c programming problems. reading data file and finding max

I have a program I have to write that reads a data file. The data file has four columns(time, altitude, velocity, acceleration). I am supposed to read this data file and find out when the rocket is falling back to Earth. I would do this by finding when the altitude starts to drop off. When the altitude drops off I am supposed to print the time that it starts to drop off to the screen.

I got a program together but it is not printing off the time that is corresponding to the time the altitude starts to drop off.
Here is the program:

Code:

#include <stdio.h>
#define FILENAME "rocket11.txt"


int main(void)
{
int max_found=0, k, lines;
double time1, time2, time3, alt1, alt2, alt3, vel, acc;
FILE * data;


/* Open file and read the number of data points.  */
data = fopen(FILENAME,"r");
if (data == NULL)
  printf("Error opening input file.  \n");
else
{


fscanf(data, "%d", &lines);


/*  Read data and compute summary information.  */
for (k=1; k<=lines; k++);
{
fscanf(data, "%lf %lf %lf %lf",&time1, &alt1, &vel, &acc);
fscanf(data, "%lf %lf %lf %lf",&time2, &alt2, &vel, &acc);
fscanf(data, "%lf %lf %lf %lf",&time3, &alt3, &vel, &acc);
}
if(alt1<alt2>alt3);
        printf("Rocket is returning to Earth at %lf (s)",time2);


}
return 0;
}

Welcome drewdude92!

Fix your indentation in future posts. Posting hard to read code like that is a good way to not get help. You get a free pass since this is your first time

Why are you reading 3 lines each time through the loop? Once is enough. Also, you will need to check for "dropping off" inside the loop, so you can check every data point. If you wait until the whole file is read, you missed 99% of the data points. You only need to know the last set of data and the current one, to see if the altitude drops. Thus, you need time, alt, vel, acc as well as prev_time, prev_alt, prev_vel and prev_acc. Read your first set of data. Then, every time through the loop, set the prev_ values equal to the current values, then read the next line in as the new current values. Then check for dropping off

Your "dropping off" check

Code:

if(alt1<alt2>alt3);

Is all kinds of broken. First, ditch that semicolon, it make the body of the if statement an empty statement, so the printf below will always execute no matter what. Second, you can't compare values like that in C. The correct way to do that would be

Code:

if (alt1 < alt2 && alt2 > alt3)

However, if you are only using alt and prev_alt, then it's just one simple comparison.

Lastly, you should really check the return value of your fscanf calls, make sure they succeeded. fscanf returns the number of items scanned. If you ask for 1 (line reading num lines), it returns 1 on success, if you ask for 4 (like reading time, alt, vel and acc), it should return 4. Anything else, and you have a problem. So something like:

Code:

if (fscanf(data, "%lf %lf %lf %lf",&time3, &alt3, &vel, &acc) != 4)
    print error
    exit program