Segmentation error when trying to access element of a multidimensional array in C++

Question

I have a two dimensional array and a one dimensional vector. My task is to write a program, that assigns to the i-th component of the vector

the first element of the i-th row of the matrix, if there is at least one negative element in that row,
the last element of the i-th row, if there isn't. Here is what I've done.

#include <iostream>
#include <cmath>
using namespace std;
    
int main() {
    double matrix[4][4] = {{1.3, -5.0, 4.4, 3.0}, {7.0, 4.0, 2.01, 1.0}, {7.0, 12.3, -8.7, 9.0}, {4.0, 1.0, 33.0, 63.3}};
    
    double vector[4];
    for(int i = 0; i < 4; i++) {
            bool rowHasNegNum = false;
            for(int j = 0; j < 4; i++) {
                if (matrix[i][j] < 0) {
                    rowHasNegNum = true;
                }
            }
            if(rowHasNegNum) {
                vector[i] = matrix[i][0];
            } else {
                vector[i] = matrix[i][3];
            }
    }
    

        
    for(int i=0; i<4; i++) {
        cout<<vector[i]<<"\t";
    }
    cout<<endl;
    
    return 0;
}

But when I compile the program, I get the following error. What's the problem?

Segmentation fault (core dumped)

unrelated, but using vector as a variable name is a terrible idea. — Mansoor
– Mansoor, Commented Feb 4, 2021 at 9:32
@KamilCuk thank you so much, I corrected it and my program worked as expected :) — Arman Grigoryan
– Arman Grigoryan, Commented Feb 4, 2021 at 9:35
@M.A strongly disagreed. It can be an entirely appropriate variable name. — Konrad Rudolph
– Konrad Rudolph, Commented Feb 4, 2021 at 10:32
@M.A The issue in that case is unambiguously the using namespace std, not the use of a perfectly sensible name. — Konrad Rudolph
– Konrad Rudolph, Commented Feb 4, 2021 at 11:22

D-RAJ · Accepted Answer · 2021-02-04 09:35:10Z

4

The fault is here for (int j = 0; j < 4; i++). Your not incrementing j, instead your incrementing i. And since j is not getting incremented, i gets incremented passing the bounds of the array. This is the reason for the segmentation fault.

The fix is simple, just swap i with j here,

for (int j = 0; j < 4; j++)

answered Feb 4, 2021 at 9:35

D-RAJ

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Comments

Vlad from Moscow · Accepted Answer · 2021-02-04 10:17:13Z

For starters neither declaration from this header

#include <cmath>

is used in the program. So remove it.

Do not use magic numbers like 4 or 3 in these statements

for(int i = 0; i < 4; i++)

ector[i] = matrix[i][3];

Instead use named constants.

In this for loop

 for(int j = 0; j < 4; i++) {
                       ^^^

there is a typo. You need to write

 for(int j = 0; j < 4; j++) {
                       ^^^

This for loop

        bool rowHasNegNum = false;
        for(int j = 0; j < 4; j++) {
            if (matrix[i][j] < 0) {
                rowHasNegNum = true;
            }
        }

should be interrupted as soon as a negative number is found.

Also instead of this for loop you could use the standard algorithm std::any_of.

In this case your program could look the following way

#include <iostream>
#include <functional>
#include<iterator>
#include<algorithm>

int main() 
{
    const size_t N = 4;
    double matrix[N][N] = 
    {
        { 1.3, -5.0,  4.4,  3.0 }, 
        { 7.0,  4.0,  2.01, 1.0 }, 
        { 7.0, 12.3, -8.7,  9.0 }, 
        { 4.0,  1.0, 33.0, 63.3 }
    };
    
    double vector[N];
    
    size_t i = 0;
    
    for ( const auto &row :matrix )
    {
        using std::placeholders::_1;
        if ( std::any_of( std::begin( row ), std::end( row ), 
                          std::bind( std::less<>( ), _1, 0 ) ) )
        {
            vector[i++] = row[0];
        }
        else
        {
            vector[i++] = row[N-1];
        }
    }
    
    for ( const auto &item : vector )
    {
        std::cout << item << ' ';
    }
    std::cout << '\n';
    
    return 0;
}

The program output is

1.3 1 7 63.3

If to use only loops then the program can look the following way

#include <iostream>

int main() 
{
    const size_t N = 4;
    double matrix[N][N] = 
    {
        { 1.3, -5.0,  4.4,  3.0 }, 
        { 7.0,  4.0,  2.01, 1.0 }, 
        { 7.0, 12.3, -8.7,  9.0 }, 
        { 4.0,  1.0, 33.0, 63.3 }
    };
    
    double vector[N];
    
    for ( size_t i = 0; i < N; i++ )
    {
        bool rowHasNegNum = false;  
        
        for ( size_t j = 0; not rowHasNegNum && j < N; j++ )
        {
            rowHasNegNum = matrix[i][j] < 0;
        }
        
        vector[i] = rowHasNegNum ? matrix[i][0] : matrix[i][N-1];       
    }
    
    for ( const auto &item : vector )
    {
        std::cout << item << ' ';
    }
    std::cout << '\n';
    
    return 0;
}

0RR · Accepted Answer · 2021-02-04 11:38:18Z

3

 for(int j = 0; j < 4; i++) {

You are incrementing the wrong variable here. A compiler warning picked this up, you should make sure you have warnings enabled.

And by incrementing i in this loop, you are accessing values outside the array resulting in the segmentation fault.

edited Feb 4, 2021 at 11:38

answered Feb 4, 2021 at 9:33

0RR

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2 Comments

Arman Grigoryan Over a year ago

Thanks, my program worked. How can I enable warnings? I'm using an online compiler

0RR Over a year ago

@ArmanGrigoryan depends on which you use, you should be able to set the compile flags somewhere where you can add -Wall which roughly stands for warnings all

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