Numbers, Floats & Percentages

Infrastructure code mostly deals with integers (ports, counts, bytes), but reporting often needs percentages and formatted output.

Percentage Calculation

In Go, dividing two ints gives an int — the decimal part is thrown away. 3 / 8 is 0, not 0.375. This is different from Python 3 where / always gives a float.

So to get a real percentage, you must convert to float64 before dividing:

passed := 7
total := 12

pct := float64(passed) / float64(total) * 100  // 58.333...
fmt.Printf("%.1f%%\n", pct)                     // "58.3%"

The float64() calls do the conversion. %% prints a literal percent sign (because % is the format specifier prefix).

Rounding

math.Round rounds to the nearest integer. But what if you want 1 decimal place? There's no math.Round(x, places). The trick: multiply to shift the decimal point, round, then divide back.

To round to 1 decimal: multiply by 10 (moves the tenths digit into the ones place), round, divide by 10:

avg := 72.6789
rounded := math.Round(avg*10) / 10  // 72.7
// 72.6789 * 10 = 726.789 → Round → 727 → / 10 = 72.7

To round to 2 decimals, multiply/divide by 100. To round to the nearest integer, just math.Round(x).

Parsing Numbers from Strings

import "strconv"

// String → int
n, err := strconv.Atoi("42")
if err != nil {
    // handle: "not a number"
}

// String → float64
f, err := strconv.ParseFloat("3.14", 64)
if err != nil {
    // handle: "not a number"
}

The second argument to ParseFloat is the bit size (64 for float64, 32 for float32). Always use 64.