CODEWITHSUNDEEP
cpointers
Ashis Ghosh
Ashis Ghosh

Reputation: 85

Not able to understand this C program

I am new to programming, not getting the code below. This program checks if a character c is in the string s. 

int is_in(char *s, char c){
  while(*s){
    if(*s==c) return 1;
    else s++;
  return 0;
}

The main thing confusing me is, how the while loop will stop, as, I think s++ will go through all over the memory, after the end of string also. Can anyone explain this please? Please correct me if I am wrong.

Upvotes: 1

Views: 86

Answers (3)

Bathsheba
Bathsheba

Reputation: 234635

The loop stops when *s is 0, i.e. at the end of the NUL-terminated string.

The idiomatic way of modelling strings in C is to terminate them with 0. Note that if s is not formed in this way, then the behaviour of your function is undefined.

Personally I'd prefer the function to be int is_in(const char *s, char c) to signify to the caller that the function doesn't modify the string.

Upvotes: 3

Govind Parmar
Govind Parmar

Reputation: 21532

Your intuition that the pointer s will continue to loop indefinitely would be correct were it not for two things:

  • C strings are terminated by a null-terminator (the character '\0'). This acts as a sentinel value for functions that process strings; this is necessary since when an array is passed to a function it decays to a pointer to its first element, losing length information.
  • The loop condition while(*s) will be false when the null terminator is reached.

In fact, while(*s) { loop-body; s++; } is a well-known idiom in C for processing strings.

Upvotes: 1

Igor S.K.
Igor S.K.

Reputation: 1039

The string char *s is supposed to end with terminating NUL. The value of NUL is zero. Zero is what *s is supposed to "expand" to eventually.

Upvotes: 0

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