Divide and Conquer in C, may reach end of non-void function - c

Question

I am trying to get this divide and conquer to work, but the compiler is giving me:
control may reach end of non-void function
I have read through similar scenarios, and understand the error implies the program might run forever without a return. I have reviewed some scenarios, which were resolved by using "else" instead of "if else" (as you should to begin with). However, that didn't help.
I am aware that using a do while loop and return in this scenario is redundant, I was fiddling with it in hopes to trick the compiler.
Where is the error?
bool search(int value, int values[], int n)
{
int sorted = 0;
int min = 0;
int max = n;
int mid = n / 2;
do
{
//mid is value
if (value == values[mid])
{
printf("value found!");
sorted = 1;
return 1;
}
//search right
else if (values[mid] < value)
{
min = mid + 1;
mid = (max - mid) / 2 ;
}
//search left
else if (values[mid] > value)
{
max = mid - 1;
mid = (max - mid) / 2;
}
// DNE
else
{
printf("value not found");
sorted = 1;
return 0;
}
}
while(sorted == 0);
}

Answer 1

Every branch of code should have a return if function suppose to return a value.
In your case after while you have to put a return. How ever, Your logic is also slightly wrong. In your code there is not any need of sorted variable and the terminating condition is also wrong as well as the way of calculating mid is also wrong. Don't worry here is your updated code:
bool search(int value, int values[], int n)
{
//int sorted = 0;
int min = 0;
int max = n;
int mid = n / 2; // S1
do
{
//mid=(max+min)/2; // S2
//mid is value
if (value == values[mid])
{
//printf("value found!");
//sorted = 1;
return 1;
}
//search right
else if (values[mid] < value)
{
min = mid + 1;
mid = (max + min) / 2 ; // S3
}
//search left
else if (values[mid] > value)
{
max = mid - 1;
mid = (max + min) / 2; //S4
}
// DNE
else
{
//printf("value not found");
//sorted = 1;
return 0;
}
}
while(min!=max);
return 0;
}
So if your function is returning 1 it mean value found otherwise value doesn't found.
you also can comment S1, S3 and S4 and uncomment S2 for minimum line of code.
And the way of finding mid will be mid=(max+min)/2.

Answer 2

Ignoring other problems in your code (the loop will never terminate under certain conditions although I didn't study it too closely), let's just look at why the compiler complains, because it's quite interesting.
Your compiler complains about that you don't have a return at the end of the function (pretty much what we can read from the warning message) after the while loop. You loop runs:
while(sorted == 0);
And everywhere you change sorted, you do this:
sorted = 1;
return X;
So it is pretty obvious to you and me that the while condition will always be true and you won't fall out of the while loop and need a return after it. Every time you make the while condition not true, you also return immediately. But the compiler doesn't know that. It probably could figure it out with a little bit more effort, but it can never be written to know in all cases and you wouldn't accept a compiler that slow anyway. The problem of fully analyzing any arbitrary bit of code to know what you and I know (that we'll never fall out of the loop) is pretty much the halting problem (if you don't know what it is, you should, google it).
In situations like this we need to work with the compiler and help it understand. I would add a return sorted; at the end of the function, change the loop condition to while (1) and replace sorted = 1; return X; with sorted = X; break; to break out of the loop and have the function return from just one place (it is much easier to read functions that have just one or very few return statements). Or just remove the sorted variable, do the returns properly and loop forever.

Answer 3

I think your compiler doesn't like your understanding of Control Flow. As a beginner this is one of the most cringing problems.
Your error explained: Control may reach end of non-void function. Whenever you branch your code (i.e. use loops, if, switch etc) you have to explicitly write what each branch will do. So in your code, you must return a bool by any means necessary.
I am unable to discern where the error might be, it would be great if someone can show me.
See we have two return x; statements, one in if block and second in else block. Now we think that control should return from either of these blocks conveniently. But your compiler is "concerned" about what will happen if control didn't enter either of these blocks and loop ends. Moreover, what will happen after the loop, how can control ever return to main? Hence the error.
Now what we can do to fix the problem is add a return 0; just before the end of the function. That will surely make the error disappear. However, there are better ways to solve this problem:
bool search(int value, int values[], int n) {
int min = 0, max = n;
int mid = n / 2;
bool is_found = 0;
while (min != max) {
if (value == values[mid]) {
is_found = 1;
break;
}
else if (value > values[mid]) {
min = mid + 1;
mid = (max + min) / 2;
}
else if (value < values[mid]) {
max = mid - 1;
mid = (max + min) / 2;
}
else break;
}
return is_found;
}