Thread: Help reading a text file and adding new lines depending on value in line

I need to code to read in text files and write a new line between lines where the value of the first column is different to the next. Any guidance on my current code would be greatly appreciated.

Code:

int main()
{
    float *doub1, *doub2;
    char *line, *rest, lines, *line2, filename[100], ch;
    FILE * newfile;
    printf("enter file name: ");
    scanf("%s", filename);
    newfile = fopen(filename, "r+");
    while (ch = fgetc(newfile) !=EOF)
    {
        if (ch == '\n')
        {
            lines++;
        }
    }
    for (int i = 0; i < lines/2; i++)
    {
        fgets(line, 100, newfile);
        sscanf(line, "%e %s", doub1, rest);
        fgets(line2, 100, newfile);
        sscanf(line, "%f %s", doub2, rest);
        if (doub2>doub1)
        {
            fputs( "\n", newfile);
        }
        doub2=doub1;
        line2=line;
    }
    return 0;
}

Yes, the first thing is use a compiler with lots of warnings, and enable those warnings.

Code:

$ gcc -Wall -Wextra foo.c
foo.c: In function ‘main’:
foo.c:8:11: warning: format not a string literal and no format arguments [-Wformat-security]
     scanf(filename);
           ^
foo.c:10:23: warning: passing argument 1 of ‘fgetc’ from incompatible pointer type [-Wincompatible-pointer-types]
     while (ch = fgetc(filename) !=EOF)
                       ^
In file included from foo.c:1:0:
/usr/include/stdio.h:531:12: note: expected ‘FILE * {aka struct _IO_FILE *}’ but argument is of type ‘char *’
 extern int fgetc (FILE *__stream);
            ^
foo.c:10:5: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
     while (ch = fgetc(filename) !=EOF)
     ^
foo.c:12:16: warning: comparison between pointer and integer
         if (ch == "\n")
                ^
foo.c:12:16: warning: comparison with string literal results in unspecified behavior [-Waddress]
foo.c:19:26: warning: passing argument 3 of ‘fgets’ from incompatible pointer type [-Wincompatible-pointer-types]
         fgets(line, 100, filename);
                          ^
In file included from foo.c:1:0:
/usr/include/stdio.h:622:14: note: expected ‘FILE * restrict {aka struct _IO_FILE * restrict}’ but argument is of type ‘char *’
 extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
              ^
foo.c:20:22: warning: format ‘%f’ expects argument of type ‘float *’, but argument 3 has type ‘double’ [-Wformat=]
         sscanf(line, "%f %s", doub, rest);
                      ^
foo.c:22:27: warning: passing argument 3 of ‘fgets’ from incompatible pointer type [-Wincompatible-pointer-types]
         fgets(line2, 100, filename);
                           ^
In file included from foo.c:1:0:
/usr/include/stdio.h:622:14: note: expected ‘FILE * restrict {aka struct _IO_FILE * restrict}’ but argument is of type ‘char *’
 extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
              ^
foo.c:23:22: warning: format ‘%f’ expects argument of type ‘float *’, but argument 3 has type ‘double’ [-Wformat=]
         sscanf(line, "%f %s", doub2, rest);
                      ^
foo.c:26:26: warning: passing argument 2 of ‘fputs’ from incompatible pointer type [-Wincompatible-pointer-types]
             fputs( "\n", filename);
                          ^
In file included from foo.c:1:0:
/usr/include/stdio.h:689:12: note: expected ‘FILE * restrict {aka struct _IO_FILE * restrict}’ but argument is of type ‘char *’
 extern int fputs (const char *__restrict __s, FILE *__restrict __stream);
            ^
foo.c:6:12: warning: variable ‘newfile’ set but not used [-Wunused-but-set-variable]
     FILE * newfile;
            ^
foo.c:19:9: warning: ‘line’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         fgets(line, 100, filename);
         ^
foo.c:20:9: warning: ‘doub’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         sscanf(line, "%f %s", doub, rest);
         ^
foo.c:20:9: warning: ‘rest’ may be used uninitialized in this function [-Wmaybe-uninitialized]
$

Better, but there are still things to fix.

Code:

$ gcc -Wall -Wextra foo.c
foo.c: In function ‘main’:
foo.c:10:5: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
     while (ch = fgetc(newfile) !=EOF)
     ^
foo.c:19:9: warning: ‘line’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         fgets(line, 100, newfile);
         ^
foo.c:20:9: warning: ‘doub1’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         sscanf(line, "%e %s", doub1, rest);
         ^
foo.c:20:9: warning: ‘rest’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Also, omitting the single line of #include <stdio.h> really doesn't save anything and p....es people off because it's extra steps to actually test.
We like to copy/paste/compile/go. If there is more work than that, you'll get comments or ignored.

You need a better compiler.

> while (ch = fgetc(newfile) !=EOF)
> warning: suggest parentheses around assignment used as truth value [-Wparentheses]
!= has higher precedence than =, so what you've written is this

while (ch = (fgetc(newfile) !=EOF))
Which translates as read a char, compare with EOF and then assign the boolean result (0 or 1) to ch.

whereas what you probably intended was this
while ((ch = fgetc(newfile)) !=EOF)
Which translates as read a char, assign to ch and then compare with EOF.


> char *line[lines], *rest[lines], filename[100], ch;
These arrays do not magically get larger simply by doing lines++ later on in the code.

> fgets(line[i], 100, newfile);
Nor does line[i] point to 100 bytes of memory.

You only need a single line buffer to read the file one line at a time. Nothing you've posted suggests that you need to hold ALL the lines in memory at the same time.


> #include <string>
This is a C++ header file.
Try to make sure you're compiling C with a C compiler.
Not any old mish-mash of syntax.