Table-Driven Tests

In this lesson, explore table-driven tests through one small Go program, and connect its output to the language rules behind it.

Table-driven tests place inputs and expected outcomes in a slice of cases. One loop executes the same assertion logic against each case, reducing repetitive test code.

Include boundary cases and clear case names. Use t.Run with each case to make failures easy to identify.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

square_test.go

package main
import "testing"
func TestSquare(t *testing.T) {
    cases := []struct{name string; n, want int}{
        {"zero",0,0},{"positive",2,4},{"negative",-3,9},
    }
    for _, tc := range cases {
        t.Run(tc.name, func(t *testing.T) {
            if got := square(tc.n); got != tc.want { t.Errorf("got %d; want %d", got, tc.want) }
        })
    }
}

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func square(n int) int { return n * n }
func main() {
	cases := []struct{ n, want int }{{0, 0}, {2, 4}, {-3, 9}}
	passed := 0
	for _, tc := range cases {
		if square(tc.n) == tc.want {
			passed++
		}
	}
	fmt.Println("Cases passed:", passed)
}

Output

Cases passed: 3

The three cases cover zero, positive and negative numbers. All expected outputs match, so the example prints Cases passed: 3.

• Do not silently skip a test case when adding entries.
• For parallel subtests, check loop-variable capture rules on the target Go release.

Add a case for -5 and an expected square of 25.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Table-Driven Tests in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.