checking for numbers

[H.]

What's the easiest way to check if an argument entered at the command
line is a number?

I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is 0
through 9, with the first digit not being 0.

But this seems like such basic functionality, I'm wondering if I'm
missing something.

[Robert Gamble]

On Jan 28, 3:29 pm, "H." <[e-mail]:

What's the easiest way to check if an argument entered at the command
line is a number?

I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is 0
through 9, with the first digit not being 0.

But this seems like such basic functionality, I'm wondering if I'm
missing something.

What is your definition of a number? Does it have to be an integer or
can it be a decimal number? Can the number be negative? Can it be of
arbitrary size of does it need to fall into the range of a specific
type? Can it start with space characters? What exactly are you
trying to do?
You might find the strto* functions helpful, otherwise you will need
to clarify your definition of a number.

Robert Gamble

[Espen Myrland]

"H." <hb****@gmail.comwrites:

What's the easiest way to check if an argument entered at the command
line is a number?

I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is 0
through 9, with the first digit not being 0.

But this seems like such basic functionality, I'm wondering if I'm
missing something.

Why not leading zeros? You can try strspn:

#include <stdio.h>
#include <string.h>
int main (void)
{
char s[] = "3345567899005431234";
char valid[] = "0123456789";

size_t res = strspn (s, valid);
if ( res != strlen (s) )
printf("Invalid argument: %s\n", s);

return 0;
}
--
espen

[Eric Sosman]

H. wrote:

What's the easiest way to check if an argument entered at the command
line is a number?

I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is 0
through 9, with the first digit not being 0.

But this seems like such basic functionality, I'm wondering if I'm
missing something.

Best bets: strtol(), strtoul(), or strtod(), depending on
the kind of "number" you want to accept. Note that these will
accept some forms of "number" that you might not want: digit
strings with leading spaces, for example, or with a leading
minus sign. But in addition to checking for digits (and such),
they'll also check the range of the converted value: a string
of a thousand consecutive nines is "numeric," but you might not
want to consider it a "number."

Another possibility is sscanf(), but I feel the strtoxx()
functions are more direct.

--
Eric Sosman
es*****@acm-dot-org.invalid

[Robert Gamble]

On Jan 28, 3:52 pm, Eric Sosman <[e-mail]:

H. wrote:

What's the easiest way to check if an argument entered at the command
line is a number?

I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is 0
through 9, with the first digit not being 0.

But this seems like such basic functionality, I'm wondering if I'm
missing something. Best bets: strtol(), strtoul(), or strtod(), depending on

the kind of "number" you want to accept. Note that these will
accept some forms of "number" that you might not want: digit
strings with leading spaces, for example, or with a leading
minus sign. But in addition to checking for digits (and such),
they'll also check the range of the converted value: a string
of a thousand consecutive nines is "numeric," but you might not
want to consider it a "number."

Another possibility is sscanf(), but I feel the strtoxx()
functions are more direct.

The scanf functions are almost always a poor choice for converting
numbers because if the number is outside the range of the expected
type it will always invoke undefined behavior.

Robert Gamble

[Peter Nilsson]

On Jan 29, 8:04 am, "Robert Gamble" <[e-mail]:

On Jan 28, 3:52 pm, Eric Sosman <[e-mail]:

H. wrote:

What's the easiest way to check if an argument entered at the command
line is a number?

Best bets: strtol(), strtoul(), or strtod(), depending on
the kind of "number" you want to accept. ...

Another possibility is sscanf(), but I feel the strtoxx()
functions are more direct.

The scanf functions are almost always a poor choice for converting
numbers

You mean programmers almost always make poor choices of the way
they invoke scanf.

because if the number is outside the range of the expected
type it will always invoke undefined behavior.

No. For example, consider: int i, r = scanf("%4d", &i);

--
Peter

[Gordon Burditt]

>What's the easiest way to check if an argument entered at the command

>line is a number?

Define "a number".

>I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is 0
through 9, with the first digit not being 0.

You forgot the leading minus sign. And perhaps leading whitespace.
And perhaps a leading 0 or 0x signifying the base.

>But this seems like such basic functionality, I'm wondering if I'm
missing something.

strtol() and family will return in the pointer pointed at by its
second argument a pointer to the next character after the number
(if one was found). If that pointer is not null and points at a
string terminator, it's a valid number. If that pointer is not
null and points at characters such as a space, tab, comma, newline,
or whatever, it may or may not be something you consider valid. If
it points at something else wierd, like an underscore, it's probably
not valid.

[Robert Gamble]

On Jan 28, 5:22 pm, "Peter Nilsson" <[e-mail]:

On Jan 29, 8:04 am, "Robert Gamble" <[e-mail]:

On Jan 28, 3:52 pm, Eric Sosman <[e-mail]:

H. wrote:
What's the easiest way to check if an argument entered at the command
line is a number?

Best bets: strtol(), strtoul(), or strtod(), depending on
the kind of "number" you want to accept. ...

Another possibility is sscanf(), but I feel the strtoxx()
functions are more direct.

The scanf functions are almost always a poor choice for converting
number.

You mean programmers almost always make poor choices of the way
they invoke scanf.

The usefulness of scanf for converting numbers is quite limited if you
don't want to risk invoking undefined behavior. Tell me, what is the
proper way to invoke scanf to convert an int whose value may be any of
those that an int can represent without invoking undefined behavior
upon encountering a value that is not?

because if the number is outside the range of the expected
type it will always invoke undefined behavior.

