if-else condition in C

Question

In C language, what is the result of x=0 if I put it in if-else condition? if it is represent false or if this assignment is finished, then represent true?

my colleague write code as:

if(condition = 0)
{
    //do process A
}
else
{
    // do process B
}

obviously, this code is wrong, I know it should be condition == 0 or ((condition=foo()) == 0) but my assumption is program should always do process A because i think if(condition = 0) should always return true since this is set value 0 to variable condition and this set process should be true. However, program always do process B, that means if use the variable condition value and my assumption is wrong.

Then I did a another test code:

if(condition = 2) //or other none-zero value
{
    //do process A
}
else
{
    // do process B
}

this time, program always do process A.

My question is why if-else condition does not use the operation value of condition but use the left variable after setting?

MD. Khairul Basar · Accepted Answer · 2014-10-16 03:13:01Z

4

when you assign 0 to variable condition it becomes false as 0 represents false and any non-zero value represents true.so, when you assign 0 else condition is executed and when you assign 2 condition represents a true statement so, it executes...

if(condition = 0)

after assigning value 0 to condition it becomes

if(condition)

and as it is false, it doesn't execute.but, when condition = 2, it works in the same way and become true .so, the if condition is executed then.

edited Oct 16, 2014 at 3:13

answered Oct 16, 2014 at 3:06

MD. Khairul Basar

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4 Comments

How Chen Over a year ago

but I think the assign operation should return true

MD. Khairul Basar Over a year ago

why it should return true ? it will return true for all the values except only for 0.

How Chen Over a year ago

I mean the assign operation is successfully done, and no error return during assignment

Colonel Thirty Two Over a year ago

@HowChen Assignment in C can't possibly result in an error (well, it could result in undefined behavior, but that's undefined). Returning a success/fail would be meaningless.

George Eco · Accepted Answer · 2014-10-16 03:12:20Z

3

You use wrong operator.

a = 10;

The "equals" operator means "assign value to".

Now to compare two operands, a and b, to see if a = b, you use another operand. That operand is double equals (==).

if(variable == 0)
{
    //do process A
}
else
{
    // do process B
}

Now look.

If the variable with name "variable" has value = 8 that is not equal to 0, thus the whole "variable == 0" expression if FALSE. So proccess B will run.

If otherwise variable is equal to 0 indeed, proccess A will be executed because "variable == 0" is true.

It's like:

if(EXPRESSION)
{
    // EXPRESSION IS TRUE
}
else
{
    //EXPRESSION IS FALSE
}

Got it? :)

answered Oct 16, 2014 at 3:12

George Eco

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2 Comments

How Chen Over a year ago

I know how to use if-else, pls look my question, I need to know why if use condition value 0 but not use the operation result of a assignment

George Eco Over a year ago

You got an expression in a parenthesis. That expression can be something (a == b) or just (b). If you now assign (b = 2) it is not 0 so not false. So it is true. Thus (b=2) is equal to just (b) with value different than 0. So it returns always true. Hope this helped you. That 2 assigns different than false value really...

Colonel Thirty Two · Accepted Answer · 2014-10-16 03:05:47Z

2

In C, assignment is an expression that returns the set value; i.e. x = 2 will result in 2.

This allows you to do something like this:

unsigned char ch;
while((ch = readFromFile(f)) != EOF) {
    // do something with ch
}

It also allows you to shoot yourself in the foot if you accidentally mistype == as =, which is why this 'feature' doesn't appear in a lot of other languages.

In your first loop, the expression condition = 0 will always result in 0, causing the else branch to be taken. Similarly, condition = 2 results in 2, which causes the true branch to be taken.

answered Oct 16, 2014 at 3:05

Colonel Thirty Two

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3 Comments

How Chen Over a year ago

but what is operation the result of x=2, true of false?

M.M Over a year ago

@HowChen read the first sentence of this post. The result of an assignment is the variable that was assigned to. if (x = 0) is the same as x = 0; if (x)

librik Over a year ago

it is 2. And when an integer appears in an if statement, then 0 means false, and anything not 0 means true! That's an important fact about C.

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