Best way to get numbers from input?

Question

I would like to loop character by character from an input file that has text and numbers.

I thought I could just loop

char count;

while( c != ' ' && c != '\n' && c != '\t' ) {
  count += c;
  c = fgetc(fp);
}

To say get "11" from the text file and then use atoi() to convert to an int but I then realized I'm just adding ASCII numbers.

I'm fairly new to C and from what I understand strings are just char arrays - does this mean I have to put "1" and "1" into a char array?

But then I would have to worry about array size and converting that to a number?

The C specification says that numerical digits always have to be consecutive, no matter the encoding. That means you can do e.g. `'1' - '0'` to get the integer value `1`. From there it's easy to create integer values no matter the base used for the input. No need to store in a string for later use with `atoi`. If you don't want to construct the number on the fly, and use `atoi`, this comment gives you a hint about that too. — Some programmer dude, Feb 03 '16 at 08:22

score 4 · Answer 1 · edited May 23 '17 at 11:52

4

While it looks like the natural solution, I usually advice against using fscanf() on potentially malformed input.

There are several issues with the function, among them:

The inability to recover gracefully from an error (short of ftell() / fseek()), because you don't know where exactly it stopped parsing input.
The ease with which it leads to using uninitialized values (if the format failed to match the input, and you didn't check the return code properly).
Some corner cases where fscanf() chokes on input where atoi() / strtol() don't ("0xz"...).

All this relegates fscanf() to read well-formatted input only, i.e. things your own program had written before in a known format.

For any kind of input that might not be in the expected format, I recommend reading a line at a time (fgets()), and parsing it in-memory, for example with strtol():

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

#define LINE_BUFFER_SIZE 256;

// ...
char line[ LINE_BUFFER_SIZE ];
fgets( line, LINE_BUFFER_SIZE, fp );
char * scanptr;
errno = 0;
long number = strtol( line, &scanptr, 0 );
if ( scanptr == line )
{
    // not a number...
}

scanptr now points to where number parsing ended.
number contains the number parsed, 0 if line did not start with a number, and LONG_MAX resp. LONG_MIN if the number found is out of long range (in which case errno == ERANGE as well).

I don't know your exact requirements, so I cannot give a good example for what to do if you don't find a number, or what to do with the text. But since you got the line (or, the first LINE_BUFFER_SIZE characters of a longer line...) in memory, you have the whole range of the string functions at your disposal. strpbrk( line, "0123456789" ) could be used to scan for the next digit, or strpbrk( line, "+-0123456789" ) if you might have +- in your input...

edited May 23 '17 at 11:52

Community

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answered Feb 03 '16 at 08:59

DevSolar

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Even though variations of this question come up from time to time and similar answers discussing `*scanf()` and `strto*()`, this is one of the best ones yet for a learner. – chux - Reinstate Monica Feb 03 '16 at 09:08
@chux: Thanks. Together with "what is Unicode, and how do I use it in C/C++", it's one of those I've been thinking about writing a FAQ entry for and never got around to doing it (yet). Seems I answer one of them every other week. ;-) – DevSolar Feb 03 '16 at 09:09
Contact me should you embark on that and want a (pre-)review. – chux - Reinstate Monica Feb 03 '16 at 09:10
Thanks DevSolar. Im trying to grasp what you wrote simply because of unfamiliarity with C. I need to iterate through a file and find numbers and words that I will later use. Fgetc seemed to be the most natural since I can continue from where I last found something. I should be able to use scanptr for that? Part of me is thinking of switching to C++ so I can just work with strings but feels like I'm quitting if I do ha – cpd1 Feb 03 '16 at 11:19
@cpd1: Going for C++ is not "quitting". It's a much more powerful language that makes many things much easier -- but comes with its own pitfalls. Generally speaking, while it's a good thing to work along some "toy" application while learning a language, it's not a good idea to try *both* learning a languagne *and* solving a "real" problem at the same time. You want to first get a "feel" for a language, and the tools it provides, so you can select the right tools for any given problem. Otherwise, you'll always play blind man's bluff, and become frustrated. – DevSolar Feb 03 '16 at 11:46
I hear you and I wish I had more time to learn before I try coding up something but I really don't. This is just a small piece of something larger I have to put together and I've spent the last few days trying to get my head around the parsing part. Likely because I'm thinking of it as how I would handle it in Java with strings etc – cpd1 Feb 03 '16 at 11:52
I hoped the scope of that [Q&A](http://stackoverflow.com/questions/35178520) would have been much smaller along the lines of "How to read / parse an integer in C?" - it is fairly broad. – chux - Reinstate Monica Feb 03 '16 at 15:45

score 1 · Answer 2 · answered Feb 03 '16 at 08:29

1

Most people would use fscanf:

int number, items_scanned;
items_scanned = fscanf(fp, "%d", &number);
if (items_scanned == 0) {
     /* Scanning for a number failed */
     printf("Scan failed!\n");
}
printf("Found number: %d\n", number);

fscanf skips whitespace. It will succeed if it finds a number and fail if it finds something else.

answered Feb 03 '16 at 08:29

Klas Lindbäck

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Thanks Klas. I have to actually iterate through the file to find other text and numbers while keeping track of them so I would need some sort of line position with the above? – cpd1 Feb 03 '16 at 11:14
You can use `ftell` to find the current position in the file. You should also consider the answer by @DevSolar which may be a better fit for your problem. – Klas Lindbäck Feb 03 '16 at 14:14

Best way to get numbers from input?

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