No. For example, consider: int i, r = scanf("%4d", &i);

Well in that case the number being converted isn't outside the range
of the expected type now is it?

Robert Gamble

[David T. Ashley]

"H." <hb****@gmail.comwrote in message
news:11*********************@h3g2000cwc.googlegrou ps.com...

What's the easiest way to check if an argument entered at the command
line is a number?

I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is 0
through 9, with the first digit not being 0.

But this seems like such basic functionality, I'm wondering if I'm
missing something.

First, you have to realize something about both users and numbers -- they
ARE out to get you. You can't turn your back on them, or they will conspire
and move against you. It is the same principle when bowling -- the 8 and 9
pins are shielded from your view by the other pins and they are conspiring
against you.

The safest approach to the problem you're describing is to write your own
function that maps from (string) to (valid, number). Proceed in two phases:

a)Force the string into the allowed character set (i.e. it should be only
digits).

b)Use a bit of FSA theory to parse the string. You need to have a firm idea
of what constitutes a "number" for your application, and it is then easy to
devise an automaton approach to parse it.

Generally, expect to write about 100 lines of code. In goes the string.
Out comes an error/success code and a number.
--
David T. Ashley (dt*@e3ft.com)
http://www.e3ft.com (Consulting Home Page)
http://www.dtashley.com (Personal Home Page)
http://gpl.e3ft.com (GPL Publications and Projects)

[CBFalconer]

"H." wrote:

>
What's the easiest way to check if an argument entered at the
command line is a number?

I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is
0 through 9, with the first digit not being 0.

But this seems like such basic functionality, I'm wondering if
I'm missing something.

A simple sequence to use is:

int i;

if (1 != sscanf(argv[i], "%d", &i)) puts("not a number");
else {
carryonjack();
}

provided you have first ensured that argv[i] exists by checking the
value of argc.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>

[Eric Sosman]

Gordon Burditt wrote:

[Gordon, it might amuse you to see what Thunderbird's spell-
checker does with your surname.]

>What's the easiest way to check if an argument entered at the command
line is a number?

Define "a number".

>I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is 0
through 9, with the first digit not being 0.

You forgot the leading minus sign. And perhaps leading whitespace.
And perhaps a leading 0 or 0x signifying the base.

Therein lies a user interface trap. If you specify zero
as the base for strtol() or its brethren, they will interpret
a leading 0 or 0x as indicating base eight or sixteen. If a
program accepts input from humans who are not C programmers
this is almost certainly a Bad Thing: Ordinary people expect
leading zeroes to be place-holders and nothing more. If you
use strtoul() this way to accept telephone numbers in three
parts, for example, you'll have trouble with 781 424 0116: it
will be accepted, but when you dial you'll reach 781 424 0078.

Recommendation: "Almost always" specify ten explicitly as
the base for strtoxx(). The extra flexibility gained by using
zero is "almost always" unwanted and potentially harmful.

--
Eric Sosman
es*****@acm-dot-org.invalid

[Jeffrey Stedfast]

Eric Sosman wrote:

Gordon Burditt wrote:

[Gordon, it might amuse you to see what Thunderbird's spell-
checker does with your surname.]

I had to try it myself, that truly was amusing ;)

Jeff

[H.]

int i;

if (1 != sscanf(argv[i], "%d", &i)) puts("not a number");
else {
carryonjack();
}

But aren't you assuming here that the digits are all 1s?

[Walter Roberson]

In article <11**********************@k78g2000cwa.googlegroups .com>,
H. <hb****@gmail.comwrote:

>

> int i;

if (1 != sscanf(argv[i], "%d", &i)) puts("not a number");
else {
carryonjack();
}

But aren't you assuming here that the digits are all 1s?

No, sscanf() returns the -number- of successfully matched items,
0 if no items were successfully matched, and EOF if there is
end of input string before the first character.

--
There are some ideas so wrong that only a very intelligent person
could believe in them. -- George Orwell

[Robert Gamble]

On Jan 28, 9:24 pm, CBFalconer <[e-mail]:

"H." wrote:

What's the easiest way to check if an argument entered at the
command line is a number?

I know that one way would be to treat the argument as a character
string, and then manually check each character to make sure it is
0 through 9, with the first digit not being 0.

But this seems like such basic functionality, I'm wondering if
I'm missing something.A simple sequence to use is:

int i;

if (1 != sscanf(argv[i], "%d", &i)) puts("not a number");
else {
carryonjack();
}

provided you have first ensured that argv[i] exists by checking the
value of argc.

A better solution would be to use one of the strto* functions as they
don't invoke UB if the number is too large and allow you to more
easily determine if the data after the conversion is valid. For
example, the string "123abc" will be deemed a number by the presented
sscanf solution whereas strtol can set a pointer to the "a" in the
string which the program can use to examine the data after the number
and determine if it is valid.

Robert Gamble

[Richard Bos]

Jeffrey Stedfast <st******@comcast.netwrote:

Eric Sosman wrote:

Gordon Burditt wrote:

[Gordon, it might amuse you to see what Thunderbird's spell-
checker does with your surname.]

I had to try it myself, that truly was amusing ;)

I don't have an up-to-date Thunderbird here, but let me guess: Gordon
Bennett!?

Richard