Is there anyway to decide the amount of array while running your code?-CodePudding

I'm trying to write a beginner c code when you get unknown amount of integers from user and process them. Since it's is not valid to initialize a array of integers without deciding the size, i wonder if i could do some trick to make my code more efficient. Currently I'm initializing an array with size of 999 and ask user not to go beyond this border. example code:

#include <stdio.h>
#include <stdlib.h>
int main()
{
int a[999];
printf("After entering numbers enter -1 to stop: \n");
    for(int i=0;i<999;i  )
    {
        printf("Enter number(%d): ",i 1);
        scanf("%d",&a[i]);
        if(a[i]==-1) break;
        if(i==998)
        {
            printf("Max limit reached.");
            break;
        }
    }
    //some code for processing the numbers
}

CodePudding user response：

2 options: a) ask the user how many items and use a Variable Length Array (added to Standard C in C99; made optional in C11) ... or b) use dynamic memory and keep growing the array.

option a)

printf("enter number of items: ");
fflush(stdout);
char input[100];
fgets(input, sizeof input, stdin); // error checking ommited
size_t n = strtol(input, 0, 10); // error checking ommited
int array[n]; // VLA
// use array

option b)

size_t reserved = 5; used = 0;
int *array = malloc(reserved * sizeof *array); // start with space for 5 items
for (;;) {
    if (used == reserved) {
        // grow array
        reserved *= 2;
        int *tmp = realloc(array, reserved * sizeof *tmp);
        if (!tmp) exit(EXIT_FAILURE); // not enough memory
        array = tmp;
    }
    array[used  ] = getint(); // get integer from user/file/network/...
    // find way to leave for loop
}
// optionaly shrink the array
if (used < reserved) {
    int *tmp = realloc(array, used * sizeof *tmp);
    if (!tmp) exit(EXIT_FAILURE);
    array = tmp;
}
// use array
free(array);

Option b) is preferred if the number of items can be large so as to not cause a stack overflow.

CodePudding user response：

One sensible option is to forget about scanf and use command-line arguments instead. Faster, potentially safer (if you add an upper-bounds check), less memory consumed.

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

int main (int argc, char* argv[])
{
  if(argc < 2) // or if(argc < 2 || argc > SOME_UPPER_LIMIT)
    return 0;

  size_t size = argc-1;
  int input [size];

  for(size_t i=0; i<size; i  )
  {
    input[i] = strtol(argv[i 1],NULL,10);
    printf("%d ", input[i]);
  }
}

Execute as for example: myprog 12 34 56 78 -> Output 12 34 56 78.

CodePudding user response：

you can use this also :

#define MAXSIZE 999

void getNums(int nums[]){
    int i = 0, temp;
    char c;
    while(((c=getchar())!='\n') && (i < MAXSIZE-1)){
        temp = c - '0';
        nums[i] = temp;
        i  ;
    }
    nums[i] = '\0';
}

int main(){
    int nums[MAXSIZE];
    getNums(nums);
    return 0;
}