how to get an unknown number of integers into array in c

Question

I need to write a program in c that gets integers as input from the user.

input example:

10 20 50 70

The user presses Enter and then the input is over.
I can't think of a condition to make it happen. I tried to write:

int grades[1000];
int i=0;

while(scanf("%d", &grades[i])!=EOF)
{
     i++;
}

It is not working.

perhaps read the documentation of scanf() instead of making assumptions about its return value. Also, do NOT use scanf(). Use fgets() along with strtol(). — The Paramagnetic Croissant
– The Paramagnetic Croissant, Commented Jan 6, 2015 at 18:14
No. This won't work. scanf() is going to block and wait for more numbers to come. Use fgets() instead to read the whole line, and then sscanf() to extract each number. — Filipe Gonçalves
– Filipe Gonçalves, Commented Jan 6, 2015 at 18:15
we have some restricting rules so we can't use most of the c functions. I found a solution int here, someone wrote a scanf using bitwise operations. did'nt fully understand that but it's short and works — LEARNER
– LEARNER, Commented Jan 7, 2015 at 17:39

Community · Accepted Answer · 2017-05-23 12:12:49Z

2

Reading a line of user input and then parsing is really the best approach as with @The Paramagnetic Croissant

If code can not pre-define an input buffer size or must parse the line while it comes in then using scanf("%d",... is OK. Non-elegant code occurs with finding the '\n'.

#define N 1000
int grades[N];
int i=0;

for (i=0; i<N; i++) {
  // Consume leading white-space, but not \n
  int ch;
  while ((ch == fgetc(stdin)) != '\n' && isspace(ch));

  // normal exit
  if (ch == '\n' || ch == EOF) break;

  ungetc(ch, stdin);
  if (1 != scanf("%d", &grades[i])) {
    // Non-numeric data
    break;
  }
  i++;
}

edited May 23, 2017 at 12:12

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answered Jan 6, 2015 at 19:14

chux

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2 Comments

Weather Vane Over a year ago

I tried something along these lines but am totally unskilled with scanf() due to a life(CPU)long aversion. Marked up.

chux Over a year ago

@Weather Vane I feel your pain. Although code can do this all with scanf(), it is ugly. Even here, code uses fgetc()/ungetc() for the finding \nemo part.

The Paramagnetic Croissant · Accepted Answer · 2015-01-06 18:24:03Z

1

If you need read an entire line, then read an entire line, simple as that. If you google "C read line", you will most probably end up reading the documentation of fgets(). Then you google "C convert string to integer", and you perceive that there exists a function called strtol() in the C standard library. Armed with these two weapons, and applying some logic, you can deduce something like this:

const size_t max_numbers = 1000; // however many

int numbers[max_numbers];
size_t index = 0;

char buf[LINE_MAX];
while (index < max_numbers && fgets(buf, sizeof buf, stdin)) {
    char *p = buf;
    char *end;
    while (index < max_numbers && *p && *p != '\n') {
        numbers[index++] = strtol(p, &end, 10);
        p = end;
    }
}

answered Jan 6, 2015 at 18:24

The Paramagnetic Croissant

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2 Comments

chux Over a year ago

After numbers[..., suggest adding some error tests like if (p == end) break; to deal with non-numeric input.

The Paramagnetic Croissant Over a year ago

@chux right. Also, one must reset and check errno, so using strtol() absolutely correctly is a bit ugly; still better than scanf() & co., though.

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