C question: why there's an "i" here ?

Question

​int​ ​main​(​int​ argc, string argv[ ])
​{
​    ​int​ trackdigit = ​0​;
​    ​if​ (argc == ​2​)
​    {
​        ​for​ (​int​ i = ​0​, n = ​strlen​(argv[​1​]); i < n; i++)
​        {
​            ​if​ (​isdigit​(argv[​1​][i])) /////// here
​            {
​                trackdigit +=​1​;
​            }

And another question: why when im doing normal if else statement and trying to check if argv[1] is digit without adding "i" it doesn't work and give me segfault

Answer 1

Command line arguments are strings, but isdigit() is only defined for single characters (which a string is composed of), hence the loop and the additional index. The expression argv[1][i] refers to the (i + 1)th character in the second command line argument, so the loop counts how many digits are present in that string.

Answer 2

May I know
Is there string type in c? Is it new feature? Shadow

Answer 3

Jayakrishna🇮🇳 Not that I know of, I assumed it to be a typedef for char* maybe.

Answer 4

Jayakrishna,
The below thread might have answer to your doubt. The code in that thread includes cs50 header, perhaps 'string' was a custom type introduced in that header.

https://www.sololearn.com/Discuss/2984614/?ref=app

Answer 5

edit:
Oh. Thats useful.
Ipang , Shadow thanks you both for the clarification..
I guess, for OP also..