C program file read/perform operation error

[mjoshi]

Hello,

I am having trouble with a simple math operation after reading from a file.
If a particular condition is met, i am copying a number(7 positions), starting from position 31.
My aim is to sum all these numbers when the condition is met.
I am testing the code on a file where the condition is met only once, as follows:

ATOM   5616  N   MET B   1     334.916 199.163  84.447  0.70172.02           N  

And yet the ouput is double what it should be: 669.831970 (right answer: 334.916)

while(fgets(s, 80 , fp1)!=NULL)
{
if((s[13] == 'N' && s[24] == ' ' && s[25] == '1'))
{
strncpy (sx, s+31, 7);
sx[7] = '/0';
x = atof(sx);
sum = sum + x;
}
}
printf("%f", sum);

Thanks in advance

[Ancient Dragon]

In the example line you posted, position 13 is the letter 'E', (in MET), not 'N'. And pos 24 is '1' (in 916).

[mjoshi]

In the example line you posted, position 13 is the letter 'E', (in MET), not 'N'. And pos 24 is '1' (in 916).

For some strange reason when i added a printf line (printf("%f",x);)
within the if statement the program is behaving well:

while(fgets(s, 80 , fp1)!=NULL)
{
if((s[13] == 'N' && s[24] == ' ' && s[25] == '1'))
{
printf("%s", s);
strncpy (sx, s+31, 7);
sx[7] = '/0';
x = atof(sx);
printf("%f",x);
sum = sum + x;
}
}

printf("%f", sum);

[r.stiltskin]

Assuming you have a line that actually does satisfy your test condition, if the following line is short enough (or even contains only a newline) so that its contents do not overwrite s[13], s[24] and s[25], the test condition will still be satisfied and the copy/add operations will be repeated.

Try this code, for example, with an input file containing these 2 lines:
line 1
l2

#include <stdio.h>

int main()
{
    FILE * infile;
    char s[80];
    infile = fopen("testfile.txt" , "r");
    while(fgets(s, 80 , infile)!=NULL)
    {
        if( s[5] == '1' )
        {
            printf("%c\n", s[5]);
        }
    }
    return 0;
}

[mjoshi]

What then is the solution to this problem? certainly there must be an alternate way of doing it.

[csurfer]

@mjoshi : Null character is represented as '\0' and not '/0' as you have done in your program.

And hope you have used double variables for sum, x etc as atof returns double.

[r.stiltskin]

The solution is simple: use strlen(s) to test if the new string is long enough. In my example I'm printing the char at s[5], so the string s must be at least 6 chars long. This code will ignore any string that is too short to satisfy my original test condition:

#include <stdio.h>
#include <string.h>

int main()
{
    FILE * infile;
    char s[80];
    infile = fopen("testfile.txt" , "r");
    while(fgets(s, 80 , infile)!=NULL)
    {
        if( strlen(s) > 5 && s[5] == '1' )
        {
            printf("%c\n", s[5]);
        }
    }
    return 0;
}