# Following a program step by step

[Programming guides](README.md)

To **trace** a program means to follow its instructions one at a time and keep track of what changes. You can do this with a pencil before running the program. Tracing is useful when an answer surprises you.

## Keep a score sheet

Imagine recording a team's score after every round. You cross out the old score and write the new one. A trace table does the same for a program's variables.

Make one column for each value you want to follow. Add a row whenever an instruction changes it. A blank entry means “not created yet,” not zero.

## Trace some coins

```pliro
# language: en
let coins = 3
set coins = coins + 2
let reward = coins * 2
set coins = 0
say reward
say coins
```

Read the right side of an assignment using the values that exist at that moment. Then store the result on the left.

| After this instruction | `coins` | `reward` |
| --- | --- | --- |
| Create `coins` | 3 | Not created |
| Add 2 | 5 | Not created |
| Create `reward` | 5 | 10 |
| Set `coins` to zero | 0 | 10 |

## Predict, then run

Does the reward become zero when the coins become zero?

```text
10
0
```

No. `reward` was calculated from the earlier value of `coins`. It remembers the resulting number, not a formula that keeps recalculating itself. Shared lists and maps need a different explanation; this example uses only numbers.

## Your challenge

Move `set coins = 0` to just before `let reward = coins * 2`. Make a fresh trace table and predict both output lines. Then run the changed version to check your prediction.

## A common mistake

Skipping over a line because it looks unimportant can hide the cause of a bug. Include every assignment in your table. If you get lost in a loop, trace just its first two rounds before trying all of them.

You can also add temporary `say` statements to show intermediate values. Give those messages a clear label so you know what you are looking at.

## Where next?

[Remembering state](state.md) · [Finding bugs](debugging.md) · [Collection sharing](../syntax/collection-sharing.md)