How do I scan in numbers to an array from a file?

问题

Say I have a file with the numbers: 1 2 3 4 5 6 7 8 9 10 -1

I want to read in that file and store all the values of that file, stopping at the control variable -1.

So if I printed that array to another file it would look like: 1 2 3 4 5 6 7 8 9 10

This gives me all the numbers, but it has -1 included. How can I drop off the -1?

   int arr[100];
   int n;
   while (scanf("%d",&arr[n]) > 0)
       n++;

回答1:

You cannot add the > 0 condition: this would give you undefined behavior. In order to ignore the negative one, you could add a separate check inside the loop, after reading the number, like this:

while (scanf("%d", &arr[n]) != EOF) {
    if (arr[n] > 0) {
        n++;
    }
}

Since the array arr has fixed size, it would be a good idea to guard against overruns, like this:

while (n < 100 && scanf("%d", &arr[n]) != EOF) {
    if (arr[n] > 0) {
        n++;
    }
}

This loop would stop if you reach 100 before the file ends. Note that you also need to initialize n to zero before the loop to avoid undefined behavior.

Here is a demo on ideone.

回答2:

int arr[100];
int n;
int temp;
do
{
    scanf("%d",&temp);
    if(temp != -1)
    {
        arr[n] = temp;
    }
   n++;
}while(temp != -1 && n < 100);

来源：https://stackoverflow.com/questions/19914298/how-do-i-scan-in-numbers-to-an-array-from-a-file