reading a document and finding a specific word in that document - c

Question

I have the following document:
WTPZ24 KNHC 032040
TCMEP4
TROPICAL STORM SIMON FORECAST/ADVISORY NUMBER 9
NWS NATIONAL HURRICANE CENTER MIAMI FL EP192014
2100 UTC FRI OCT 03 2014
THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT.
TROPICAL STORM CENTER LOCATED NEAR 18.8N 110.6W AT 03/2100Z
POSITION ACCURATE WITHIN 20 NM
REPEAT...CENTER LOCATED NEAR 18.8N 110.6W AT 03/2100Z
AT 03/1800Z CENTER WAS LOCATED NEAR 18.6N 110.1W
FORECAST VALID 04/0600Z 19.2N 112.2W
MAX WIND 65 KT...GUSTS 80 KT.
64 KT... 10NE 0SE 0SW 10NW.
50 KT... 30NE 30SE 20SW 20NW.
34 KT... 50NE 50SE 40SW 40NW.
FORECAST VALID 04/1800Z 20.0N 114.3W
MAX WIND 75 KT...GUSTS 90 KT.
64 KT... 20NE 10SE 10SW 20NW.
and I am trying to make a program in C that will read through the file and find the word string FORECASt VALID. Everytime a line has the words FORECAST VALID I want the program to print the line and "===>" before the line. For the rest of the document I just want it to simply print the line.
The final output put by the program should look like this:
REPEAT...CENTER LOCATED NEAR 18.8N 110.6W AT 03/2100Z
AT 03/1800Z CENTER WAS LOCATED NEAR 18.6N 110.1W
===>FORECAST VALID 04/0600Z 19.2N 112.2W
MAX WIND 65 KT...GUSTS 80 KT.
64 KT... 10NE 0SE 0SW 10NW.
50 KT... 30NE 30SE 20SW 20NW.
34 KT... 50NE 50SE 40SW 40NW.
===>FORECAST VALID 04/1800Z 20.0N 114.3W
MAX WIND 75 KT...GUSTS 90 KT.
64 KT... 20NE 10SE 10SW 20NW.
I do have some snippets of code that print the document verbatim without the ===> but I just need help in how to isolate out FORECAST VALID from a line so as to put the ===> in those lines:
FILE *fr; /* declare the file pointer */
main()
{
int n;
char line[800];
char buf[255];
fr = fopen ("wtnt23.knhc.201410141453", "rt"); /* open the file for reading */
while(fgets(line, 800, fr) != NULL)
{
sscanf (line, "%s", buf);
if (buf="FORECAST")
{
printf("===>%s",line);
}
else
{
printf("%s",line);
}
fclose(fr); /* close the file prior to exiting the routine */
} /*of main*/

Answer 1

if (buf="FORECAST") is totally wrong. it is not compare and you can't copy to char array with a =...
You just need to do like this:
while(fgets(line, 800, fr) != NULL)
{
if(0 == strncmp(line, "FORECAST VALID", strlen("FORECAST VALID")) )
{
printf("===>%s",line);
}
else
{
printf("%s",line);
}
}

Answer 2

I would use std::strings and iostreams rather than char arrays and FILE*.
Something a bit like:
int main()
{
std::ifstream ifs("wtnt23.knhc.201410141453");
std::string line;
while(std::getline(ifs, line)) // loop as long as getline() succeeds
{
if(line.find("FORECAST VALID") == 0) // line starts with this?
{
// output ===>
}
// output line + linefeed
}
}