Short-circuit evaluation on C

The && operator uses lazy evaluation. If either side of the && operator is false, then the whole expression is false.

C checks the truth value of the left hand side of the operator, which in your case is 0. Since 0 is false in c, then the right hand side expression of the operation, (a = b = 777), is never evaluated.

The second case is similar, except that || returns true if the left hand side expression returns true. Also remember that in c, anything that is not 0 is considered true.

Hope this helps.

From the C Standard (6.5.13 Logical AND operator)

3 The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.

and

4 Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation; if the second operand is evaluated, there is a sequence point between the evaluations of the first and second operands. If the first operand compares equal to 0, the second operand is not evaluated.

In this expression statement

x = 0 && (a = b = 777);

the first operand compares equal to 0. So the second operand is not evaluated that is the values of the variables a and b are not changed. So the variable x will be set to 0 according to the paragraph #3 of the section.

From the C Standard (6.5.14 Logical OR operator)

3 The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.

and

4 Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; if the second operand is evaluated, there is a sequence point between the evaluations of the first and second operands. If the first operand compares unequal to 0, the second operand is not evaluated.

In this expression statement

x = 777 || (a = ++b);

the first operand compares unequal to 0. So the second operand is not evaluated that is the values of the variables a and b are not changed.. So the variable x will be set to 1 according to the paragraph #3 of the section.

If you will change the order of the operands in the expressions like

x = (a = b = 777) && 0;
x = (a = ++b) || 777;

you get the expected by you result.