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.

[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************ **********@k78g 2000cwa.googleg roups.com>,
H. <hb****@gmail.c omwrote:

>

> 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 <cbfalco...@yah oo.comwrote:

"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******@comca st.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