Loop and sum giving unexpexted result - c

Question

This is my code
with the help of pointers i try to find and compare the sum of the main and second diagonal of a 5x5 matrix.The numbers are randomly generated
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main {
int i, s1, s2, j, a[5][5];
srand (time (NULL));
printf ("The matrix is:\n");
for (i = 0; i < 5; i++) {
printf ("\n\n");
for (j = 0; j < 5; j++) {
*(*(a + i) + j) = rand ();
printf ("%d ", *(*(a + i) + j));
}
}
for (i = 0; i < 5; i++) {
s1 += a[i][i]; // main diagonal
s2 += a[i][4 - i]; // second diagonal
}
printf ("\n\nThe sum 1:%d\nThe sum 2:%d", s1, s2);
if (s1 == s2) {
printf ("They are the same");
}
return 0;
}
The problem is that the sum of the main diagonal is bigger by 51 than it should be

Answer 1

s1 and s2 both are uninitialised. You need to initialize them with 0 before using in statements s1+=a[i][i]; and s2+=a[i][4-i];.
int i, s1 = 0, s2 = 0, j, a[5][5];
I would also suggest to use
a[i][j] = rand();
instead of
*(*(a+i)+j)=rand();
to access array members.

Answer 2

You have not initialised s1,s2 to zero
s1+=a[i][i]; // main diagonal
s2+=a[i][4-i]; // second diagonal
Prior to these statements

Answer 3

You could do as follow:
*(*(a+i)+j)=rand(); ----> a[i][j]=rand()%100;
printf("%d ",*(*(a+i)+j)); -----> printf("%d ", a[i][j]);
add s1 = s2 = 0;
Then the following could work.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main()
{
int i,s1,s2,j,a[5][5];
srand(time(NULL));
printf("The matrix is:\n");
for(i=0;i<5;i++)
{
printf("\n\n");
for(j=0;j<5;j++)
{
a[i][j]=rand()%100;
printf("%d ", a[i][j]);
}
}
s1 = s2 = 0;
for(i=0;i<5;i++)
{
s1+=a[i][i]; // main diagonal
s2+=a[i][4-i]; // second diagonal
}
printf("\n\nThe sum 1:%d\nThe sum 2:%d\n",s1,s2);
if (s1==s2)
{
printf("They are the same\n");
}
return 0;
}

Answer 4

For starters according to the C Standard function main without parameters shall be declared like
int main( void )
Secondly it is better to use a named constant instead of magic numbers. So you could define for example named constant N for the magic number 5. In this case your program will be more flexible and can be modified easy.
For the sums it is better to use a more larger integer type as long long int. Otherwise an overflow can occur.
You should initialize variables near the point in the program where the variables are used. If your compiler supports C99 then you may declare and initialize variables where they are used.
You could limit the maximum value that elements of the matrix can have.
The program can look the following way.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define N 5
#define VALUE_MAX 100
int main(void)
{
int a[N][N];
size_t i, j;
long long int s1, s2;
srand( ( unsigned int )time( NULL ) );
printf( "The matrix is:\n" );
for ( i = 0; i < N; i++ )
{
printf ( "\n\n" );
for ( j = 0; j < N; j++ )
{
*( *( a + i ) + j ) = rand() % VALUE_MAX;
printf ( "%d ", *( *( a + i ) + j ) );
}
}
s1 = 0ll;
s2 = 0ll;
for ( i = 0; i < N; i++ )
{
s1 += a[i][i]; // main diagonal
s2 += a[i][N - i - 1]; // second diagonal
}
printf( "\n\nThe sum 1: %lld\nThe sum 2: %lld\n", s1, s2 );
if ( s1 == s2 )
{
puts( "They are the same" );
}
return 0;
}
Its output might look like
The matrix is:
55 81 79 14 75
89 70 41 27 73
94 14 79 85 45
66 92 27 90 67
14 85 79 53 83
The sum 1: 377
The sum 2: 287
Or if your compiler supports C99 then the program can look like
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define N 5
#define VALUE_MAX 100
int main(void)
{
int a[N][N];
srand( ( unsigned int )time( NULL ) );
printf( "The matrix is:\n" );
for ( size_t i = 0; i < N; i++ )
{
printf ( "\n\n" );
for ( size_t j = 0; j < N; j++ )
{
*( *( a + i ) + j ) = rand() % VALUE_MAX;
printf ( "%d ", *( *( a + i ) + j ) );
}
}
long long int s1 = 0ll;
long long int s2 = 0ll;
for ( size_t i = 0; i < N; i++ )
{
s1 += a[i][i]; // main diagonal
s2 += a[i][N - i - 1]; // second diagonal
}
printf( "\n\nThe sum 1: %lld\nThe sum 2: %lld\n", s1, s2 );
if ( s1 == s2 )
{
puts( "They are the same" );
}
return 0;
}