Pset4 whodunit question

Question

I'm trying to change the blue in clue.c. Right now, I made all the red into white, but i'm confused why my code doesn't change the remaining into black? Here's what i have. I'm trying to say, if it's not a white pixel, change it to black.

if(triple.rgbtRed ==  0xff){

            triple.rgbtBlue = 0xff;
            triple.rgbtGreen = 0xff;
        }

if(triple.rgbtBlue != 0xff && triple.rgbtRed!= 0xff && triple.rgbtGreen != 0xff){
            triple.rgbtBlue = 0x00;
            triple.rgbtGreen = 0x00;
            triple.rgbtRed = 0x00;
        }

Also, for the very beginning of the program, why do we need to remember the file names with char* infile = argv[1]; char* outfile = argv[2];. Can't we just use argv[1] and argv[2] in place of all of them?

Answer 1

Think about what your test is saying:

if(triple.rgbtBlue != 0xff && triple.rgbtRed!= 0xff && triple.rgbtGreen != 0xff){

This will be true only if all three triples are not 0xff. If any one of them is 0xff, then the test is false. That means that a pure red, green or blue pixel will not be turned to black. If you want to test for a pixel that is not white, you need to change the test to this:

if( !(triple.rgbtBlue == 0xff && triple.rgbtRed== 0xff && triple.rgbtGreen == 0xff) ){

When you're trying to test for a negative of a complex condition, it is simpler and better to just apply the not operator to the whole condition, as in !(white). It's possible to do it by testing each individual condition, but it's a complex process to convert, and I don't recommend it for anyone that doesn't know how to fill out a truth table or hasn't had a course in digital logic.

As for using argv[x], yes, you can use them directly. Accessing the individual characters is the same as accessing an element in a 2 dimensional array. Assigning a different variable to them just makes it easier to access the data as a 1-dimensional array instead of 2d. It also makes for more descriptive var names to facilitate programming and debugging. Don't you think that infile is more meaningful than argv[1]? wouldn't it make it easier to understand what the code was doing?

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