动态数组：使用C将数字分为偶数和奇数

Question

My program should first ask for the number of integers to read and dynamically allocate an array just big enough to hold the number of values I read. Then, I need to categorize these values into odd and even.

This is the code I've tried

#include <stdio.h> 
#include <stdlib.h>
int main(void){

//declare variables
int i; //loop counter
int count; //integer amount from user
int j = 0; int k = 0;

// read in integer count from user
printf("enter the amount of numbers: ");
scanf("%d", &count);

// declare pointer 
int *number = malloc(sizeof(int)*count);
int *evens = malloc(sizeof(int)*count);
int *odds = malloc(sizeof(int)*count);

// declare variable
//int odds_count = 0;
//int evens_count = 0;

//loop to read in numbers from user
for (i=0; i<count; i++){
    printf("enter number %02d: ",i+1);
    scanf("%d",(number+i));
    printf("you entered %d\n", *(number+i)); //--- entered values are correct here
    if (*(number+i)% 2 ==0){
        *(number+i) = *(evens+j);
        j++;
        //evens_count++;
    } else {
        *(number+i) = *(odds+k);
        k++;
    }
    printf("you entered %d\n", *(number+i));  //---entered values become 0
}
//print array elements
printf("\nEntered array elements are:\n");
for(i=count;i>0;i--)
{
    printf("%d ",*(number+i));
}
printf("\n");

// print out even numbers
printf("even numbers: ");
for (i=0;i<j;i++){
    printf("%5d",*(evens+i));
}
printf("\n");

// print out odd numbers
printf("odd numbers: ");
for (i=0;i<k;i++){
    printf("%5d",*(odds+i));
}
printf("\n");

return 0;
}

No matter what input I enter, the output only displays 0. Eg:

Input- 1, 2, 3
Output-
Evens: 0
Odds: 0 0

Please help me with this. Thanks in advance!

Answer 1

While getting the input from user, try putting an '&' symbol before your variable name. I think that should work.

scanf("%d",&(number+i));

Happy coding :)

Answer 2

Your biggest issues regarding assigning values occurs when you attempt to assign evens[j] and odds[j] to numbers[i] before the elements of either evens or odds have been initialized. Following your call to malloc for each evens and odds the block of memory allocated contains whatever garbage values happen to be in that memory region at the time of allocation. malloc does not initialize the content of the memory in any way and leave the values indeterminate. If you want to both allocate and zero all bytes, you can use calloc instead of malloc .

This problem occurs here:

    if (*(number+i)% 2 ==0){
        *(number+i) = *(evens+j);
        j++;
        //evens_count++;
    } else {
        *(number+i) = *(odds+k);
        k++;
    }

Look carefully, you check whether numbers[i] is even/odd with % 2 == 0 , but then attempt to overwrite the value at numbers[i] with either evens[j] or odds[k] -- this is backwards . The intent would be to assign the value in numbers[i] to either evens[j] or odds[k] , eg

    if (number[i] % 2 ==0){
        evens[j] = number[i];
        j++;
        //evens_count++;
    } else {
        odds[k] = number[i];
        k++;
    }

Next, your use of variables is jumbled. You don't need all the different i, j, k counters declared at the beginning of main() . For the past 20 years, since C99, you can declare the loop variable within the for loop declaration itself. This eliminates the chance that your use of your loop variable will conflict with another use of i, j, k somewhere else in the program.

You have count, odd_count & even_count , those are the only counters you need.

Further, you need to VALIDATE every User Input and every Allocation. Otherwise, you risk invoking Undefined Behavior with the slip of a keystroke or when (not "if") an allocation returns NULL . Validate every critical step.

Putting those measures to use, you can simplify your declaration to:

    int count,          /* integer amount from user     */
        odd_count = 0,  /* odd count (initialized zero) */
        even_count = 0, /* even count (initialized zero)*/
        *number,        /* declare pointers             */
        *evens,
        *odds; 

You can validate your input with:

    /* read in integer count from user (VALIDATE every input) */
    printf("enter the amount of numbers: ");
    if (scanf("%d", &count) != 1) {
        fputs ("error: invalid integer input.\n", stderr);
        return 1;
    }
    ...
    for (int i = 0; i < count; i++) {   /* loop count times for input */
        printf ("enter number %2d: ", i + 1);
        if (scanf ("%d", &number[i]) != 1) {    /* validate EVERY input */
            fputs ("error: invalid integer input.\n", stderr);
            return 1;
        }
        if (number[i] % 2 == 0)     /* assign, increment evens count */
            evens[even_count++] = number[i];
        else                        /* same for odds */
            odds[odd_count++] = number[i];
    }

And you can validate your allocations:

    /* allocate count integers each pointer (VALIDATE every allocation) */
    if ((number = malloc (count * sizeof *number)) == NULL) {
        perror ("malloc-number");
        return 1;
    }
    if ((evens = malloc (count * sizeof *evens)) == NULL) {
        perror ("malloc-evens");
        return 1;
    }
    if ((odds = malloc (count * sizeof *odds)) == NULL) {
        perror ("malloc-odds");
        return 1;
    }

