Quarterback Rating Function with Arrays and Structs is acting strange - c++

Question

First of all, I should state that this is a homework assignment, so while questions that give a direct answer will give me a good grade, I would prefer to know why something doesn't work, and a reason why/how I should fix it with your solution.
So here is the background for this function. I have a quarterback struct with the following information. There are ten games, which all are stored in the struct and its arrays:
struct QuarterBack{
string name;
int completions[kNumGames];
int attempts[kNumGames];
int yards[kNumGames];
int touchdowns[kNumGames];
int interceptions[kNumGames];
};
Now my goal for this problem is to use the information stored in these structs to compute the NFL Style passer ratings. For reference, wikipedia gives the following:
So here is the code I am using. It has some excessive parenthesis that I was trying to use to make sure my control was correct, but other than that I am stumped as to why I am not getting more correct answers. Below the code I will post an example file and output.
/**
* #brief printPasserRating prints the passer rating of all players
* #param players is the array holding all the players
*/
void printPasserRating(QuarterBack *players, int numPlayers){
for(int player = 0; player < numPlayers; player++){
double passerRating = 0;
int sumCompletions = 0, sumAttempts = 0, sumYards = 0,
sumTouchdowns = 0, sumInterceptions = 0;
for(int game = 0; game < kNumGames; game++){
sumCompletions += players[player].completions[game];
sumAttempts += players[player].attempts[game];
sumYards += players[player].yards[game];
sumTouchdowns += players[player].touchdowns[game];
sumInterceptions += players[player].interceptions[game];
}
double a = 0, b = 0, c = 0, d = 0;
double nums[4] = {a, b, c, d};
nums[0] = static_cast<double>((sumCompletions / sumAttempts) - 0.3) * 5;
nums[1] = static_cast<double>((sumYards / sumAttempts) - 3) * 0.25;
nums[2] = static_cast<double>(sumTouchdowns / sumAttempts) * 20;
nums[3] = 2.375 - (static_cast<double>(sumInterceptions / sumAttempts) * 25);
for(int letter = 0; letter < 4; letter++){
nums[letter] = mm(nums[letter]);
}
passerRating = (nums[0] + nums[1] + nums[2] + nums[3]) / 0.06;
cout << players[player].name << "\t" << passerRating << endl;
}
showMenu(players, numPlayers);
}
Here is the example file. Ignore the 4, as it is for a separate part of the problem. Each row is a game, and it is listed as: completions, attempts, yards, touchdowns, then interceptions.
4
Peyton Manning
27 42 462 7 0
30 43 307 2 0
32 37 374 3 0
28 34 327 4 0
33 42 414 4 1
28 42 295 2 1
29 49 386 3 1
30 44 354 4 3
25 36 330 4 0
24 40 323 1 0
Tom Brady
29 52 288 2 1
19 39 185 1 0
25 36 225 2 1
20 31 316 2 0
18 38 197 0 1
25 43 269 1 1
22 46 228 0 1
13 22 116 1 1
23 33 432 4 0
29 40 296 1 1
Drew Brees
26 35 357 2 1
26 46 322 1 2
29 46 342 3 1
30 39 413 4 0
29 35 288 2 0
17 36 236 2 1
26 34 332 5 0
30 51 382 2 2
34 41 392 4 0
30 43 305 1 1
Eli Manning
24 35 360 1 2
25 46 340 2 3
26 44 350 3 1
34 35 460 1 2
25 36 240 2 3
16 34 250 3 1
24 35 360 1 0
35 56 340 2 2
36 44 350 3 0
34 45 360 1 1
And here is the output that the function is giving me:
Any help is much appreciated, and if you need more information to help me, feel free to comment and ask. Also, as this is a homework assignment, don't assume that I am just incompetent even if I make a silly mistake. I was told that Stack Overflow has no stupid questions, and I really hope that the community can live up to that.

Answer 1

This math is unlikely to do what you want:
nums[0] = static_cast<double>((sumCompletions / sumAttempts) - 0.3) * 5;
nums[1] = static_cast<double>((sumYards / sumAttempts) - 3) * 0.25;
nums[2] = static_cast<double>(sumTouchdowns / sumAttempts) * 20;
nums[3] = 2.375 - (static_cast<double>(sumInterceptions / sumAttempts) * 25);
Where you've put the cast will cast the result of the division to be double after the division has been performed. But, the division itself will be an integer division.
You want something more like this:
nums[0] = (static_cast<double>(sumCompletions) / sumAttempts - 0.3) * 5.0;
nums[1] = (static_cast<double>(sumYards) / sumAttempts - 3) * 0.25;
nums[2] = (static_cast<double>(sumTouchdowns) / sumAttempts) * 20.0;
nums[3] = 2.375 - (static_cast<double>(sumInterceptions) / sumAttempts) * 25.0;
By casting one of the terms in the divide to double, the division itself upgrades to double.
Alternately, you could just declare all of these variables to be double and avoid the casts entirely. That would make the code much easier to follow. Or, just make sumAttempts into a double, as it is common to all of the four divides.

Answer 2

I think the issue is in code like this:
static_cast<double>((sumCompletions / sumAttempts) - 0.3)
Here, sumCompletions and sumAttempts are ints. While you're trying to do a cast to a double to avoid integer division, the cast is on the complete value of the expression rather than on the numerator or denominator. This means that the division performed is integer division, which then has 0.3 subtracted and the result, which is already a double, is then cast to a double.
To fix this, cast the numerator or denominator, not the quotient itself:
static_cast<double>(sumCompletions) / sumAttempts - 0.3
Hope this helps!