How to make this while loop finish on new line

Question

#include <stdio.h>

int main() {
    int lista[9999];
    int a;
    int i = 0;

    while (scanf(" %d", &a) == 1)
    {
        lista[i] = a;
        i++;
    }
    return 0;
}

I am expecting an input of integers and spaces in between them, I do NOT want to read this with one scanf because there will be some calculations done depending on the integer. How to make this loop finish on a new line? if (a == "\n") doesn't seem to work.

Answer 1

You cannot change the behavior of scanf() for %d on newline characters. There are alternatives:

• you can read the whole input line with fgets() (or getline() on POSIX systems) and parse the numbers on the line with strtol() or sscanf().

• you can read one integer at a time as a string with scanf("%[-+0-9]", buf) and convert them with atoi(), strtol() or sscanf().

• you can read one character at a time with getchar() and convert the numbers on the fly, stopping at the newline character.

Here is an example using scanf() and strtol() with format validation:

#include <errno.h>
#include <limits.h>
#include <stdio.h>

int main(void) {
    int lista[9999];
    char buf[20];
    int i = 0;
    while (i < 9999) {
        scanf("%*[ ]");  // skip spaces if any
        if (scanf("%19[-+0-9]", buf) != 1)
            break;
        errno = 0;
        char *p;
        long n = strtol(buf, &p, 10);
        if (p == buf || errno != 0 || *p != '\0'
        ||  n < INT_MIN || n > INT_MAX) {
            printf("invalid number: %s\n", buf);
            continue;
        }
        int a = (int)n;
        lista[i] = a;
        i++;
    }
    if (scanf("%1[\n]", buf) != 1) {
        // no trailing newline
        printf("invalid input\n");
    }
    return 0;
}

Here is an alternative using just getchar():

#include <stdio.h>

int main(void) {
    int lista[9999];
    int i = 0;
    int c = getchar();
    while (i < 9999) {
        while (c == ' ')
            c = getchar();
        int sign = c;
        if (c == '+' || c == '-')
            c = getchar();
        if (!isdigit(c))
            break;
        unsigned value = c - '0';
        while (isdigit(c = getchar()))
            value = value * 10 + (c - '0'); 
        a = (sign == '-') ? -value : value;
        lista[i] = a;
        i++;
    }
    if (c != '\n' && c != EOF) {
        // extra input on the line
        printf("invalid input\n");
    }
    return 0;
}