Calculate average sea level change - c

Question

This the assignment I'm trying to do.
Download the sea level data from http://climate.nasa.gov/vital-signs/sea-level (Links to an external site.). Create a program that does the following:
a. Tell the user that the program uses data from NASA to predict sea level from the years 2020 to 2050.
b. Store the sea level data in an array. You only need to use one data point for each year from 1993 to the present year. Use the last column for each row (the Global Mean Sea Level GMSL with annual and semi-annual signal removed).
c. Find the average annual change in sea level over all the years specified in the data. (Hint - use a loop to store the annual change in an array over the years, and then use a loop to compute the average annual change).
d. Assume a linear increase and compute the predicted sea level rise for the years 2020, 2025, 2030, 2035, 2040, 2045, and 2050. Store these results in their own array. (Hint - just use the average you computed in part c as the annual change for the future years).
e. Display the results for the user and be sure to reference the data set as specified in the data file so the user knows where the data came from.
Sample output:
The predicted Global Mean Sea Level is
2020 64.32
2025 68.98
2030 73.51
2035 78.12
2040 83.43
2045 88.12
2050 93.04
These predictions were made using data provided by XXXXXXXXXX
This is the code so far. However It seems to not use all of the data in the array to find the average change in sea level.
#include <stdio.h>
#include <stdlib.h>
int main()
{
//creates a file object to read data
FILE* infile = fopen("nasa.txt","r");
//checks if file exists
if(infile == NULL)
{
printf("File does not exist.\n");
return -1;
}
//create 2d array to store years and their sea levels
int level[50][2];
//number of elements in array
int n = 0,i;
char word[5];
//read data from file word by word
while(fscanf(infile, "%s", word) != EOF)
{
if(word != ' ' && word != '\n')
{
//convert string to int and store in array
level[n][0] = atoi(word);
//store sea level
fscanf(infile, "%s", word);
level[n][1] = atoi(word);
//increment n
n++;
}
}
//store avg change
float avg=0;
for(i=1;i<n;i++)
{
//add difference of consecutive elements
avg += level[i][1] - level[i-1][1];
}
//calculate mean
avg = (float)avg/n;
int c = 7; //number of predictions
//array to store results
float predictions[][2] = {{2020,0},{2025,0},{2030,0},{2035,0},
{2040,0},{2045,0},{2050,0}};
//predict future sea levels
for(i=0;i<c;i++)
{
//multiply avg change by number of years
predictions[i][1] = level[n-1][1] +
(predictions[i][0] - level[n-1][0])*avg;
}
//print avg change
printf("Average change in sea level year over year is: %f mm\n",avg);
//print predictions
for(i = 0;i<c;i++)
{
printf("Predicted sea level change since 1993 for the year %.0f: %.2f mm\n",
predictions[i][0],predictions[i][1]);
}
printf("These predictions were made using data provided by the National Aeronautics and Space Administration.");
return 0;
}
Sea level change data
1993 4
1994 7
1995 11
1996 14
1997 21
1998 20
1999 19
2000 22
2001 27
2002 31
2003 34
2004 36
2005 40
2006 42
2007 43
2008 47
2009 48
2010 54
2011 53
2012 59
2013 65
2014 68
2015 75
2016 83
2017 85
2018 88
2019 94

Answer 1

However It seems to not use all of the data in the array to find the average change in sea level.
avg = (float)avg/n; is same as avg = (float)(level[n-1][1] - level[0][1])/n.
Only the end-points matter.
The loop that accumulates the differences is canceling out the in-between year's values.
In a mid-year, if you add +100, it makes one year difference +100 more, and the next year difference 100 less. The running sum of the differences is not affect by that +100 in the end.
All the mid-year values could be 0 and one would get the same average.