Lab 5: Advanced pointers Solution

If this lab is confusing right now, we’ll be talking more about this stuff on Wednesday/Thursday. And/or go to recitation and have the TAs help you!
Pointers are super important when writing low-level programs! GET COMFORTABLE WITH EM!
In this lab, you’ll be writing a generic array-filtering function. This will make use of pointer arithmetic, function pointers, and pointer casting. This is an actually useful function! Filtering values out of an array is a very common operation.
Starting off  
Get the starting file here. Rename it properly, and upload to thoth.
Open it and read the comments. All of them.

Predicates  
“Predicate” is a common programming term which means “something that gives a yes-or-no answer.”
The filter function’s predicate must be a function which:
    • takes a const void* which points to a value from the array
    • returns an integer:
        ◦ 0 for false (ignore the item)
        ◦ nonzero for true (put the item in the output array)
            ▪ (This is common in C, because it didn’t use to have bool.)

Writing the predicate function  
The less_than_50 function should interpret its parameter as a pointer to a float, and as the name implies, return a “true” (nonzero) value if it is less than 50.
Since the parameter is a const void*, you’ll have to cast the parameter to a different pointer type.
Have a look at how I wrote the comparison function in the qsort.c example to get an idea of how to write this.
Hint: in C, comparison operators give an integer value. They give 1 if they’re true, and 0 if they’re false.

Writing the filter function  
You didn’t read the comments, did you. 😤
Have a look at the code in main:
float filtered[NUM_VALUES];
int filtered_len = filter(filtered, float_values, NUM_VALUES, sizeof(float), &less_than_50);

printf("there are %d numbers less than 50:\n", filtered_len);

for(int i = 0; i < filtered_len; i++)
    printf("\t%.2f\n", filtered[i]);

Look at the float_values array and think about what the output should look like. (There are 6 numbers less than 50, right?)
The filter function should work like this:
    • for each item in the input array:
        ◦ call the pred function with a pointer to that element
        ◦ if it returned “true”:
            ▪ use memcpy to copy that item from the input array to the output array (see below)
In addition it should:
    • keep a count of how many items “passed the test” (predicate returned “true”)
    • return that count
memcpy  
memset is used to fill in a blob of bytes with a value. memcpy is used to copy blobs of bytes from one place to another. It’s a very common function.
        memcpy(dest, src, length);
This will copy length bytes from the memory pointed to by src into the memory pointed to by dest.
“Walking pointers”  
You’re used to using [] to access values from arrays. But you can’t use [] on a void*. Instead, an easier technique is to use a “walking pointer.”
Instead of keeping a pointer to the beginning of an arrays, we move the pointer along, item by item, to access the array. Like this.

But there’s a catch: you can’t do pointer arithmetic on void pointers either!!
So if you want to move a void* over by n bytes, you have to:
    • cast it to a char*
    • add n to that
    • store it back into the void*
All this can be done on one line. Don’t overcomplicate things.
Good luck, but some likely mistakes:  
If you don’t move the pointer along the input array, you’ll get something like:
there are 10 numbers less than 50:
        31.94
        31.94
        31.94
        ...etc...
If you don’t move the pointers by the right number of bytes, you might get something like:
there are 6 numbers less than 50:
        127.76
        36.10
        0.00
        ...etc...
If you moved the input pointer right, but forgot to move the output pointer along:
there are 6 numbers less than 50:
        19.60
        0.00
        ...etc...
If you didn’t count properly, or maybe you didn’t respond to the predicate properly:
there are 0 numbers less than 50:

Submission  
Please make sure the driver (the main function) is the default one I gave you before you submit.
Then submit as usual.