数组打印出错误的数字

Question

I'm new to C++ and I have to make a program that lets the user enter a specified number of test scores and calculate the average, highest, and lowest scores. (It doesn't have to check if values are between 1 and 100.) For some reason, when it goes to print out the average score and sum of the scores, it just prints out random exponential numbers that are never the same. I haven't gotten to printing out the lowest and highest scores yet since I'm not sure if I'm even doing the average right. Again, I'm new to C++ so I'm sure I messed something up. Here's the code:

#include <iostream>
using namespace std;

int loopLimit = 0; //number of scores or how many times it will loop
double *testScores = {0}; //array scores are stored in

int main () {
    cout << "How many test scores are you entering?" << endl;
     cin >> loopLimit;

     testScores = new double[loopLimit]; //changes array to needed size

     for (int i = 0; i < loopLimit; i++) {
         cout << "Enter test score #" << (i + 1) << endl;
         cin >> *testScores;
     }

    double sum = 0.0;
    double average = 0.0;
    //double max = 0.0; //making these functions later
    //double min = 0.0; 

     sum += testScores[loopLimit];
     average = sum / loopLimit;
     //Sum is here for testing purposes at the moment

     cout << "Sum = " << sum << " Average = " << average << endl;

     return 0; 
}

Example output 1:

How many test scores are you entering?
3
Enter test score #1
100
Enter test score #2
100
Enter test score #3
100
Sum = 8.29874e-310 Average = 2.76625e-310

Example output 2:

How many test scores are you entering?
3
Enter test score #1
100
Enter test score #2
100
Enter test score #3
100
Sum = 8.94176e-310 Average = 2.98059e-310

Expected output:

How many test scores are you entering?
3
Enter test score #1
100
Enter test score #2
100
Enter test score #3
100
Sum = 300.0 Average = 100.0

I've been at this all week, and I honestly got nothing at this point.

Answer 1

OK, let's go over your code.

#include <iostream>
using namespace std;

Why are you importing all identifiers in std:: here? using namespace std; is not recommended.

int loopLimit = 0; //number of scores or how many times it will loop
double *testScores = {0}; //array scores are stored in

Bad: You should avoid global variables unless they're absolutely necessary. 99.9% of the time they're not. These could easily be local variables in main .

testScores is not an array, it's a pointer. Initializing it with {0} is just a weird way of writing testScores = nullptr; .

int main () {
    cout << "How many test scores are you entering?" << endl;
     cin >> loopLimit;

     testScores = new double[loopLimit]; //changes array to needed size

You're using manual memory management here. Not technically wrong, but it would be much easier and less error prone to use a std::vector instead.

     for (int i = 0; i < loopLimit; i++) {
         cout << "Enter test score #" << (i + 1) << endl;
         cin >> *testScores;

This line stores every input in *testScores , ie the location pointed to by testScores , which corresponds to the first index of the array allocated by new above. This means only testScores[0] is initialized (ending up containing the last number the user input), every other index is uninitialized.

You should use cin >> testScores[i] instead.

     }

    double sum = 0.0;
    double average = 0.0;
    //double max = 0.0; //making these functions later
    //double min = 0.0; 

     sum += testScores[loopLimit];

This is an invalid memory access. As testScores points to a dynamic array of size loopLimit , the valid array indices go from 0 to loopLimit-1 . Therefore testScores[loopLimit] accesses memory past the bounds of the array.

Also, it's just one element you're adding here. Even if this were a valid array index, this could would still make no sense. You should loop over all array elements here (like your for loop above), or just do this part of the calculation in your other loop. In fact, there's no need to store all numbers in memory if all you're interested in is their sum (which you can compute directly as you're reading the input).

     average = sum / loopLimit;

average is computed from sum , which has a garbage value, so it's garbage too.

     //Sum is here for testing purposes at the moment

     cout << "Sum = " << sum << " Average = " << average << endl;

... and this is why you're getting garbage output.

     return 0;

Here you're leaking the memory allocated with new . This is not really a problem in this case because your program is about to exit anyway, but in general you want delete[] testScores; here (unless you use a std::vector , which takes care of cleaning up for you).

}

Answer 2

A couple of things, first the line

     cin >> *testScores;

is not storing the test scores in an array (I think this is what you want to do) instead at every iteration the new test score is being stored in the first element ie testScores[0], rewriting the old value.

Next, The line

sum += testScores[loopLimit]; 

looks outside the array. This means that you are looking at a random place in memory which probably has junk in it. That explains why you are seeing random numbers outputted.

Try to fix those two issues. If you don't need to save the test scores you can away keep a running sum of them, that would eliminate the need to store everything in an array.

Answer 3

I don't know why you want to write such a messed up code for just calculating average.But here's my approach. Since you entered loopLimit value as 3 .So according to your code, sum will contain have the value of sum[3] .That is a garbage value .Since indexing starts with 0 and not 1 .So your sum will have a garbage value and that is the reason why you are getting such incorrect value .I have modified your code and it works fine for me.

int loopLimit;
cout << "How many test scores are you entering?" << endl;
    cin >> loopLimit;
 double scores[loopLimit];
 double sum=0.0;
 for (int i = 0; i < loopLimit; i++) 
{
         cout << "Enter test score #" << (i + 1) << endl;
         cin >> scores[i];
    sum+=scores[i];
     }
 double avg=sum/loopLimit;
 cout << "Sum = " << sum << " Average = " << avg << endl;
     return 0; 

Try avoiding using pointers during the early stage of programming as it can really mess up with your brain and can create problems.Try to go for a simple approach.

Answer 4

The possible issue is that you're doing this

double* testScores = {0};

which will initialize it to nullptr, potentially letting you read and write garbage data. What you should use instead is probably a dynamic resizing array such as

std::vector<double> testScores;

and to add a score to the list

double inScore = 0;
std::cin >> inScore;
testScores.push_back(inScore); // This adds to the list of scores.

Answer 5

 #include <iostream>
 using namespace std;

int loopLimit = 0; //number of scores or how many times it will loop
double *testScores = {0}; //array scores are stored in

int main () {

double sum = 0.0;
double average = 0.0;

 cout << "How many test scores are you entering?" << endl;
 cin >> loopLimit;

 testScores = new double[loopLimit]; //changes array to needed size

 for (int i = 0; i < loopLimit; i++) {
     cout << "Enter test score #" << (i + 1) << endl;
     cin >> testScores[i];
     sum += testScores[i];
}

 average = sum / loopLimit;

 cout << "Sum = " << sum << " Average = " << average << endl;
 delete [] testScores;
 return 0; 
}

*melpomene is right...that pointer " testScores" wont increment through your for loop so the array occupies garbage also put sum += testScores[i] in the loop to sum your input.