C：字符数组的索引并使用strcmp

Question

I accidentally ran expensive simulations with an extra printf statement hidden in a for loop, creating huge ~10 GB files instead of ~10 KB files.

Re running the simulations isn't the best option so I've decided to take a crack at fixing the problem even though I don't have any formal training with C.

I'm trying to fix the problem by creating a program that reads these huge files, and prints out only the lines of the form (x, y, t, n) where (x, y, t, n are int, int, long double, long double). The other lines are a combination of strings, numbers and spaces that basically describe certain parameters, but all the (millions) of unwanted lines start with the letter "i".

So I thought I'd try to follow the example of this code here: Skip a line while reading a file in C

int main(int argc, char *argv[])
{
     FILE *f;

     if((f = fopen(argv[1], "r")) == NULL)
     {
         printf("Couldn't open file. \n");
         return(0);
     }

     char buf[500];

     while(fgets(buf, sizeof(buf), f) != NULL)
     {

         if(strcmp(&buf[0], "i") != 0)
         {
             printf("%s \n", buf);
         }

     }
}

So it seems the line

if(strcmp(&buf[0], "i") != 0)

Doesn't really work, as even if the first element of a line is "i" the printf of the line still prints. Interestingly if I add this inside the if statement:

printf("%c \n", buf[0]);

"i" can still print, even though I was trying to make that the condition for failing the if statement. Interestingly, the line

printf("%s \n", &buf[0]);

Just prints the same thing as

printf("%s \n", buf);

I tried to re-write

if(strcmp(&buf[0], "i") != 0)

as

if(strcmp(buf[0], "i") != 0)

but just received an error.

How do I correctly compare the first element of each line to a chosen character "i"?

Answer 1

you can use strncmp() instead of strcmp() for comparing the n elements of string.

if statement should be like if((strncmp(buf,"i",1))!=0) .Now it will compare only one element ie the 1st element of string stored in buf to i .

But for first element you can simply use if(buf[0]=='i') .