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I want to process user input as an integer, but it seems as though C has no way to get an int from stdin. Is there a function to do this? How would I go about getting an int from the user?

Jon Chasteen
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5 Answers5

24
#include <stdio.h>

int n;
scanf ("%d",&n);

See http://www.cplusplus.com/reference/clibrary/cstdio/scanf/

Marc W
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6

scanf() is the answer, but you should certainly check the return value since many, many things can go wrong parsing numbers from external input...

int num, nitems;

nitems = scanf("%d", &num);
if (nitems == EOF) {
    /* Handle EOF/Failure */
} else if (nitems == 0) {
    /* Handle no match */
} else {
    printf("Got %d\n", num);
}
dwc
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  • Mine number goes to negative or random sometimes, why is this?This is if my integer is big, still within 32 characters but still big – P.Andrews Dec 22 '15 at 10:37
  • An integer is usually 2 bytes long. The leftmost bit is used to store the sign of the number (0 for positive, 1 for negative), so we're left with 15 bits for storing the number itself. Because of this, integers in C can range from -32768 (-2^15 or 1111111111111111) to 32768 (2^15 or 0111111111111111). When you enter a number larger than the largest possible value, the leftmost bit gets assigned a value and the integer no longer represents a value you were hoping for. If you want to enter bigger integers use `long int` or even 'long long int' as it can hold much bigger numbers. – PoVa Jun 25 '17 at 14:20
2

Aside from (f)scanf, which has been sufficiently discussed by the other answers, there is also atoi and strtol, for cases when you already have read input into a string but want to convert it into an int or long.

char *line;
scanf("%s", line);

int i = atoi(line);  /* Array of chars TO Integer */

long l = strtol(line, NULL, 10);  /* STRing (base 10) TO Long */
                                  /* base can be between 2 and 36 inclusive */

strtol is recommended because it allows you to determine whether a number was successfully read or not (as opposed to atoi, which has no way to report any error, and will simply return 0 if it given garbage).

char *strs[] = {"not a number", "10 and stuff", "42"};
int i;
for (i = 0; i < sizeof(strs) / sizeof(*strs); i++) {
    char *end;
    long l = strtol(strs[i], &end, 10);
    if (end == line)
        printf("wasn't a number\n");
    else if (end[0] != '\0')
        printf("trailing characters after number %l: %s\n", l, end);
    else
        printf("happy, exact parse of %l\n", l);
}
ephemient
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1

The standard library function scanf is used for formatted input: %d int (the d is short for decimal)

#include <stdio.h>
int main(void)
{
  int number;
  printf("Enter a number from 1 to 1000: ");

  scanf("%d",&number); 
  printf("Your number is %d\n",number);
  return 0;
}
Matthew Flaschen
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TStamper
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-1
#include <stdio.h>

main() {

    int i = 0;
    int k,j=10;

    i=scanf("%d%d%d",&j,&k,&i);
    printf("total values inputted %d\n",i);
    printf("The input values %d %d\n",j,k);

}

from here

Kostas Konstantinidis
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    This looks very suspicious, it both stores the result of scanf() in i, and reads a value into i at the same time. Not recommendable. – unwind May 14 '09 at 20:10