variables, so you can vary the number of characters shown.
For added niceness, this function prints a correct number of spaces when the phrase is found before the minimum number of before characters, so the results line up nicely. Also, since printing out characters such as tab and newlines will mess up your output, I replaced them with ?.
There is, admittedly, some repetition in print_range but in this case I went for clarity, rather than brevity.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_LENGTH 10000000
void print_range (char *source_text, int startindex, int before, int after, int phrase_length)
{
int i;
if (before > startindex)
{
for (i=0; i<before-startindex; i++)
printf (" ");
startindex = before;
}
for (i=0; i<before; i++)
{
if (strchr ("\t\r\n", source_text[startindex-before+i]))
printf ("?");
else
printf ("%c", source_text[startindex-before+i]);
}
for (i=0; i<phrase_length; i++)
{
if (strchr ("\t\r\n", source_text[startindex+i]))
printf ("?");
else
printf ("%c", source_text[startindex+i]);
}
for (i=0; i<after; i++)
{
if (!source_text[startindex+phrase_length+i])
break;
if (strchr ("\t\r\n", source_text[startindex+phrase_length+i]))
printf ("?");
else
printf ("%c", source_text[startindex+phrase_length+i]);
}
printf ("\n");
}
int main (int argc, char *argv[]){
char *text = (char *) malloc (MAX_LENGTH);
char *word = argv[1];
int rep = 0;
if (!text)
return -1;
if(argc < 2)
{
printf("Usage: GET <website> | ./word_counter <word>\n");
exit(1);
}
fread(text, 1, MAX_LENGTH, stdin);
text[MAX_LENGTH] = 0;
const char *tmp = text;
do
{
tmp = strstr(tmp, word);
if (!tmp)
break;
print_range (text, tmp-text, 16,16, strlen(word));
rep++;
tmp++;
} while (1);
free (text);
printf ("Word count: %d\n", rep);
return 0;
}
Result of running this on its own source code:
~/Documents $ ./wordcounter printf < wordcounter.c
tindex; i++)????printf (" ");???starti
-before+i]))????printf ("?");???else??
"?");???else????printf ("%c", source_t
before+i]);??}??printf ("{");??for (i=
rtindex+i]))????printf ("?");???else??
"?");???else????printf ("%c", source_t
tindex+i]);??}??printf ("}");??for (i=
_length+i]))????printf ("?");???else??
"?");???else????printf ("%c", source_t
length+i]);??}??printf ("\n");?}??int
argc < 2)??{??? printf("Usage: GET <we
?free (text);???printf ("Word count: %
Word count: 12