c++ programming arrays to of find number of integers in range

Question

The question is attached and so I've tried to solve this but the output I'm getting is large numbers which is probably garbage data and therefore is definitely not correct.

Here is my code

#include <iostream>
#include <fstream>
using namespace std;
const int size=8;
 void readData(char filename[], int list[], int size)
 {

     ifstream fin;
     fin.open("HW4_Q1data.txt");
int value=0;
     for(int i=0;i<30;i++)
     {
         fin>>filename[i];
         value=filename[i];
     if (value >= 0 && value <= 24)
       list[0]++;

       else if (value >= 25 && value <= 49)
       list[1]++;

       else if (value >= 50 && value <= 74)
       list[2]++;

       else if (value >= 75 && value <= 99)
       list[3]++;

       else if (value >= 100 && value <= 124)
       list[4]++;

       else if (value >= 125 && value <= 149)
       list[5]++;

       else if (value >= 150 && value <= 174)
       list[6]++;

       else if (value >= 175 && value <= 200)
       list[7]++;
     }

       fin.close();
 }
 void print(int list[], int size)
 {
     cout << "   Range"<<'\t'<<"# of students"<<endl;
cout << "0-24:    " <<'\t'<<list[0] << endl;
cout << "25-49:   " << '\t'<<list[1] << endl;
cout << "50-74:   " <<'\t'<< list[2] << endl;
cout << "75-99:   "   <<'\t'<< list[3] << endl;
cout << "100-124: " <<'\t'<< list[4] << endl;
cout << "125-149: " <<'\t'<<list[5] << endl;
cout << "150-174: " <<'\t'<< list[6] << endl;
cout << "175-200: " <<'\t'<< list[7] << endl;

 }
int main()
{

    char filename[70];
    int list[size];

    readData(filename,list,size);
    print(list,size);

    return 0;
}

Answer 1

As VTT said, you do not initialize list values, therefore it has garbage values there. You should initialize them with 0.

int main() {
    // filename stuff
    int list[size];
    for(int i = 0 ; i < size; i++) {
        list[i] = 0;
    }
    // read and write functions
}

Also, as I far I can read from the question char filename[] should be used as the name of the file: fin.open(filename);

EDIT

Most of the program, to fix your problems:

void readData(char filename[], int list[], int size)
{
    ifstream fin;
    fin.open(filename);
    int value;
    for(inti= 0; i<30; i++) {
        fin>>value;
        if (value >= 0 && value <= 24)
           list[0]++;
        // ... other if else stuff
    }

    fin.close()
}

// print stuff
int main() {
   int list[size] = {0}; // initialize the list with 0.
   readData(*filename*, list, size);  // replace *filename* with the real file name
   print(list,size);

   return 0;

}

Answer 2

Your assumption is correct, you are getting random (undefined behavior as standard says) data as the array values are uninitialised. There are several ways to initialise static arrays:

You can initialise static array with curly braces

int list[10] = {1,2,3,4,5,6,7,8,9,10} // will put 1,2...10 to your array

If you specify less elements in the brackets the compiler will put 0 in there, therefore initialising your array to zeroes (which you might often do) can be achieved easily:

int list[10] = {}

If you want to use any number, you can easily use the fill function:

fill(std::begin(arr), std::end(arr), 25); //will fill all elements to 25

using std::end for static array is convenient, because sometimes you cannot exactly say how many elements are there. You can define array with 10 elements as before, or you can have a curly brackets at the definition and then let the compiler to count the elements for you

int list[] = {32,5,0,10};

In C (and so in C++ using C style code) you can get this information in a bit hacky way:

int arraySize = sizeof(list)/sizeof(list[0]);

In your case it might be even more convenient to use C++ std::array rather than plain old static array. It is a simple wrapper around static array with few good methods and overloaded operators.

Regarding your assignment I believe when programming in C++ you should not pass char pointers, rather references to string, since C++ provides these higher level classes that are generally safer and more convenient to use. I have written my solution using some C++ features.

#include <iostream>
#include <fstream>
#include <array>

constexpr int arraySize = 8;
constexpr int maxNumber = 200;
using arrayType = std::array<int, arraySize>;

bool readData(const std::string &fileName, arrayType &results, count int numCount){
    results.fill(0);

    std::ifstream inFile(fileName, std::ifstream::in);
    if (!inFile.is_open())
        return false;

    for(int i = 0; i < numCount; i++){
        int val;
        if(!(inFile >> val) || val < 0 || val > maxNumber)
            return false;
        results[std::min(maxNumber-1,val)/25]++; //Since 200 is included and the range contains 201 numbers
    }

    return true;
}

void print(const arrayType &count){
    std::cout << "Range" << '\t' << "# of Students" << std::endl;

    const auto printLine = [](const int minVal, const int maxVal, const int count){
      std::cout << minVal << "-" << maxVal << '\t' << count << std::endl;  
    };

    const int segmentSize = maxNumber/arraySize;
    for(int i = 0; i < arraySize-1; i++){
        printLine(i*segmentSize, (i+1)*segmentSize-1, count[i]);
    }
    printLine((arraySize-1)*segmentSize, maxNumber, count[arraySize-1]);
}

int main()
{
    constexpr int numberCount = 30;
    std::string fileName = R"(/mnt/d/Prog/C++/in2.txt)";
    std::array<int,arraySize> frequences;

    readData(fileName, frequences, numberCount);
    print(frequences);

    return 0;
}

Beside different language features used I also tried to remove the part with long list of if statements as it can be written with one line and such way follow the DRY principle, which is a good things to follow (less bugs, shorter code, lower chance of bugs in future as someone changes the code not at all places).