c: scanf reading integers

Question

This scanf statement will read in numbers from input like 10ss as 10 - how do I rewrite it to ignore an int that's followed by other characters? If 10ss is read, it should ignore it and stop the for loop.

for (int i = 0; scanf("%d", &val)==1; i++)

`scanf` is not good for *bad* input. It always looks only one character ahead. By the time it meets `s` it is already too late. Of course you could try something like capture the following character using a scan set if it is non-whitespace, and checking the return value, but I'd suggest that you would look at the myriad duplicates about how to ensuring that a string is only an integer using `strtol`. — Antti Haapala -- Слава Україні, Jan 17 '18 at 05:06
Use `getchar()` to read the offending non-numeric, then try `scanf("%d", &val)==1` again. — chux - Reinstate Monica, Jan 17 '18 at 05:20
Read documentation of [scanf](http://en.cppreference.com/w/c/io/fscanf). — Basile Starynkevitch, Jan 17 '18 at 05:35
What should happen with input like `"123 456"` or `"123-456"` ? — chux - Reinstate Monica, Jan 17 '18 at 15:15
Does this answer your question? [How to scanf only integer?](https://stackoverflow.com/questions/26583717/how-to-scanf-only-integer) — Germano Massullo, Aug 18 '20 at 14:00

AnT stands with Russia · Answer 1 · 2018-01-17T15:20:56.030

4

You can try reading the next character and analyzing it. Something along the lines of

int n;
char c;
for (int i = 0; 
     (n = scanf("%d%c", &val, &c)) == 1 || (n == 2 && isspace(c)); 
     i++)
  ...

// Include into the test whatever characters you see as acceptable after a number

or, in an attempt to create something "fancier"

for (int i = 0; 
     scanf("%d%1[^ \n\t]", &val, (char[2]) { 0 }) == 1; 
     i++)
  ...

// Include into the `[^...]` set whatever characters you see as acceptable after a number

But in general you will probably run into limitations of scanf rather quickly. It is a better idea to read your input as a string and parse/analyze it afterwards.

edited Jan 17 '18 at 15:20

answered Jan 17 '18 at 05:32

AnT stands with Russia

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@chux: I tried initially, but suppressed assignments are not included into the count returned by `scanf`. So, the version with `*` will return `1` even for `10ss` input, which is not what is needed in this case. – AnT stands with Russia Jan 17 '18 at 15:18

chux - Reinstate Monica · Answer 2 · 2018-01-17T15:09:23.970

how do I rewrite it to ignore an int that's followed by other characters?

The robust approach is to read a line with fgets() and then parse it.

// Returns
//   1 on success
//   0 on a line that fails
//   EOF when end-of-file or input error occurs.
int read_int(int *val) {
  char buf[80];
  if (fgets(buf, sizeof buf, stdin) == NULL) {
    return EOF;
  }
  // Use sscanf or strtol to parse.
  // sscanf is simpler, yet strtol has well defined overflow funcitonaitly
  char sentinel;  // Place to store trailing junk
  return sscanf(buf, "%d %c", val, &sentinel) == 1;
}


for (int i = 0; read_int(&val) == 1; i++)

Additional code needed to detect excessively long lines.

c: scanf reading integers

2 Answers2