incr: Counting Without expr

Part I · Module 3 · Chapter 3.9 · incr: Counting Without expr

1. The Command

set errors 0

incr errors
puts "after incr: $errors"

after incr: 1

incr takes a variable name and adds one to it. Note there is no $ on the name — incr changes the variable, so it needs the name, not the value. Same shape as set.

The long way round would be:

set errors [expr {$errors + 1}]

Both work. incr is shorter, says what it means, and has no expression in it — so there is no bracing decision to get wrong. For counting, prefer it.

2. Any Step, Including Negative

A second argument sets the step:

set errors 0

incr errors
puts "after incr:      $errors"

incr errors 5
puts "after incr 5:    $errors"

incr errors -2
puts "after incr -2:   $errors"

after incr:      1
after incr 5:    6
after incr -2:   4

There is no separate decrement command. A negative step is how you count down.

3. It Returns the New Value

Like set, incr hands back what it produced:

set count 4
puts "incr returns: [incr count]"
puts "count is now: $count"

incr returns: 5
count is now: 5

Useful when you want to count and use the number in one step, though two plain lines are usually clearer.

4. It Creates a Missing Variable

Here is the first thing that surprises people:

incr fresh
puts "fresh = $fresh"

incr other 7
puts "other = $other"

fresh = 1
other = 7

Neither variable existed. incr treated the missing value as zero and created the variable.

That is convenient — you can count without initialising first. It is also a hazard, because a typo in the variable name will not produce an error:

set error_count 0

incr eror_count          ;# typo, and Tcl is perfectly happy

puts "error_count = $error_count"
puts "eror_count  = $eror_count"

error_count = 0
eror_count  = 1

No error, two variables, and a count that stays at zero forever. Compare this with reading a misspelled variable, which fails immediately — Chapter 3.2 §8.

5. It Requires an Integer

The second surprise:

set x "abc"
incr x

expected integer but got "abc"

Expected. But so is this:

set x 1.5
incr x

expected integer but got "1.5"

incr does whole numbers only. A float is an error, not a rounded result.

If you are accumulating a non-integer quantity, use expr:

set total 0.0
set total [expr {$total + 1.5}]
set total [expr {$total + 2.25}]

puts "total = $total"

total = 3.75

The step may also not be a float — incr x 0.5 fails the same way.

6. The Shape You Will Write Most

A counter has three parts, and they are usually spread across a script:

set checked 0
set failed 0

incr checked
incr checked
incr failed
incr checked

puts "checked $checked, failed $failed"

checked 3, failed 1

Initialise, increment where the event happens, report at the end. Once you reach Module 9 the increments will sit inside a loop, but the shape is this.

7. Common Mistakes

Writing incr $count. That increments the variable named by the value of count. Use incr count.

Relying on auto-creation. It hides typos. §4.

Expecting it to handle decimals. §5. Use expr.

Looking for a decrement command. incr x -1.

Using expr for simple counting. It works, but incr is clearer and has no bracing question.

8. Exercises

Exercise 1 — count up and down. Start a variable at 10. Increment by 1, then by 5, then by -3, printing after each. Predict each value first.

Exercise 2 — see the auto-creation. Increment a variable you never set, and print it. Then explain in one sentence why this is both convenient and risky.

Exercise 3 — reproduce the typo bug. Write the §4 typo example. Confirm there is no error. Then say what you would have to do to notice this in a real script.

Exercise 4 — meet both errors. Trigger incr on a value of abc and on a value of 2.5. Are the two messages the same shape? What does that tell you about what incr checks?

Exercise 5 — accumulate a decimal. Add 0.1 to a running total five times, then print it. Which command did you have to use, and why could incr not do it?

Exercise 6 — two counters. Write a script that counts how many of the values 5 12 7 20 3 are greater than 10, without a loop — just repeat the comparison by hand using if and incr. The point is the counter shape, not the comparison.

9. Remember

• incr name adds one to a variable in place. No $ on the name.
• A second argument sets the step, and a negative step counts down. There is no decrement command.
• It returns the new value.
• It creates a variable that does not exist, starting from zero — convenient, and it hides typos.
• Initialise counters anyway. One line, and the script documents itself.
• It requires whole numbers. A float is an error, not a rounding. Use expr for decimals.
• Prefer it to expr for counting — shorter, clearer, and no bracing decision.

Next: the arithmetic result that catches more people than any other in Tcl.