QUESTION 73 · Expressions and conversions

What is implicit type conversion? Give an example.

2 marks

Answer

Implicit type conversion is a conversion performed automatically by C without a written cast.

In double value = 5;, integer 5 is converted to double, giving 5.0.
A conversion can lose information: converting 3.8 to int discards the fractional part.

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QUESTION 74 · Expressions and conversions

Explain implicit type conversion, integer promotions and mixed-type arithmetic.

5 marks

Answer in brief

Implicit type conversion is a conversion performed automatically according to C rules, without writing a cast at that point.

Where conversions occur

Initialisation: In double value = 5;, integer 5 is converted to double.
Assignment: In int count = 3.8;, the fractional part is discarded and count becomes 3.
Mixed arithmetic: In 5 + 2.5, the integer is converted to double and the result is 7.5.
Function arguments: Arguments can be converted to the parameter types. Extra promotion rules apply to functions such as printf.

Integer promotions

Small integer types such as char and short commonly become int before arithmetic. If int cannot represent all values of the original type, the promotion uses unsigned int. This allows many calculations on smaller integer values to use a standard arithmetic type.

Advantages

Convenience: The programmer does not need to write a cast for every permitted conversion.
Mixed calculations: Values of different arithmetic types can be used together according to the language rules.

Limitations and example

Automatic conversion can lose information. Converting 3.8 to int discards .8. Mixing signed and unsigned integers can also produce unexpected comparisons, so check the types. In double average = 7 / 2;, integer division happens first and gives 3; only then is 3 converted to 3.0. Use 7.0 / 2 or convert an operand before division to obtain 3.5.

Reference table

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Where conversions occur
Context	Example	Effect
Integer promotion	A short used in arithmetic	Often promotes to int; unsigned int is used when int cannot represent all values.
Mixed arithmetic	int + double	The integer value is converted for double arithmetic.
Assignment	double result = 5;	The integer value is converted to double.
Narrowing assignment	int count = 3.9;	A representable floating value is truncated toward zero, producing 3.
Variadic function arguments	printf("%f", a_float)	float is promoted to double.

Key points

Implicit does not mean harmless or lossless.
Signedness matters in comparisons and arithmetic.