wrong output in sort + find bigges in Array

Question

Today I created a program which have 3 Functions:

sortArray(array,length);
removeDuplicateInArray(array, length);
max = findMax(array, length);

The program works fine but, if I run it more times let say, three times, the output is only one OK, other two is different and i think that somehow has to do with the length of the array in findMax function, because i remove duplicates and the array does not have the same size . I'm not sure if there is the problem.

The program is this:

#include<stdio.h>

void sortArray(int *array, int length){
    int i, j, k, temp;

    for(i=0;i<length-1;i++){
        for(k=0;k<length-i-1;k++){
            if(array[k]<array[k+1]){
                temp=array[k];
                array[k]=array[k+1];
                array[k+1]=temp;
            }
        }
    }

    for(j=0;j<length;j++){
        printf("%d ",array[j]);
    }
    printf("\n\n");
}

void removeDuplicateInArray(int *array, int length){
    int i,j,k;

    for (i = 0; i < length; i++) {
      for (j = i + 1; j < length;j++) {
         if (array[j] == array[i]) {
            for (k = j; k < length; k++) {
               array[k] = array[k + 1];
            }
            length--;
         }else{
            j++;
         }
      }
   }

   for (i = 0; i < length; i++) {
      printf("%d ", array[i]);
   }
   printf("\n\n");
}

int findMax(int *array, int length){
    int i;
    int max = array[0];

    for(i=1;i<length;i++){
        if(max == array[i]){
            continue;
        }

        if(max<array[i]){
            max = array[i];
        }
    }
    return max;
}

int main(void){
    int array[] = {-9,-7,-3,-1,9,7,3,1,-8,-6,-4,-10,-2,8,6,4,2,5,-5,-10};
    int length = sizeof array / sizeof array[0];
    int max;

    sortArray(array,length);
    removeDuplicateInArray(array, length);
    max = findMax(array, length);

    printf("Max = %d\n", max);
    return 0;
}

and the output is:

michi@michi-laptop:~$ ./program 
9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -10

9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 

Max = 9
michi@michi-laptop:~$ ./program 
9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -10 

9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 

Max = 9
michi@michi-laptop:~$ ./program 
9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -10 

9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 

Max = 2034093120
michi@michi-laptop:~$ ./program 
9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -10 

9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 

Max = 912874208
michi@michi-laptop:~$ ./program 
9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -10 

9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 

Max = 1269451840
michi@michi-laptop:~$ ./program 
9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -10 

9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 

Max = 1946221408
michi@michi-laptop:~$ ./program 
9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -10 

9 8 7 6 5 4 3 2 1 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 

Max = 9

The output should be 9, but the output is not always 9

instead of passing the array length as a parameter, why not get the length of the array in each function? — devlin carnate
– devlin carnate, Commented Aug 25, 2015 at 17:44
@devlin carnate and how do you get the length of that array if i pass it as pointer ? — Michi
– Michi, Commented Aug 25, 2015 at 17:45
in function: remove....array() this line: 'array[k] = array[k + 1];'will access beyond the upper boundary of the array. This is undefined behaviour and can lead to a seg fault event. — user3629249
– user3629249, Commented Aug 25, 2015 at 20:01
this line: 'if(array[k]<array[k+1]){' in the sortarray() function is not correct. It should be: 'if(array[k] > array[k+1]){' to sort the array ascending — user3629249
– user3629249, Commented Aug 25, 2015 at 20:02

M Oehm · Accepted Answer · 2015-08-25 18:00:30Z

7

The removeDuplicateInArray function changes the length of the array, but the calling function, main in your case, doesn't know about the new length.

You can either return the new length from the function:

int removeDuplicateInArray(int *array, int length)
{
    // code as above

    return length;
}

and call it like this:

length = removeDuplicateInArray(array, length);

or you can pass the length as a pointer, which will reflect the canges:

void removeDuplicateInArray(int *array, int *plength) ...
{
    int length = *plength;

    // use and modify length as above

    *plength = length;
}

and call it like this:

removeDuplicateInArray(array, &length);

I prefer the second variant, because it doesn't allow you to accidentally forget about the return value.

The garbage value you see is shifted in from beyond the array's bounds, because you loop up to k < length and access athe element at index k + 1, which might be length, which is one element beyond the array.

edited Aug 25, 2015 at 18:00

answered Aug 25, 2015 at 17:50

M Oehm

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9 Comments

The Paramagnetic Croissant Over a year ago

I don't think you need those voids before the function name when you call the function.

Michi Over a year ago

@M Oehm this means that if i use a pointer (int *lenght) i will have to use cast "error: comparison between pointer and integer [-Werror]|"

M Oehm Over a year ago

@TheParamagneticCroissant: Oh, I've left them there for the compiler to find them. (Fixed, thanks!)

pm100 Over a year ago

you could return the new length

M Oehm Over a year ago

@Michi: No, you won't need casts. But you must use *length inside the function where you usually would use just length.

|

Alex Lop. · Accepted Answer · 2015-08-25 17:51:00Z

3

In removeDuplicateInArray pass the length by pointer so it will have the proper value later in the main void removeDuplicateInArray(int *array, int *length).

answered Aug 25, 2015 at 17:51

Alex Lop.

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