Skip to content

For statements#

Code Example

Runnable Example in Jac and JacLib

# For Statements

with entry {
    # Basic for-in loop
    for x in [1, 2, 3] {
        print(x);
    }

    # For-in with range
    for i in range(5) {
        print(i);
    }

    # For-to-by loop
    for i=0 to i<5 by i+=1 {
        print(i);
    }

    # For-to-by counting down
    for i=10 to i>0 by i-=1 {
        print(i);
    }

    # For-to-by with different step
    for i=0 to i<10 by i+=2 {
        print(i);
    }

    # For with else clause (executes when loop completes normally)
    for x in [1, 2, 3] {
        print(x);
    } else {
        print("completed");
    }

    # For with break (else clause skipped)
    for x in range(10) {
        if x == 3 {
            break;
        }
        print(x);
    } else {
        print("not reached");
    }

    # For with continue
    for x in range(5) {
        if x % 2 == 0 {
            continue;
        }
        print(x);
    }

    # Nested for loops
    for i in range(3) {
        for j in range(2) {
            print(f"{i},{j}");
        }
    }

    # For-in over string
    for char in "abc" {
        print(char);
    }

    # For-in over dictionary keys
    d = {"a": 1, "b": 2};
    for key in d {
        print(key);
    }

    # Mix for-in and for-to-by in nested loops
    for i in ["a", "b"] {
        for j=0 to j<2 by j+=1 {
            print(f"{i}{j}");
        }
    }
}

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
# For Statements

with entry {
    # Basic for-in loop
    for x in [1, 2, 3] {
        print(x);
    }

    # For-in with range
    for i in range(5) {
        print(i);
    }

    # For-to-by loop
    for i=0 to i<5 by i+=1 {
        print(i);
    }

    # For-to-by counting down
    for i=10 to i>0 by i-=1 {
        print(i);
    }

    # For-to-by with different step
    for i=0 to i<10 by i+=2 {
        print(i);
    }

    # For with else clause (executes when loop completes normally)
    for x in [1, 2, 3] {
        print(x);
    } else {
        print("completed");
    }

    # For with break (else clause skipped)
    for x in range(10) {
        if x == 3 {
            break;
        }
        print(x);
    } else {
        print("not reached");
    }

    # For with continue
    for x in range(5) {
        if x % 2 == 0 {
            continue;
        }
        print(x);
    }

    # Nested for loops
    for i in range(3) {
        for j in range(2) {
            print(f"{i},{j}");
        }
    }

    # For-in over string
    for char in "abc" {
        print(char);
    }

    # For-in over dictionary keys
    d = {"a": 1, "b": 2};
    for key in d {
        print(key);
    }

    # Mix for-in and for-to-by in nested loops
    for i in ["a", "b"] {
        for j=0 to j<2 by j+=1 {
            print(f"{i}{j}");
        }
    }
}

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
from __future__ import annotations
from jaclang.runtimelib.builtin import *
for x in [1, 2, 3]:
    print(x)
for i in range(5):
    print(i)
i = 0
while i < 5:
    print(i)
    i += 1
i = 10
while i > 0:
    print(i)
    i -= 1
i = 0
while i < 10:
    print(i)
    i += 2
for x in [1, 2, 3]:
    print(x)
else:
    print('completed')
for x in range(10):
    if x == 3:
        break
    print(x)
else:
    print('not reached')
for x in range(5):
    if x % 2 == 0:
        continue
    print(x)
for i in range(3):
    for j in range(2):
        print(f'{i},{j}')
for char in 'abc':
    print(char)
d = {'a': 1, 'b': 2}
for key in d:
    print(key)
for i in ['a', 'b']:
    j = 0
    while j < 2:
        print(f'{i}{j}')
        j += 1
Jac Grammar Snippet 
137
138
for_stmt: KW_ASYNC? KW_FOR assignment KW_TO expression KW_BY assignment code_block else_stmt?
       | KW_ASYNC? KW_FOR atomic_chain KW_IN expression code_block else_stmt?

Description

For Statements in Jac

For loops provide iteration over collections and sequences. Jac offers two distinct loop styles: the for-in pattern for iterating collections, and the for-to-by pattern for explicit counter control.

Basic For-In Loop

The simplest iteration pattern uses for-in to loop through collections (lines 5-7):

The loop variable x takes each value from the list in sequence. This works with any iterable: lists, tuples, sets, dictionaries, strings, and ranges.

For-In with Range

Lines 10-12 demonstrate using range() to generate numeric sequences. This produces values 0 through 4. The range() function is lazy, generating values on demand rather than creating a full list in memory.

For-To-By Loop (Jac's C-Style Loop)

Lines 15-17 show Jac's unique three-part loop syntax. This loop has three components: - Initialization (i=0): Sets the starting value - Condition (to i<5): Loop continues while true - Increment (by i+=1): Executed after each iteration

This provides explicit control similar to C's for(int i=0; i<5; i++) but with more readable syntax.

Counting Down

Line 20-22 shows decrementing with for-to-by. The loop starts at 10, continues while i>0, and decrements by 1 each iteration.

Custom Step Values

Lines 25-27 demonstrate non-unit steps. This produces even numbers: 0, 2, 4, 6, 8.

For-Else Clause

Lines 30-34 introduce the else clause, which executes only if the loop completes normally (without break). This pattern is useful for search operations: if you break when finding an item, the else clause indicates "not found."

Breaking Out of Loops

Lines 37-44 show how break exits the loop immediately and skips the else clause. Output: 0, 1, 2 (the else block doesn't execute).

Continue Statement

Lines 47-52 demonstrate continue, which skips to the next iteration. This prints only odd numbers: 1, 3.

Nested Loops

Lines 55-59 show loops within loops. Both for-in and for-to-by loops can be nested and mixed freely.

Iterating Strings

Lines 62-64 demonstrate character iteration. Strings are iterable sequences of characters.

Dictionary Iteration

Lines 67-70 show that iterating a dictionary yields its keys. Use .values() for values or .items() for key-value pairs.

Mixed Loop Types

Lines 73-77 demonstrate combining different loop styles. The outer loop uses for-in while the inner uses for-to-by.

Loop Control Flow Summary

Statement Effect Else Clause Behavior
break Exit loop immediately Skipped
continue Skip to next iteration Not affected
Normal completion Loop finishes naturally Executes (if present)

For Loop Variations

Form Syntax Use Case
for-in for var in iterable Iterate collections
for-to-by for i=start to cond by step Explicit counter control
for-else for ... { } else { } Detect uninterrupted completion

Loop Flow Visualization

flowchart TD
    Start([Start For Loop]) --> Check{Condition<br/>True?}
    Check -->|Yes| Body[Execute Body]
    Body --> Continue{Continue<br/>Statement?}
    Continue -->|Yes| Update
    Continue -->|No| Break{Break<br/>Statement?}
    Break -->|Yes| End([Exit Loop])
    Break -->|No| Update[Update/Next Item]
    Update --> Check
    Check -->|No| Else{Else<br/>Clause?}
    Else -->|Yes, No Break| ElseBody[Execute Else]
    Else -->|No| End
    ElseBody --> End

Common Patterns

Filtering during iteration:

Aggregating values:

Finding with for-else:

Key Differences from Python

  1. Braces required: Jac uses { } for loop bodies, not indentation
  2. Semicolons required: Each statement ends with ;
  3. For-to-by syntax: Unique to Jac, provides C-style explicit control
  4. Same else clause: Works identically to Python