comparison signed vs unsigned

[Timothee Groleau]

Hi all,

When should I be worried about doing a comparison of signed vs unsigned
ints? Could someone give me a example where such a comparison would lead to
unwanted results?

Thanks,
Tim.

[Rolf Magnus]

Timothee Groleau wrote:

Hi all,

When should I be worried about doing a comparison of signed vs unsigned
ints? Could someone give me a example where such a comparison would lead
to unwanted results?

#include <iostream>

int main()
{
int a = -100;
unsigned int b = 100;
std::cout << a << " is "
<< (a < b ? "" : "not") << " less than " << b << '\n';
}

[Rolf Magnus]

Rolf Magnus wrote:

Timothee Groleau wrote:

Hi all,

When should I be worried about doing a comparison of signed vs unsigned
ints? Could someone give me a example where such a comparison would lead
to unwanted results?

#include <iostream>

int main()
{
int a = -100;
unsigned int b = 100;
std::cout << a << " is "
<< (a < b ? "" : "not") << " less than " << b << '\n';
}

Another example:

#include <iostream>

int main()
{
int a = -1;
unsigned int b = std::numeric_li mits<unsigned int>::max();
std::cout << a << " is " << (a == b ? "" : "not")
<< " equal to " << b << '\n';
}

[Timothee Groleau]

Rolf Magnus wrote:

#include <iostream>

int main()
{
int a = -1;
unsigned int b = std::numeric_li mits<unsigned int>::max();
std::cout << a << " is " << (a == b ? "" : "not")
<< " equal to " << b << '\n';
}

Thanks Rolf!

[Olaf van der Spek]

On Sat, 06 May 2006 12:29:41 +0200, Rolf Magnus <ra******@t-online.de>
wrote:

Timothee Groleau wrote:

Hi all,

When should I be worried about doing a comparison of signed vs unsigned
ints? Could someone give me a example where such a comparison would lead
to unwanted results?

#include <iostream>

int main()
{
int a = -100;
unsigned int b = 100;
std::cout << a << " is "
<< (a < b ? "" : "not") << " less than " << b << '\n';
}

What's the rationale for doing an unsigned comparion instead of a
signed comparison in case of mixed arguments?

[Alf P. Steinbach]

* Olaf van der Spek:

On Sat, 06 May 2006 12:29:41 +0200, Rolf Magnus <ra******@t-online.de>
wrote:

Timothee Groleau wrote:

Hi all,

When should I be worried about doing a comparison of signed vs unsigned
ints? Could someone give me a example where such a comparison would lead
to unwanted results?

#include <iostream>

int main()
{
int a = -100;
unsigned int b = 100;
std::cout << a << " is "
<< (a < b ? "" : "not") << " less than " << b << '\n';
}

What's the rationale for doing an unsigned comparion instead of a
signed comparison in case of mixed arguments?

Integer promotion comes into play. That's primarily designed for
assignment, promoting to types with ever widening range of values that
can be represented. 'unsigned' can /represent/ all 'int' values (you
can convert from 'int' to 'unsigned' with well-defined unique result)
but not vice versa.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[john]

Alf P. Steinbach wrote:

Integer promotion comes into play. That's primarily designed for
assignment, promoting to types with ever widening range of values that
can be represented. 'unsigned' can /represent/ all 'int' values (you
can convert from 'int' to 'unsigned' with well-defined unique result)
but not vice versa.

How is the int value -1 represented in unsigned int?

[Alf P. Steinbach]

* john:

Alf P. Steinbach wrote:

Integer promotion comes into play. That's primarily designed for
assignment, promoting to types with ever widening range of values that
can be represented. 'unsigned' can /represent/ all 'int' values (you
can convert from 'int' to 'unsigned' with well-defined unique result)
but not vice versa.

How is the int value -1 represented in unsigned int?

Modulo 2^n, where n is the number of bits in the value representation of
unsigned int.

For the exact value (if that's not obvious), see the earlier messages in
this thread, where it was used as an example.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Jim Langston]

"john" <[e-mail] > wrote in message
news:T7******** ***********@new sb.telia.net...

Alf P. Steinbach wrote:

Integer promotion comes into play. That's primarily designed for
assignment, promoting to types with ever widening range of values that
can be represented. 'unsigned' can /represent/ all 'int' values (you can
convert from 'int' to 'unsigned' with well-defined unique result) but not
vice versa.

How is the int value -1 represented in unsigned int?

The easiest way for you to figure that out is to try it.

std::cout << (unsigned int)(-1)

or to make sure
int MyInt = -1;
std::cout << (unsigned int) MyInt;

[john]

Jim Langston wrote:

The easiest way for you to figure that out is to try it.

std::cout << (unsigned int)(-1)

or to make sure
int MyInt = -1;
std::cout << (unsigned int) MyInt;

Ok ,thanks, i see.

But why is the opposite, unsigned to int, undefined?
Shouldn't it just be the other way around?