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Are these two lines of C code for PIC16F1947 the same?

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T

treez

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Hello,
Our software engineer tells us that the following two lines are the same in every way. Is this true? Do they both execute the same?
(as you can see , one has "!=" and the other has "=!".
The C compiler is the free one that comes with microchip's MPLAB software.



if(eepromWriteCount != BACKUP_1_eepromWriteCount)
Click to expand...

if(eepromWriteCount =! BACKUP_1_eepromWriteCount)
Click to expand...
 

I think you can answer the question yourself applying average C language knowledge.

The first is a pure comparison, the second is an assignment changing the value of eepromWriteCount.

Even if eepromWriteCount is discarded, it's a different if expression argument. First is true if both variables are unequal, the second if BACKUP_1_eepromWriteCount is zero.
 

Agreed.... shoot the software engineer.....

!= means "if not equal do the following" (a comparison)
=! means "make it equal to the following if it isn't true" (assigns a value to eepromWriteCount of zero or one)

Brian.
 

Anyway, an assignment within a conditional expression is something totally not recommended, and would be expected the compiler issue a warning in this case, since it is not a standard implementation, which could be interpreted differently from one compiler to another.
 

Anyway, an assignment within a conditional expression is something totally not recommended, and would be expected the compiler issue a warning in this case, since it is not a standard implementation, which could be interpreted differently from one compiler to another.
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I almost agree, except for a potentially different interpretation.

IMHO, the C language specification is crystal clear in this relation: an assignment is an assignment, the if statement is evaluating the assigned value. The compiler should issue a warning though, because the assignment is possible inadvertent.

Writing if (a = b) instead of if (a == b) is a pretty popular fault.
 

I wasn't suggesting it was good programming practice to use it at all, especially in an 'if' statement.

"if(a == !b)" reads as "if a is equal to the inverted state of b".
"if(a != b)" reads as "if a is not equal to b".
"if(a =! b) reads as "assign the value to a of the inverted state of b"., the 'if' is ambiguous as no query is made inside the braces.

However in the first post the implication is that both a and b are numeric values rather than boolean. I think the line is intended to ask if the working and backup values are different so "!=" is the operator to use.

Brian.
 

"if(a =! b) reads as "assign the value to a of the inverted state of b"., the 'if' is ambiguous as no query is made inside the braces.
Click to expand...
IMHO it's bad programming style, but not ambiguous. It's just a shortcut of
Code C - [expand]
1
2

a = !b;
if (a) ..
 

However in the first post the implication is that both a and b are numeric values rather than boolean. I think the line is intended to ask if the working and backup values are different so "!=" is the operator to use.
Click to expand...
Thanks yes the software guy now wants to use "!="

The following of his code indeed shows that they are not boolean types, so yes i suppose it would be a mystery how the compiler would deal with this "=!"..


// EEPROM VAR (part of eeprom where data change often)

volatile int32_t eepromWriteCount; // number of writes for this block

volatile int32_t BACKUP_1_eepromWriteCount;
Click to expand...
 

The following of his code indeed shows that they are not boolean types, so yes i suppose it would be a mystery how the compiler would deal with this "=!"..
Click to expand...
Again, it's not a mystery, the C standard is crystal clear, both about the comparison "!=" and the logical not "!" applied to numerical types. Most of this thread discussion can be safed by reading a C text book like Kernighan & Ritchie.

As for the "!" operation, the result is 1 if the argument is equal to zero else 0.
 

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