What is the result of an assignment expression in C?

Question

In the following code:

int c;
while((c=10)>0)

What does c = 10 evaluate to? Is it 1 which indicates that the value 10 is assigned to variable c successfully, or is it 10? Why?

score 17 · Accepted Answer · answered May 15 '13 at 14:23

17

c = 10 is an expression returning 10 which also assigns 10 to c.

answered May 15 '13 at 14:23

Bathsheba

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if it assigns 10 to c, but should c = 10 return 1? – user2131316 May 15 '13 at 14:24
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That would not be nice, think about the statement a = c = 10; Wouldn't you want a to be 10, not 1? – Bathsheba May 15 '13 at 14:25
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@user2131316: the *expression* `c = 10` has the value of `c` after the assignment ([N1570](http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf), 6.5.15/3). Assignment expressions are not Boolean expressions. – John Bode May 15 '13 at 15:03
It doesn't return 10, it returns `c` which is 10 for the first call. The difference is that the value of `c` will most likely change in the `while` block. – ramazan polat Aug 31 '20 at 08:34

score 13 · Answer 2 · answered Aug 30 '14 at 03:08

13

It is said in C99 6.5.16

An assignment operator stores a value in the object designated by the left operand. An        
assignment expression has the value of the left operand after the assignment, but is not an 
lvalue.

answered Aug 30 '14 at 03:08

Gurity

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Best answer - clear, precise and short – martinkunev Dec 25 '19 at 10:45
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To follow up @martinkunev's answer: this answer is also better because it references the standard directly, and more directly speaks to the question of the type of the expression's value, which matches the type of the left-hand operand. (Consider *a=b=c* where *a* and *c* are *unsigned int* and *b* is *unsigned short*.) – Max Barraclough May 02 '20 at 10:51
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This also means that `int x = 10; int y = (x += 1);` results in `x = 11, y = 11`, **not** `x = 11, y = 1`. – Qix - MONICA WAS MISTREATED Nov 19 '20 at 03:23

score 2 · Answer 3 · 2013-05-15T14:34:27.443

2

Assignment returns with the assigned value. In case c=10 is 10. Since 10!=0, in c it means also true so this is an infinite loop.

It is like you would write

while(10)

Plus You've made the assignment.

If You follow this logic, You can see, that

while(c=0)

would be a loop that never executes its statement or block.

edited May 15 '13 at 14:34

answered May 15 '13 at 14:22

Ashish Rawat · Answer 4 · 2013-05-15T14:42:33.360

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This is an infinite loop. It first assign 10 to c, then compare it with c > 0, then again loop starts, assign 10 to c, compare it with c>0 and so on. Loop never ends. This is equivalent to the following:

while(c=10);

/* Because c assign a garbage value, but not true for all cases maybe it assign 0 */
while(c);

Edit: It will not return 10 because compiler return only true or false value, so it return true or 1 instead of 10.

edited May 15 '13 at 14:42

answered May 15 '13 at 14:25

Ashish Rawat

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Why will c be assigned a garbage value? – Suvarna Pattayil May 15 '13 at 14:31
cause it's not initialized yet and compiler automatically assign some value to this. As all addresses in memory have some value before it initialized – Ashish Rawat May 15 '13 at 14:31
`c = 10` will it not assign the value `10` to `c`? – Suvarna Pattayil May 15 '13 at 14:34
It assign 10 to c in 1st statement not in second – Ashish Rawat May 15 '13 at 14:35
Oh, I thought its one single block – Suvarna Pattayil May 15 '13 at 14:36

score 1 · Answer 5 · edited May 23 '17 at 12:02

1

while((c=10)>0)

c = 10 should return 10.

Now, for while(10>0) 10>0, the > operator returns 1 (non-zero value).

edited May 23 '17 at 12:02

Community

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answered May 15 '13 at 14:33

Suvarna Pattayil

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What is the result of an assignment expression in C?

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