Control Flow

Control flow is where XE starts to feel like a small usable language instead of only a syntax demo. This chapter covers branching, looping, and loop control.

if, elif, and else

XE conditionals look like this:

score = 82

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
else:
    print("Keep going")

Rules:

• conditions use truthiness: false, none, 0, "", [] and {} are false, everything else is true
• each branch is an indented block
• elif chains are supported
• else is optional

repeat N times

Use repeat for a fixed-count loop:

repeat 3 times:
    print("XE")

The count must be a non-negative integer at runtime.

These fail:

• negative counts
• decimal counts such as 2.5
• non-number counts

while

Use while when the number of iterations depends on a condition:

count = 0

while count < 3:
    print(count)
    count = count + 1

The condition is checked before each iteration.

for ... in ...

XE allows for loops over:

• lists
• text values, one character at a time
• maps, which visit their keys in insertion order
• range(...), which counts numbers

Count with range:

for i in range(5):
    print(i) # 0 to 4

for i in range(10, 0, -2):
    print(i) # 10, 8, 6, 4, 2

Example over a list:

total = 0

for item in [1, 2, 3, 4]:
    total = total + item

print(total)

Example over text:

for ch in "XE":
    print(ch)

For text, each iteration variable receives a one-character text value.

break

break exits the nearest loop immediately.

count = 0

while true:
    count = count + 1
    if count == 3:
        break

print(count)

break outside a loop is a compiler error.

continue

continue skips the rest of the current iteration and starts the next one.

count = 0
total = 0

while count < 5:
    count = count + 1
    if count == 3:
        continue
    total = total + count

print(total)

continue outside a loop is also a compiler error.

pass

A block cannot be empty; write pass where there is nothing to do:

for item in [1, 2, 3]:
    pass

try and catch

try runs a block, and if a runtime error happens inside it, runs the catch block instead of stopping the program. The name after catch (optional) receives the error message:

fun safe_divide(a, b):
    try:
        return a / b
    catch err:
        print("could not divide:", err)
        return 0

print(safe_divide(1, 0))

Raise your own errors with error("message").

Scope inside control-flow blocks

Blocks do not create a new scope, as in Python. A name assigned inside an if, loop or try block is visible after it:

score = 70
if score >= 50:
    result = "pass"
else:
    result = "fail"
print(result)

Inside a function, a variable must be assigned on every path before it is read; otherwise compiling fails with "might not be assigned yet". A for loop's own variable belongs to the loop.

Short-circuit behavior

Logical and and or use short-circuit evaluation:

• left and right only evaluates right if left is truthy
• left or right only evaluates right if left is falsey

This matters when the right-hand side contains function calls or conversions that may fail.

Next steps

• Continue with Functions and Scope
• Then read Runtime Behavior and Errors