Find indexes of max numbers in array

Question

i have a program in c that finds the first and second max in an array, but i want to get the indexes of these elements. Here is my code:

#include <stdio.h>
int main(){
    int max1,max2,n;
    scanf("%d",&n);
    int a[n],i;
    int i_m1,i_m2;
    i_m1 = i_m2=0;
    for(i = 0;i < n;i++){
        scanf("%d",&a[i]);
    }
    max1 = max2 = 0;
    for(i = 0;i < n;i++){
        if(a[i]>max1){
            max1=a[i];
            //i_m1++;
        } else if(a[i]>max2 && a[i]<max1){
            max2=a[i];
            //i_m2++;
        }
    }
    printf("%d %d\n",max1,max2);
    //printf("%d %d\n",i_m1,i_m2);
    for(i = 0;i < n;i++){
        printf("%d ",a[i]);
    }
    return 0;
}

I first enter the number of elements in the array then the array. For example if i have an array

number of elements: 5
elements in array: 
3 4 2 5 1
indexes:
0 1 2 3 4
max1=5, max2=4
i_m1=3, i_m2=1

How can i get the indexes i_m1 and i_m2 ?

Answer 1

For starters even this if-else statement

for(i = 0;i < n;i++){
    if(a[i]>max1){
        max1=a[i];
        //i_m1++;
    } else if(a[i]>max2 && a[i]<max1){
        max2=a[i];
        //i_m2++;
    }
}

is wrong.

If the condition of the if statement

    if(a[i]>max1){

is true then you need also assign max1 to max2 if max1 is not equal to max2 before changing max1.

The approach can look the following way

if ( n < 2 )
{
    i_m1 = 0;
    i_m2 = 0;
}
else
{
    if ( a[0] < a[1] )
    {
        i_m1 = 1;
        i_m2 = 0;
    }
    else
    {
        i_m1 = 0;
        i_m2 = 1;
    }

    for ( i = 2; i < n; i++ )
    {
        if ( a[i_m1] < a[i] )
        {
            if ( a[i_m2] < a[i_m1] ) i_m2 = i_m1;
            i_m1= i;
        }
        else if ( a[i_m2] < a[i] )
        {
            i_m2 = i;
        }
    }
}

printf( "max1 = %d max2 = %d\n", a[i_m1], a[i_m2] );
printf( "i_m1 = %d i_m2 = %d\n", i_m1, i_m2 );        

Here is a demonstration program.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main( void )
{
    enum { N = 10 };
    int a[N];
    int n = N;
    
    srand( ( unsigned int )time( NULL ) );
    
    for ( int i = 0; i < n; i++ )
    {
        a[i] = rand() % N;
    }

    for ( int i = 0; i < n; i++ )
    {
        printf( "%d ", a[i] );
    }
    
    putchar( '\n' );
    
    //  Below the algorithm for a variable length array
    
    int i_m1 = 0, i_m2 = 0;

    if ( !( n < 2 ) )
    {
        if ( a[0] < a[1] )
        {
            i_m1 = 1;
            i_m2 = 0;
        }
        else
        {
            i_m1 = 0;
            i_m2 = 1;
        }

        for ( int i = 2; i < n; i++ )
        {
            if ( a[i_m1] < a[i] )
            {
                if ( a[i_m2] < a[i_m1] ) i_m2 = i_m1;
                i_m1= i;
            }
            else if ( a[i_m2] < a[i] )
            {
                i_m2 = i;
            }
        }
    }

    printf( "max1 = %d max2 = %d\n", a[i_m1], a[i_m2] );
    printf( "i_m1 = %d i_m2 = %d\n", i_m1, i_m2 );      
}

Its output might .look like

0 3 5 0 3 8 8 2 5 6 
max1 = 8 max2 = 8
i_m1 = 5 i_m2 = 6

Answer 2

You're thinking too hard about what to do with i as evidenced by the commented out i_m1++ and i_m2++.

If max1=a[i]; then it follows that i_m1 = i; and same for max2/i_m2.

Answer 3

Also logic error.

Once a new max1 found, the old value should become the new max2.

    if(a[i]>max1){
        max2 = max1; // add
        max1 = a[i];
    } else if(a[i]>max2 && a[i]<max1){
        max2=a[i];
    }