C program to find each of the indices of the maximum element in an array (there are multiple elements with the same max value in different positions)

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What I want: Suppose there is an array of some size. First, I want to find the maximum element in the array, and if there is more than one element with the same maximum value, then I want to store each of the indices of the maximum element without altering the ordering of the elements in the array.

Example: Let's say there are two classes: class A & class B. In an exam, each student of class A scored differently from the others, so we could easily find the maximum score and who scored it in the said exam. However, in the same exam, two students from class B scored the maximum score. Now, we have to identify those two students (with the help of their roll numbers, say) and the score they scored. I don't know how to do that!

What I know: I know how to code a C program to find the maximum value in an array with its index value. But, if there are multiple elements with the same max value in different positions in an array, then I think I would need a new array to store the indices of the max element. Once I achieve that, I could easily find the max value with the aid of any of the stored indices.

My attempt:

#include<stdio.h>
int main(void)
{
    size_t arr[5] = {2,8,10,7,10};
    size_t i,j,max = arr[0];
    size_t indices[5]={0};
    for(j = 0;j < 5; ++j)
    {
        for(i = 0;i < 5; ++i)
        {
            if(arr[i] >= max)
                indices[j] = i;
        }
        printf("Max Value: %zu ----> Position: %zu\n",arr[indices[j]],indices[j]+1);
    }
    return 0;
}

The output it generates:

Max Value: 10 ----> Position: 5
Max Value: 10 ----> Position: 5
Max Value: 10 ----> Position: 5
Max Value: 10 ----> Position: 5
Max Value: 10 ----> Position: 5

(I knew that it won't work, and it didn't as well. The index 3 is getting lost in the process!)

The desired output should read something like this:

Max Value: 10 ----> Position: 3
Max Value: 10 ----> Position: 5

Please suggest how I can code a C program that can perform the desired task.

c arrays
3 Answers

First thing, you are never overwritting max variable, so you are comparing each value on the array against arr[0], in this case 2.

Once fixed this you have multiple solutions for your problem.

Easiest one (though not the most efficient): iterate once to get the maximum value, and iterate again to get each occurence of that value.

Alternative: iterate only once. If arr[i] < max do nothing. If arr[i] == max store its index. If arr[i] > max update max, clear indices list and store current index.

Also, take care when storing indices, as 0 represents the first element of an array and should not be used as "empty" value.

Edit:

To clarify last sentence, keep in mind that in C, arrays are indexed from 0 to SIZE-1, instead of 1 to SIZE. Imagine that you make a variable to store an array index, let's call it index, and you initialize it like:

int index = 0;

Even though 0 is usually a neutral value, in this case means "the first element of the array". This doesn't necessarily mean that your code will break, but may lead to some tricky errors and poor clarity of the code.

Since indexes are always 0 or higher, using -1 as neutral value can be a good idea.

user_195622580

First loop to find the maximum element, then loop to find positions having value equal to that maximum.

#include<stdio.h>
int main(void)
{
    size_t arr[5] = {2,8,10,7,10};
    size_t max = arr[0];
    size_t indices[5]={0};
    
    for(size_t i = 0; i < 5; ++i) if(arr[i] > max) max = arr[i];
    
    size_t top = 0; // top contains count of elements with val = max
    for(size_t j = 0; j < 5; ++j)
    {
        if(arr[j]==max){
            indices[top] = j;
            printf("Max Value: %zu ----> Position: %zu\n",arr[indices[top]],indices[top]+1);
            top++;
        }
    }
    return 0;
}
user_144600510

The following code should produce the results you desire:

#include <stdio.h>

int main(){

int j, i, max, element, maxElement = 0;
int arr[5] = {2,8,10,7,10};
for(i = 0;i < 5; i++){ // loop to find highest index
    element = arr[i];
    if(element > max)max = element;} // highest index stored in 'max'

for(j = 0; j < 5; j++){ // second loop to find instances where 'max' is matched
    maxElement = arr[j];
    if(arr[j] == max)
    printf("arr[%d] has max of %d\n",j,max);
    }
return 0;

}

This creates the output:

arr[2] has max of 10

arr[4] has max of 10

user_111984820
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