Split Temporary Variable

Before you begin: Local variables, assignment, and arithmetic.

Split Temporary Variable gives separate names to unrelated values that previously reused one local variable. Each name can then keep one meaning throughout its lifetime. 1

Original version: one variable changes its meaning

A route planner describes travel time and cost. To keep the arithmetic simple, distance is an exact multiple of speed, speed is positive, and prices are whole coins.

public static class Journey
{
    public static string Describe(int distance, int speed, int pricePerUnit)
    {
        int value = distance / speed;
        string timeText = value + " hours";
        value = distance * pricePerUnit;
        return timeText + "; " + value + " coins";
    }
}

value first means hours, then coins. The assignment is legal, but the reader must track which meaning is active at each line.

def describe(distance, speed, price_per_unit):
    value = distance // speed
    time_text = str(value) + " hours"
    value = distance * price_per_unit
    return time_text + "; " + str(value) + " coins"

The same name first denotes hours, then coins. // performs integer division; the stated exact-multiple assumption avoids rounding differences between languages.

Refactored version: one name for each quantity

public static class Journey
{
    public static string Describe(int distance, int speed, int pricePerUnit)
    {
        int hours = distance / speed;
        string timeText = hours + " hours";
        int cost = distance * pricePerUnit;
        return timeText + "; " + cost + " coins";
    }
}

hours remains a duration and cost remains an amount. The calculations and their order stay the same.

def describe(distance, speed, price_per_unit):
    hours = distance // speed
    time_text = str(hours) + " hours"
    cost = distance * price_per_unit
    return time_text + "; " + str(cost) + " coins"

The names now identify stable meanings. describe(100, 50, 3) still returns 2 hours; 300 coins.

Do not split a value that is genuinely being accumulated

A running total legitimately changes as each item is added. A loop counter legitimately advances. Those assignments update one concept rather than assigning unrelated concepts to the same name.

Follow the values reaching each use before renaming. Split at the assignment where the meaning changes, and update only the uses belonging to that value. A compiler or editor can help, but blindly renaming every occurrence would recreate the original confusion under a different name.

The improvement is modest but cumulative: a reader no longer has to remember that a variable called value stopped meaning time three lines ago.

Sources

Martin Fowler — Split Variable, including the older Split Temp terminology.