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to_lower function entering an infinite loop

For Beginners C++

Started by MarcusAseth September 09, 2017 03:58 PM

27 comments, last by MarcusAseth 6 years, 7 months ago

MarcusAseth

Author

September 09, 2017 03:58 PM

I am super confused by this...

I was in the middle of an experiment, maybe is just that I am tired and I'm missing the obvious here (so facepalm is ready), but things are not working as expected, Image below (I'll also paste the code in case you want to try it out yourself)


#include <iostream>
#include <ostream>
using namespace std;

void to_lower(char* s)
{
	char* text = s;
	while (s != '\0')
	{
		char letter = *s;
		if (letter > 'A' && letter < 'Z')
		{
			*s = 'a' + (letter - 'A');
		}
		s++;
	}
	cout << text << endl;
}

int main()
{
	char* text = new char[12] {"Hello World"};
	to_lower(text);
	delete[] text;
	return 0;
}

As you see in the debug image, the content of s[0] is 'l', but "char letter = *s;" is yelding a 'e'...what is this madness?! O_O

EDIT: forget what I asked above, I just understood that the yellow arrow means that the line isn't executed yet, I just had to step forward once

So, since that question turned useless I renamed this topic and I have a new question: my function enters an infinite loop.

Apparently the test "while (s != '\0')" is not good because when s points to '\0', the content of s is "" according to the debugger... so how do I test for '\0'?

My portfolio: https://www.artstation.com/artist/marcusaseth

Kylotan

10,510

September 09, 2017 04:14 PM

You're comparing a pointer, i.e. a memory address, against what you're expecting to see when dereferencing the pointer, i.e. the number 0 or '\0'. You need to dereference the pointer.

Also, your if statement is wrong.

And, you're wasting your time duplicating a standard language function.

MarcusAseth

Author

September 09, 2017 04:17 PM

10 minutes ago, Kylotan said:

You're comparing a pointer, i.e. a memory address, against what you're expecting to see when dereferencing the pointer, i.e. the number 0 or '\0'. You need to dereference the pointer.

Also, your if statement is wrong.

And, you're wasting your time duplicating a standard language function.

ops, missed that dereference in the while, I knew I was tired x_x

Fixed my >= and <= , thanks!

I disagree on the last point though, I don't think this time is wasted, we are probably assigning value to different things

My portfolio: https://www.artstation.com/artist/marcusaseth

Alpha_ProgDes

6,963

September 09, 2017 05:06 PM

Though you wouldn't do this in production code, I agree you should know/understand how to handle pointers.

Beginner in Game Development? Read here. And read here.

MarcusAseth

Author

September 09, 2017 05:14 PM

9 minutes ago, Alpha_ProgDes said:

Though you wouldn't do this in production code, I agree you should know/understand how to handle pointers.

Exactly my goal here

What I'm currently writing is probably ugly and a bit buggy, but I'm getting useful practice, today first time I use const_cast and write my template, so...IMPROVEMENT (for better or worse...)


char* findx(const char* s, const char* x)
{
	if(s == nullptr || x == nullptr)
	{ return nullptr; }

	char* Tmp = const_cast<char*>(x);
	int FindSize = 0;
	while (*Tmp++ != '\0') { FindSize++; };

	int ID = 0;
	int firstOccurrence = 0;
	bool isMatch = false;
	
	while (s[ID] != '\0')
	{
		if (s[ID] == x[0])
		{
			firstOccurrence = ID;
			for (size_t i = 0; i < FindSize; i++)
			{
				if (s[ID + i] != x[i]) { break; }
				if (i == FindSize - 1) { isMatch = true; }
			}
			if (isMatch) { break; }
		}
		ID++;
	}
	return (isMatch ? const_cast<char*>(s) + firstOccurrence : nullptr);
}

My portfolio: https://www.artstation.com/artist/marcusaseth

Alberth

10,209

September 09, 2017 05:43 PM

Using const cast looks horribly wrong, "find" shouldn't change the string!

You have "char *tmp" and return a "char *", change that to "const char *" too.

At a higher level, you are accepting const char * values as parameters, users don't expect that you break your promise, and sneakily return a modifiable point in their const string as return value.

MarcusAseth

Author

September 09, 2017 05:50 PM

The Exercise in the book I'm following gave as only instructions:

Quote

Write a function, char* findx(const char* s, const char* x), that finds the first occurrence of the C-style string x in s.

I don't know how to return a char* out of a const char*, so I tried removing the const, just to fullfill the assignment

My portfolio: https://www.artstation.com/artist/marcusaseth

Alberth

10,209

September 09, 2017 06:24 PM

Book is just wrong

h8CplusplusGuru

166

September 13, 2017 10:20 PM

Here you go friend


//take two char* (strings) and return char* to first occurence of second string
char* findx(char* str, char* strx ) {
	
	if (!str || !strx) return nullptr;

	char* found = nullptr;

	for (int s = 0, x = 0; str[s] != '\0' && strx[x] != '\0'; s++) {

		if (str[s] == strx[x]) {
			if( !found ) found = &str[s];
			x++;
		} else {
			found = nullptr;
			x = 0;
		}
	}

	return found;
}

MarcusAseth

Author

September 14, 2017 04:02 AM

@h8CplusplusGuru much more clean/compact than mine, regardless if I had done it without the const char required by the exercise, thanks for showing me

My portfolio: https://www.artstation.com/artist/marcusaseth

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