i tried to define string end function in K&R excercises

i tried to define strend(s,t) function which returns 1 if string t at the end of s string and zero otherwise , this is my code .

strend(s,t)

typedef enum state
{
Not_occured ,
occured
}State;
char a="Hello world zone";
char b="ne";
int main(void)
{
int x = 0 ;
x = strend(a,b);
printf("%d",x);
return 0;
}

int strend(char *s, char *t)
{
while(*++s);
while(*++t);
while(*t-- == *s--)
if(!(*t))
return occured;
return Not_occured;
}


modified code

int strend(char *s, char *t)
{
char *ptr = s;
while(*++s);
while(*++t);
while(*t-- == *s--)
if(s == ptr)
return occured;
return Not_occured;
}


why prefix worked here while(*++s); , while(*++t); and postfix doesn't work?

while(*++s);

while(*++t);

t

if (!(*t))

while(*++s);

b = "";

while(*s) s++;

while(*++s);

while(*s++);

while(*s) s++;

1 Answer
1

if(!(*t)) assumes there is a nul character before the first character in the string. Not only is this an incorrect assumption, but it also tries to access memory outside of the array bounds.

if(!(*t))

Also, while(*t-- == *s--)...What happens when the 2 strings are identical or t is longer than s?

while(*t-- == *s--)

t

s

Here's a simple solution:

int strend(char *s, char *t) {
if (s == NULL || t == NULL) return Not_occured;

size_t s_len = strlen(s);
size_t t_len = strlen(t);

if (t_len <= s_len) {
return 0 == strcmp(&s[s_len - t_len], t) ? occured : Not_occured;
}
return Not_occured;
}


Since it's now clearer that the original question is about the pre- and post-fix operations, I've updated.

First, understand the difference between the two:

int x = 1;
printf("%d", x++);
// Prints 1 because x is evaluated before the inc
x = 1;
printf("%d", ++x);
// Prints 2 because x is evaluated after the inc


Now, some operator precedence:

postfix > dereference
prefix == dereference, right-to-left assoc.


What happens with *s++:

*s++

s++

s

s0

s

s0

This is the same as:

char *s0 = s;
s += 1;
char c = *s0;


What happens with *++s:

*++s

s

s

This is the same as:

s += 1;
char c = *s;


Hope that makes sense.

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