Then it is simply a matter of looping i = 0; i < count i = 0; i < count to output the number , i = 0; i < even_count i = 0; i < even_count to output evens and finally i = 0; i < odd_count i = 0; i < odd_count to output odds , eg

    puts ("\n numbers\n--------");      /* output each array on its own */
    for (int i = 0; i < count; i++)
        printf ("%8d\n", number[i]);

    puts ("\n   evens\n--------");
    for (int i = 0; i < even_count; i++)
        printf ("%8d\n", evens[i]);

    puts ("\n    odds\n--------");
    for (int i = 0; i < odd_count; i++)
        printf ("%8d\n", odds[i]);

And Finally, you can output all 3 arrays at once with:

    /* output all arrays together */
    puts ("\nnumbers      even      odd\n-------- -------- --------");
    for (int i = 0; i < count; i++) {       /* loop printing number */
        printf ("%8d", number[i]);
        if (i < even_count) {               /* if i < even_count */
            printf (" %8d", evens[i]);      /* output evens */
            if (i < odd_count)              /* if odds too, output them */
                printf (" %8d\n", odds[i]);
            else
                putchar ('\n');             /* no more odds output '\n' */
        }
        else if (i < odd_count)             /* if only odds left, output */
            printf ("%18d\n", odds[i]);
        else
            putchar ('\n');
    }

At the end, don't forget to free the memory you allocate, eg

    free (number);  /* don't forget to free what you allocated */
    free (evens);
    free (odds);

Putting it altogether, you could do:

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

int main(void) {

    int count,          /* integer amount from user     */
        odd_count = 0,  /* odd count (initialized zero) */
        even_count = 0, /* even count (initialized zero)*/
        *number,        /* declare pointers             */
        *evens,
        *odds; 

    /* read in integer count from user (VALIDATE every input) */
    printf("enter the amount of numbers: ");
    if (scanf("%d", &count) != 1) {
        fputs ("error: invalid integer input.\n", stderr);
        return 1;
    }

    /* allocate count integers each pointer (VALIDATE every allocation) */
    if ((number = malloc (count * sizeof *number)) == NULL) {
        perror ("malloc-number");
        return 1;
    }
    if ((evens = malloc (count * sizeof *evens)) == NULL) {
        perror ("malloc-evens");
        return 1;
    }
    if ((odds = malloc (count * sizeof *odds)) == NULL) {
        perror ("malloc-odds");
        return 1;
    }

    for (int i = 0; i < count; i++) {   /* loop count times for input */
        printf ("enter number %2d: ", i + 1);
        if (scanf ("%d", &number[i]) != 1) {    /* validate EVERY input */
            fputs ("error: invalid integer input.\n", stderr);
            return 1;
        }
        if (number[i] % 2 == 0)     /* assign, increment evens count */
            evens[even_count++] = number[i];
        else                        /* same for odds */
            odds[odd_count++] = number[i];
    }

    puts ("\n numbers\n--------");      /* output each array on its own */
    for (int i = 0; i < count; i++)
        printf ("%8d\n", number[i]);

    puts ("\n   evens\n--------");
    for (int i = 0; i < even_count; i++)
        printf ("%8d\n", evens[i]);

    puts ("\n    odds\n--------");
    for (int i = 0; i < odd_count; i++)
        printf ("%8d\n", odds[i]);

    /* output all arrays together */
    puts ("\nnumbers      even      odd\n-------- -------- --------");
    for (int i = 0; i < count; i++) {       /* loop printing number */
        printf ("%8d", number[i]);
        if (i < even_count) {               /* if i < even_count */
            printf (" %8d", evens[i]);      /* output evens */
            if (i < odd_count)              /* if odds too, output them */
                printf (" %8d\n", odds[i]);
            else
                putchar ('\n');             /* no more odds output '\n' */
        }
        else if (i < odd_count)             /* if only odds left, output */
            printf ("%18d\n", odds[i]);
        else
            putchar ('\n');
    }

    free (number);  /* don't forget to free what you allocated */
    free (evens);
    free (odds);
}

Example Use/Output

$ ./bin/number_evens_odds
enter the amount of numbers: 10
enter number  1: 21
enter number  2: 22
enter number  3: 23
enter number  4: 24
enter number  5: 119
enter number  6: 121
enter number  7: 131
enter number  8: 140
enter number  9: 141
enter number 10: 143

 numbers
--------
      21
      22
      23
      24
     119
     121
     131
     140
     141
     143

   evens
--------
      22
      24
     140

    odds
--------
      21
      23
     119
     121
     131
     141
     143

numbers      even      odd
-------- -------- --------
      21       22       21
      22       24       23
      23      140      119
      24               121
     119               131
     121               141
     131               143
     140
     141
     143

Look things over and let me know if you have further questions.