Use <constant> == <value>

This tip submitted by Webmaster on 2005-01-24 06:53:34. It has been viewed 22266 times.
Rating of 5.8 with 165 votes

When starting out, to avoid using a single equal sign instead of == to check equality, you can put the constant value on the left hand side:

int a;
if(3 == a)

instead of

int a;
if(a == 3)

In this way, if you accidentally write

if(3 = a)

then your compiler will generate an error about an "invalid lvalue", meaning that you can't assign the value of a to the number 3. (It already has a value.)

Another way of avoiding this problem is to turn on all compiler warnings and look for something like:

"suggest parentheses around assignment used as truth value"

This indicates that the result of an assignment (e.g., a = 3) is being used to check for truth. In some cases, this is reasonable -- for instance, when reading characters:

while((g = getchar()) != '\n')

Since the value of an assignment is the value assigned to the variable, this will work as expected. (For the same reason, something like a = 3 will always be true because it will evaluate to 3.

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