CODEWITHSUNDEEP
clong-integerunsigned-integer
kron maz
kron maz

Reputation: 131

why is a negative long value greater than a positive unsigned long value?

Example code:

long num1 = -1;
unsigned long num2 = 1;

if (num1 > num2)
    printf("Num1 is greater\n");
else
    printf("Num2 is greater\n");

Why does this evaluate num1 > num2 as true instead of it being false?

Upvotes: 2

Views: 283

Answers (3)

Souvik Ghosh
Souvik Ghosh

Reputation: 1

You are comparing two variables of different data types, i.e. long & unsigned long.

In this case, during comparison of two variables, compiler is implicitly typecasting the variable num1 to unsigned long as two variables of different data types can't be compared.

So, num1 is being converted to 65534 [65535 - 1].

Thus, you are getting Num1 is greater

Upvotes: 0

dbush
dbush

Reputation: 223739

When using a long and unsigned long together in an expression, the ‘longvalue is converted tounsigned long. This results in the value -1 becoming the largestunsigned long` value, which is greater than 1.

Upvotes: 0

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726559

This is the result of an integer promotion rule described in C99 standard, section 6.3.1.8:

if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type.

This is exactly what happens in your example: long and unsigned long have the same rank, so the signed long is converted to unsigned long.

Since -1 cannot be represented as an unsigned, the following rules come into play:

When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged. Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

The result is, therefore, 1- + ULONG_MAX-1, which equals ULONG_MAX. That is why the result of converting -1 to unsigned is greater than 1.

Upvotes: 6

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