为什么我得到这个 output？

Question

#include <stdio.h>

#define MAX_SIZE 1000     // Maximum array size 

int main() {
    int arr[MAX_SIZE], size, i;
    int max1, max2;
    scanf("%d", &size);
    max1 = max2 = -99999;
    /* Input array elements */ 
    for (i = 0; i < size; i++) {
        scanf("%d", &arr[i]);
    }
    /*
     * Check for first largest and second
     */
    for (i = 0; i < size; i++) {
        if (arr[i] > max1) {
            max2 = max1;
            max1 = arr[i];
        } else
        if (arr[i] > max2 && arr[i] < max1) {
            max2 = arr[i];
        }
    }
    printf("%d", max2);
    return 0;
}

when I used test case 20 10 40 4 100 output is correct 40 , but when I use 1 2 3 4 5 then output is max2 = -99999 .

Can anyone please explain to me why I am not getting this? I traced but did not get why?

Answer 1

In your program, the first input is the size of the array. So, your inputs should actually be 5 20 10 40 4 100 and 5 1 2 3 4 5 (note the extra 5 in front) so they have the correct size. Your first input is correct because it read in 20 as the size and tried to read in 20 numbers, and there weren't enough numbers so it just read in 0 for the rest, and 40 was still the second-largest. For your second input, it read in 1 as the size, so there was no second-largest number, so it printed out -99999.

Answer 2

The output is somewhat consistent with the input, assuming the input comes from a text file:

• the first number read is the number of entries to handle.
• for the first case, size is read as 20 and the program tries to read 20 numbers into arr[0] through arr[19] . As you do not test the return value of scanf() , you do not detect the invalid or missing input past the last one in the file 100 . scanf() fails and leaves arr[i] unchanged from index 4 on. arr is unintiialized, so the contents is undefined and so is the behavior of the program. It so happens that there are no numbers in arr[4] through arr[19] that are larger than 40. So the second largest number is printed as 40 .
• for the second case, size is read as 1 and only a single number is read into arr[0] , the remaining input being ignored. The loop sets max1 = 2 and max2 = max1 , whose initial value is -99999 . Hence the output.

You should provide 5 20 10 40 4 100 and 5 1 2 3 4 5 as input to get the expected behavior consistently.

You should also be more defensive with user input and report unexpected cases:

#include <limits.h>
#include <stdio.h>

#define MAX_SIZE 1000     // Maximum array size 

int main() {
    int arr[MAX_SIZE], size, i;
    int max1, max2, has_max;
    if (scanf("%d", &size) != 1) {
        printf("missing count of numbers\n");
        return 1;
    }
    if (size < 0 || size > MAX_SIZE) {
        printf("invalid count of numbers: %i\n", size);
        return 1;
    }
    /* Input array elements */ 
    for (i = 0; i < size; i++) {
        if (scanf("%d", &arr[i]) != 1) {
            printf("invalid input: got %i numbers instead of %i\n", i, size);
            size = i;
            break;
        }
    }
    /*
     * Check for first largest and second
     */
    max1 = max2 = INT_MIN;
    has_max = 0;
    for (i = 0; i < size; i++) {
        if (arr[i] > max1) {
            max2 = max1;
            max1 = arr[i];
            has_max++;
        } else
        if (arr[i] > max2 && arr[i] < max1) {
            max2 = arr[i];
            has_max++;
        }
    }
    if (has_max < 2) {
        printf("no second highest value\n");
    } else {
        printf("%d\n", max2);
    }
    return 0;
}

Note the method to track if there are at least 2 different values in the array. Try and see if this improvement is sufficient for all cases...