在条件中忽略“> 0”吗？

Question

I have recently inherited an old project to make some optimization and add new features. In this project I have seen this type of condition all over the code:

if (int_variable)

instead of

if (int_variable > 0)

I have used the first option only with boolean type of variables.

Do you think the first option is a "correct" way to check if a number is positive?

Answer 1

负数也计算为true ，因此您应该使用if (int_variable > 0)来检查数字是否为正。

Answer 2

否。第一种选择是检查数字是否为非零的正确方法。

Answer 3

Any non-zero value will be considered true , so your version with > is a bit sketchy if the variable truly is int , ie signed. For clarity I prefer to make that explicit in many cases, ie write

if (int_variable != 0)

This is perhaps a bit convoluted (basically computing a Boolean where one can be automatically inferred), I would never use if (bool_variable == true) for instance but I think the integer case makes the test clearer. Reading the two cases out loud works way better for the integer.

Answer 4

if (int_variable) has the same meaning as if (int_variable != 0) .

This is not the same as if (int_variable > 0) unless you otherwise know that the value will never be negative.

Answer 5

When using an integer value as a truth value in C, zero yields false while any non-zero value yields true .

In your case, assuming int_variable is signed, (int_variable) is not equivalent to (int_variable > 0) since int_variable being negative will yield true and false , respectively.

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So

if (int_variable) { … }

is not a correct way of checking whether int_variable is positive.

Answer 6

In C any expression which evaluates to integer 0 or a null pointer is considered false. Anything else is true.

So basically something like

int i = ...;
if ( i )

Test if i is 0. However, if i is negative, the condition will also be true, so this is no replacement for i > 0 . Unless i is unsigned . Simply because they cannot be negative. But your compiler may warn about such constructs as they often show a wrong concept.

Coming to coding style, if ( i ) is bad style if you are really comparing to the value 0 . You better compare explicitly: if ( i != 0 ) . However, if i contains some boolean result, you should use a "speaking" name (eg isalpha('A') to use a ctypes.h function instead of a variable) and do not compare. Compare the readbility:

char ch = ...;

if ( isalpha(ch) )

if ( isalpha(ch) != 0 )

For the latter you have to think a second about the comparison - unnecessarily.