Practice question · Multiple choice
if x = 5 is a common bug in languages where it is legal, while if x == 5 is what was meant. Why is the mistake so easy to make and so hard to spot?
Hints
- In C, what does the expression (x = 5) evaluate to?
- Ask what error message the programmer receives.
Show the answer
B. Because assignment is an expression, so the mistaken version compiles and runs.
Why
In C and its relatives assignment is an expression whose value is the assigned value, so if (x = 5) assigns, evaluates to 5, and takes the branch, silently, with nothing to report. Language designers have responded in several ways, which shows how serious it is: Python makes assignment a statement, Java demands a boolean, and C programmers invented if (5 == x).
Practise Conditionals and Logical Branching
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