c++ - isdigit, how to distinguish between a character and a number

Question

I want to check if my array has only numbers how can I check it out?

i try this example but this not working and i dont Understand why.

int main() {

char* str = new char[9];
cin >> str;




cout << isdigit(str[0])<<endl;


system("pause");
return 0;
}

input:

1234

output: 4

what i want is:

input: 1234

print: ok

input:
12fg

print: not ok (because the character in my array)

I look forward to your help and explanation

thank's.

`isdigit` returns: _"Non-zero value if the character is a numeric character, zero otherwise."_ so you need to write a bit of code. — Richard Critten, Mar 30 '17 at 20:05
why would you expect it to print 'ok'. isdigit returns either 0 or not 0 to indicate that something is a digit — pm100, Mar 30 '17 at 20:05

pm100 · Answer 1 · 2017-03-30T20:11:36.310

1

try this

if(isdigit(str[0]))
  cout << 'ok'
else
  cout << 'not ok'

extra credit 1. Using this validate the whole string

extra credit 2. Find a different way to verify the whole string (there are many)

extra credit 3. Harder, find out why you got '4' from isdigit(str[0])

edited Mar 30 '17 at 20:11

answered Mar 30 '17 at 20:06

pm100

1

This does not check the whole string. – NathanOliver Mar 30 '17 at 20:08
@NathanOliver -- it does the same check as the code in the question. – Pete Becker Mar 30 '17 at 20:08
@PeteBecker Which is wrong. The OP is trying to find a string that only contains digits – NathanOliver Mar 30 '17 at 20:09
I did not rewrite his whole program to do what he wants. I simply unblocked his current confusion. He will discover for himself that he needs to do more – pm100 Mar 30 '17 at 20:09
wow - down votes too , its tough trying to help people :-) – pm100 Mar 30 '17 at 20:23
@NathanOliver -- it answers the question that's as asked, which was why he got 4 and not ok. – Pete Becker Mar 30 '17 at 21:07

score 1 · Answer 2 · answered Mar 30 '17 at 20:11

1

Try something like this:

const char *s = "1234";

while (isdigit(*s)) s++;
const char *result = (*s) ? "not ok" : "ok";

answered Mar 30 '17 at 20:11

Stephan Lechner

IMHO thats way to fancy for such a beginner question. Took me a while to work out what its doing, I mean its clever but really obscures the basic logic – pm100 Mar 30 '17 at 20:14

2 Answers2