How can I read a varying amount of integer input in C?

Question

I will be getting three lines of input. The first line will give me 2 integers and the third line will give me 1 integer. But the second line can give me any number of integers ranging between 1 to 100. For example, the input could be:

2 1
5 6 1 9 2
10

or could be:

10 4
5 6
9

I can read the second line of integer input into an integer array for a fixed number of integers, but cannot do so for a varying number of integers. I suppose, in this case, I should use a while loop which will break when scanf() finds a newline. How do I code that?

somewhat related http://stackoverflow.com/questions/15089140/c-how-to-read-inputs?rq=1 — Fredrik Pihl, Nov 06 '15 at 14:19
If there is a reasonably small upper bound on the number of ints on a line you could just declare an array big enough to accommodate them all. `int numbers[100];` uses a negligible amount of memory. — John Coleman, Nov 06 '15 at 14:21
http://stackoverflow.com/questions/30125445/multiple-inputs-by-user-dynamically-at-runtime#comment48361721_30125445 — Spikatrix, Nov 06 '15 at 14:29

score 3 · Accepted Answer · edited May 23 '17 at 11:59

3

Read the line into a buffer (@John Coleman) using using fgets() or getline().
Parse the string looking for whitespace that may contain a '\n', exit loop if found. Then call strtol() or sscanf() to read the 1 number. Check that function's return value for errors too.
Repeat above steps.

edited May 23 '17 at 11:59

Community

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answered Nov 06 '15 at 14:46

chux - Reinstate Monica

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Shabbir Khan · Answer 2 · 2015-11-08T15:40:59.210

1

I am actually a newbie in programming and am unaware of most of the functions. The only string functions I know of are strlen() and strcmp(). And my i\o function knowledge is limited to printf() and scanf().

Anyhow, I solved my problem in this way:

int a[101];
int i, num;
char ch;

for (i = 0; i < 101; i++)
    a[i] = 0;

while (1)
{
    scanf("%d%c", &num, &ch);
    i = num;
    a[i] = num;
    if (ch == '\n')
        break;
}

This works! The value of num had to be equal to the value i because my program needed it.

edited Nov 08 '15 at 15:40

answered Nov 06 '15 at 15:00

Shabbir Khan

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Notes: 1) `for (i = 0; i < 101; i++) a[i] = 0;` writes outside `a[]` which is _undefined behavior_ 2) `scanf("%d%c", &num, &ch);` does not check the result of `scanf()`, so code does not know the values of `num`, `ch` are valid. – chux - Reinstate Monica Nov 06 '15 at 15:19
@chux Thank you for pointing those things out. I fixed the first thing you mentioned. Next, I didn't check if `num` and `ch` are valid or not because it was competitive programming problem and we were only given integer inputs. However, your insight will be useful in real life problems. I appreciate that. – Shabbir Khan Nov 06 '15 at 15:30
@chux I fixed it according to your suggestion. – Shabbir Khan Nov 08 '15 at 15:41

How can I read a varying amount of integer input in C?

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