CS50 Filter grayscale check50 - cs50

Question

The grayscale code seems to run fine with programs having whole number as average. But gives error with complex averages, where the result is different from expected code by just 1.
// Convert image to grayscale
void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
double avgcolor;
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
avgcolor = (image[i][j].rgbtRed + image[i][j].rgbtBlue + image[i][j].rgbtGreen) / 3;
image[i][j].rgbtRed = image[i][j].rgbtBlue = image[i][j].rgbtGreen = round(avgcolor);
}
}
return;
}
Error Message
:( grayscale correctly filters single pixel without whole number average
Cause
expected "28 28 28\n", not "27 27 27\n"
Log
testing with pixel (27, 28, 28)
running ./testing 0 1...
checking for output "28 28 28\n"...
Expected Output:
28 28 28
Actual Output:
27 27 27
I get such errors in two other cases. It could be a minor issue with the round function. Ive gone through the code several times but still cant find the cause of error.

Answer 1

You are dividing two integers, so C will compute your average, which may not be a whole number, and then drop what comes after the decimal point. Because image[i][j].rgbtRed + image[i][j].rgbtGreen + image[i][j].rgbtRed will always be an integer, dividing this integer value by another integer, 3, will return yet another integer, regardless of any decimal point. In other words, if image[i][j].rgbtRed + image[i][j].rgbtGreen + image[i][j].rgbtRed/3 = 27.66 then avgcolor will equal 27. A solution to this would be to divide the color values by 3.0, a float, instead. An integer divided by a float can return a float, but not an integer divided by an integer.
Try this code, where you do an integer by float division with 3.0:
// Convert image to grayscale
void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
double avgcolor;
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
avgcolor = (image[i][j].rgbtRed + image[i][j].rgbtBlue + image[i][j].rgbtGreen) / 3.0;
image[i][j].rgbtRed = image[i][j].rgbtBlue = image[i][j].rgbtGreen = round(avgcolor);
}
}
return;
}