Check for correct input via scanf - c

Question

I have an integer 'n' that is responsible for amount of numbers to enter later on.
I need to check for any incorrect inputs here.
The first scanf for 'n' works fine, but the second has some flaws.
The case is following:
n = 3 (e.g.) - doesn't matter
Then I want to scanf: 1 2 3 4
It will be scanned first 3 values but I need to throw out an error.
Same goes with: 1 2 3.5 - (last number is float but it still reads 3)
and with a char: 1 2 3g
if (scanf("%d", n) == 1 && getchar() == '\n') {
if (n > NMAX || n < 0) {
error = 1;
return;
}
for (int i = 0; i < n; i++) {
if (scanf("%d", p) == 1) {
The ONLY correct input is the exact same amount of integer numbers (equals to 'n') to scan in a for loop.
UPD: I can use only <stdio.h> library as well as only scanf.

Answer 1

Create a helper function to read an int and validate it.
The key is that to validate input, form a helper function that you can improve as needed, like using fgets() rather than scanf() once the poor "only scanf" requirement is removed.
// Sample
// Return 1 on success
// Return EOF on end-of-file
// Else return 0
int read_int(int *i, int min, int max, int line) {
long long num;
int result = scan("%18lld", &num); // Accept many out of int range input
if (result == EOF) return result; // or if (result < 0)
if (line) {
// consume the rest of the line
int ch;
while ((ch = getchar()) != '\n' && ch != EOF) {
if (!isspace(ch)) { // or `ch != ' ' && `ch != '\t' && ...
result == 0;
}
}
}
if (result == 1) {
if (num < min || num > max) {
return 0;
}
*i = (int) num;
}
return result;
}
Sample usage
if (read_int(&n, 0, NMAX, 1) != 1) {
error = 1;
return;
}
for (int i = 0; i < n; i++) {
if (read_int(&p, INT_MIN, INT_MAX, i + 1 == n) != 1) {
error = 1;
return;
}
...
Note: read_int() does not catch all errors, just many of them. Easy enough to improve, once all OP's limitations and goals are known.

Answer 2

Try using scansets with scanf. %1[\n] will scan up to one character that is a newline. %99[ \t] will scan up to 99 characters that are space or tab. If the character is not a newline, space or tab, it is replaced in the input stream.
If scanf with %d can't scan an int, it will return 0. scanf could also return EOF.
fgets and parse with strtol or others is the better solution.
#include <stdio.h>
int main ( void) {
char space[100] = "";
char newline[2] = "";
int number = 0;
int count = 0;
int quantity = 3;
printf ( "enter %d integers\n", quantity);
while ( 1) {
if ( 1 == scanf ( "%d", &number)) {
++count;
}
else {
printf ( "could not parse an integer\n");
break;
}
scanf ( "%99[ \t]", space);
if ( 1 == scanf ( "%1[\n]", newline)) {
if ( count == quantity) {
printf ( "scanned %d integers\n", quantity);
break;
}
else if ( count > quantity) {
printf ( "too many integers\n");
break;
}
else printf ( "enter another integer\n");
}
}
return 0;
}

Answer 3

Check if this works
while(n-1)
{
scanf("%d ",p);
n-=1;
}
scanf("%d",p);
//After this you can scan again to check if there is anything extra in input
//and throw error accordingly