Not initialised and memory errors (strings and pointers) - c

Question

Here is what I need to do: delete all occurrences of a number that appears most frequently in a given string
Here is what I've done: wrote two functions; the second one extracts all integers from a string into an array, finds the most frequently repeated one, calls the first function to find that number in a string, deletes all its occurrences in a given string
The problem is it works alright when I compile it, but doesn't pass the series of auto-generated tests and displays "access to an uninitialised value" and "memory error" in lines I marked with <------.
I know this is not exactly the "minimum reproducible code" but I'm hoping someone could point out what the problem is, as I run into a lot of similar errors when working with pointers.
char* find_number(char* string,int search)
{
int sign=1;
int number=0,temp=0;
char* p = string;
while(*string != '\0') {<----------
p=string;
if(*string=='-') sign=-1;
else if(*string==' ') {
string++;
continue;
} else if(*string>='0' && *string<='9') {
temp=0;
while(*string != '\0' && *string>='0' && *string<='9') {
temp=temp*10+*string-'0';
string++;
}
number=temp*sign;
if(number==search) {
return p;
}
} else {
sign=1,number=0;
}
string++;
}
return NULL;
}
char* delete_most_frequent(char* string)
{
//writing all integers in a string to an array
char* pointer=string;
char* s = string;
int temp=0, sign = 1,i=0,array[1000],number=0,counters[1001]= {0},n=0;
while (*s != '\0') {<------------
if (*s == '-') sign = -1;<----------
else if (*s >= '0' && *s <='9') {<----------
temp = 0;
while (*s != '\0' && *s >= '0' && *s <= '9') {
temp = temp * 10 + *s - '0';
s++;
}
number=sign*temp;
if(number>=0 && number<=1000) {
array[i]=number;
i++;
}
}
number=0;
sign=1;
s++;
}
n=i;//size of the array
//finding the number that occurs most frequently
int max=0;
for (i=0; i<n; i++) {
counters[array[i]]++;
if(counters[array[i]]>counters[max]) {
max=array[i];
}
}
char* p=find_number(string,max);//pointer to the first digit of wanted number
//deleting the integer
while (*string != '\0') {
if (p != NULL) {
char *beginning = p, *end = p;
while(*end>='0' && *end<='9')
end++;
//while (*beginning++ = *end++);
while(*end != '\0'){
*beginning = *end;
beginning++;
end++;
}
*beginning = '\0';
} else string++;
p=find_number(string,max);
}
return pointer;//pointer to the first character of a string
}
int main()
{
char s[] = "abc 14 0, 389aaa 14! 15 1, 153";
printf("'%s'", delete_most_frequent(s));
return 0;
}

Answer 1

chopping your code down to just what is likely causing the problem, you have pattern that looks like
while(*string != '\0') {
:
while(*string != '\0' ...) {
:
string++;
}
:
string++;
}
so you have two nested while loops, both of which are advancing the pointer looking for a NUL terminator to end the loop. The problem is that if the inner loop gets all the way to the NUL (it might stop earlier, but it might not), then the increment in the outer loop will increment the pointer past the NUL. It will then happily run through (probably invalid) memory looking for another NUL that might not exist. This is a hard one to catch as in most test cases you write, there are likely multiple NULs (soon) after the string, so it will appear to work fine -- you almost have to specifically write a test case to trigger this failure mode to catch this.
One fix would be to check you're not yet at the null before incrementing -- if (*string) string++; instead of just string++;