Keep Getting a Segmentation Fault On This? - c++

Question

I keep getting the segmentation fault (core dumped) on the code below. Any ideas on why this is happening. The code is designed to read numbers from a text document, convert them to integers, perform radix sort, and print out the array.
#include <cstdlib>
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <time.h>
#include <sstream>
using namespace std;
int getMax(int arr[], int n)
{
int max = arr[0];
for (int i = 1; i < n; i++)
if (arr[i] > max)
max = arr[i];
return max;
}
void countSort(int arr[], int n, int exp)
{
int output[n];
int i, count[10] = {0};
for (i = 0; i < n; i++)
count[(arr[i] / exp) % 10]++;
for (i = 1; i < 10; i++)
count[i] += count[i - 1];
for (i = n - 1; i >= 0; i--)
{
output[count[(arr[i] / exp) % 10] - 1] = arr[i];
count[(arr[i] / exp) % 10]--;
}
for (i = 0; i < n; i++)
arr[i] = output[i];
}
void radixsort(int arr[], int n)
{
clock_t clockStart;
clockStart = clock();
int m = getMax(arr, n);
for (int exp = 1; m / exp > 0; exp *= 10)
countSort(arr, n, exp);
cout << "\nTime taken by radix sort: " << (double)(clock() - clockStart) / CLOCKS_PER_SEC << endl;
}
int StrToInt(string sti)
{
int f;
stringstream ss(sti); //turn the string into a stream
ss >> f;
return f;
}
int main()
{
int arr[10000];
int i = 0;
int result;
string line = "";
ifstream myfile;
myfile.open("integers2.txt");
if(myfile.is_open())
{
while(!myfile.eof())
{
getline(myfile, line);
result = StrToInt(line);
arr[i] = result;
//cout<< arr[i] <<"\n";
i++;
}
}
int n = sizeof(arr)/sizeof(arr[0]);
radixsort(arr, n);
for (int i = 0; i < n; i++)
{
cout << arr[i] << "\n";
}
return 0;
}
Contents of the text file I am using for input:
1244
3455
6565
55
765
8768
687
879

Answer 1

Your program has undefined behavior, because it uses more entries of the array than you initialized with data. You pass the length of the entire array for n even though only a small portion of it, from 0 to i, has been initialized.
Change the code to use n in place of i in the reading loop, and pass that n unmodified to the sort function. This is going to fix the problem (demo).
int n = 0;
myfile.open("integers2.txt");
if(myfile.is_open()) {
while (myfile >> arr[n]) {
n++;
}
}
radixsort(arr, n);

Answer 2

Here's your working code:
#include <cstdlib>
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <time.h>
#include <sstream>
using namespace std;
int getMax(int arr[], int n)
{
int max = arr[0];
for (int i = 1; i < n; i++)
if (arr[i] > max)
max = arr[i];
return max;
}
void countSort(int arr[], int n, int exp)
{
int output[n];
int i, count[10] = {0};
for (i = 0; i < n; i++)
count[(arr[i] / exp) % 10]++;
for (i = 1; i < 10; i++)
count[i] += count[i - 1];
for (i = n - 1; i >= 0; i--)
{
output[count[(arr[i] / exp) % 10] - 1] = arr[i];
count[(arr[i] / exp) % 10]--;
}
for (i = 0; i < n; i++)
arr[i] = output[i];
}
void radixsort(int arr[], int n)
{
clock_t clockStart;
clockStart = clock();
int m = getMax(arr, n);
for (int exp = 1; m / exp > 0; exp *= 10)
countSort(arr, n, exp);
cout << "\nTime taken by radix sort: " << (double)(clock() - clockStart) / CLOCKS_PER_SEC << endl;
}
int StrToInt(string sti)
{
int f;
stringstream ss(sti); //turn the string into a stream
ss >> f;
return f;
}
int main()
{
const int MAX_SIZE = 10;
int arr[ MAX_SIZE ] = { 0 };
//int i = 0;
//int result = 0;
string line = "";
ifstream myfile;
myfile.open("integers2.txt");
if(!myfile.is_open())
{
cerr << "Could not open file!\n";
return -1;
}
cout << "Reading integers...\n";
int index = 0;
//while ( index < SIZE && getline( myfile, line ) )
while ( index < MAX_SIZE && myfile >> arr[ index ] )
{
//getline( myfile, line );
//result = StrToInt( line );
//arr[index] = std::stoi( line );
cout << arr[index] <<"\n";
index++;
}
cout << "Sorting integers...\n";
//int n = sizeof(arr) / sizeof(arr[0]);
radixsort( arr, index );
for ( int i = 0; i < index; i++ )
{
cout << arr[i] << "\n";
}
return 0;
}
Some points:
Check std::stoi for string to integer conversion; BTW, you don't need to do that. Just read it directly like this: while ( file >> integer ).
Need to check if the file is open; return ERROR otherwise; in your case, the rest of the code was executing anyway even if the file was not open i.e. code after if ( myfile.open() ) { ... }
while( !myfile.eof() ) is bad practice. See: Why is iostream::eof inside a loop condition considered wrong?
You don't need to calculate the size like int n = sizeof(arr) / sizeof(arr[0]); because you already know the size. Just use a const for this.
While reading from file, you also need to validate the maximum size of your array. You should read what the size allows you. Take care of out-of-bounds read / write errors.
Use <ctime> instead of <time.h>.