Converting Between Data Types

A Decimal Does Not Get Rounded

Suppose a program receives the value 7.9 but needs an integer. The int() function does not choose the nearest integer. Instead, it removes the decimal portion completely. That means 7.9 becomes 7, even though 8 is the nearer rounded value. This single rule explains many results that surprise beginners.

Following the Decimal Portion

Truncation means chopping off the decimal part rather than changing the whole-number part according to a rounding rule. The decimal portion is removed regardless of how large it is. Therefore, 7.1 and 7.9 both become 7, and 3.99999 also becomes 3. For a negative value such as -7.9, removing the decimal portion leaves -7.

Accepted and Rejected Inputs

The int() function works with floating-point values and with strings that represent whole numbers. It raises an error for strings that contain a decimal portion, such as a decimal string, and it also raises an error for invalid input. The important distinction is between a numeric value that has a decimal portion and a string whose characters contain a decimal point: int() can handle the former, but a decimal string must use an additional conversion step.

The Two-Step Decimal String Path

A decimal string such as '3.14' cannot be passed directly to int(). The source-prescribed solution is to convert the string to a floating-point value first and then convert that result to an integer: int(float('3.14')). The first conversion makes the decimal value available to the second conversion; int() then removes the decimal portion and produces 3.

int(float('3.14'))

3

Comparing Conversion Roles

int(), float(), and str() should not be treated as interchangeable names. In the supplied material, int() is the operation that produces an integer, float() is used as the intermediate step that turns the decimal string '3.14' into a value int() can process, and str() is identified as a separate conversion function. The source pack does not provide a detailed rule for the exact result of str(), so the reliable rule for this lesson is to focus on int()'s integer result and float()'s role in the two-step decimal-string conversion.

Mistakes to Avoid

• Assuming int() rounds to the nearest integer

  int() removes the decimal portion instead of rounding it.

  Fix: Apply truncation: 3.99999 becomes 3.

• Expecting a decimal string to work directly

  The source states that int() raises an error for decimal strings.

  Fix: Convert the string to a float first, then pass that result to int().

• Treating a float and a decimal string as the same input

  One is a floating-point value and the other is a string containing decimal characters.

  Fix: Use int() directly for the float, but use int(float('3.14')) for the decimal string.

• Ignoring invalid input

  Invalid input causes int() to raise an error.

  Fix: Check whether the input is a supported float or whole-number string before relying on conversion.

Check Your Prediction

1. Use int() when the desired result is an integer. Remember that it truncates by removing the decimal portion, so 7.9 becomes 7 and 3.99999 becomes 3. It works with floats and whole-number strings, but decimal strings and invalid input raise an error. To process a decimal string such as '3.14', convert it to a float first and then apply int().