one question about array

[vsovereign]

Hi there,

if there is a code like this :

int c, ndigit[10];

if (c >= '0' && c <= '9')
	  ++ndigit[c-'0'];

++ndigit[c-'0'] is that the same with :

ndigit[10] = ndigit[10] + ndigit[c-'0']?

sorry if this is a stupid question, I'm just a newbie

if full program is needed (a short program this is) to answer my question, I will put it online. Just tell me so.

thanks!

[KBriggs]

++ndigit[c-'0']

Is the same as

ndigit[c-'0'] = ndigit[c-'0'] + 1

[MK27]

++ndigit[c-'0'] is that the same with :

ndigit[10] = ndigit[10] + ndigit[c-'0']?

No. The ASCII value of '0' is actually 48:
ASCII Table / Extended ASCII Codes
'9' is 57. If you don't understand what the ascii table is about, ask.

So that code could be written:

if (c >= 48 && c <= 57)
	  ++ndigit[c-48];

Thus c-48 will be between 0 and 9. ++ means to increment that value in the array by one. Here it is in front of the variable, meaning "pre-increment" (increment before the value is used in the statement context). Since there is no statement context, it means the same as ndigit[c-48]++.

[iMalc]

So that code could be written:

if (c >= 48 && c <= 57)
	  ++ndigit[c-48];

"could" but very much should not! It's purely an exercize in obfuscation.

Do not do that!

[vsovereign]

No. The ASCII value of '0' is actually 48:
ASCII Table / Extended ASCII Codes
'9' is 57. If you don't understand what the ascii table is about, ask.

So that code could be written:

if (c >= 48 && c <= 57)
	  ++ndigit[c-48];

Thus c-48 will be between 0 and 9. ++ means to increment that value in the array by one. Here it is in front of the variable, meaning "pre-increment" (increment before the value is used in the statement context). Since there is no statement context, it means the same as ndigit[c-48]++.

so that means that if c = 0 -> c = 48 in ASCII

so

ndigit[48-48] = ndigit[48-48] + 1
     ndigit[0] = ndigit[0] + 1

if we initialize ndigit[10] :

for (i = 0; i < 10; i = i + 1)
	 ndigit[i] = 0;

that means ndigit[0] = 0

so

ndigit[0] = ndigit[0] + 1
     ndigit[0] = 1

is that correct?

[KBriggs]

if we initialize ndigit[10] :

Careful with your notation. ndigit[10] does not exist. ndigit is an array of 10 elements with indices 0-9 inclusive. Saying ndigit[10] is saying "the 11th entry in ndigit" which does not exist.

What you mean to say was: "if we initialize all of the entries of ndigit to 0". The [10] is not part of the variable name, it just tells you how many elements are in it.

c = 0 does not mean c=48 in ASCII. c='0' does. Note the quotes, which makes it the zero character, rather than the number 0. The character '0' has value 48 "in ASCII."

[vsovereign]

Careful with your notation. ndigit[10] does not exist. ndigit is an array of 10 elements with indices 0-9 inclusive. Saying ndigit[10] is saying "the 11th entry in ndigit" which does not exist.

What you mean to say was: "if we initialize all of the entries of ndigit to 0". The [10] is not part of the variable name, it just tells you how many elements are in it.

c = 0 does not mean c=48 in ASCII. c='0' does. Note the quotes, which makes it the zero character, rather than the number 0. The character '0' has value 48 "in ASCII."

Yeah, but if we input c = 0, won't it changed to c = 48 because it's getchar() ?

so it will be

ndigit[c-'0'] = ndigit[c-'0'] +1 --> I input c = 0

ndigit[c-'0'] = ndigit[48-48] +1 
ndigit[c-'0'] = ndigit[0]+1

I also don't understand this : does ndigit[0] + 1 = ndigit[1] ?
if yes, how come if I input 0, then it will be enterd into ndigit[1] instead of ndigit[0] ?

help will be appreciated.

[quzah]

I also don't understand this : does ndigit[0] + 1 = ndigit[1] ?
if yes, how come if I input 0, then it will be enterd into ndigit[1] instead of ndigit[0] ?

No, you are only looking at part of the line. The whole thing translates something like:

int x;
...
x = x + 1;

The first part:

ndigit[ 0 ] =

Is the same as above when I did:

x =

Then the next part is the remainder:

ndigit[ 0 ] + 1;

Is the same as:

x + 1;

Again, taken all together:

int x;
...
x = x + 1;

Is the same as:

int ndigit[ SOMESIZE ];
...
ndigit[ SOMEPLACE ] = ndigit[ SOMEPLACE ] + 1;

Quzah.