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Loop Labels

  • What Is a Loop Label? A loop label assigns an identifier to a loop.

  • Syntax Write a label and colon before the loop.

  • Example The code below prints the multiplication table of 1 to 5 except 3. See how specifying a loop label helps you to skip the table of 3.
fn main() {
 'outer:for i in 1..5 { //outer loop
    println!("Muliplication Table : {}", i);
   'inner:for j in 1..5 { // inner loop
        if i == 3 { continue 'outer; } // Continues the loop over `i`.
        if j == 2 { continue 'inner; } // Continues the loop over `j`.
        println!("{} * {} = {}", i, j, i * j);
   }
 }
}

output:

Muliplication Table : 1
1 * 1 = 1
1 * 3 = 3
1 * 4 = 4
Muliplication Table : 2
2 * 1 = 2
2 * 3 = 6
2 * 4 = 8
Muliplication Table : 3
Muliplication Table : 4
4 * 1 = 4
4 * 3 = 12
4 * 4 = 16

Explanation

  • Outer for Loop
    • A for loop is defined on line 2.
    • The loop has a label outer . It takes i as an iterator that iterates over values from 1 to 4.
  • Inner for Loop
    • A for loop is defined on line 3
    • The loop has a label inner. It takes j as an iterator that iterates over values from 1 to 5.
    • For each i the inner loop iterate j times and prints the product i * j.
    • When the outer loop increments i to 3 and the inner loop starts from j = 1, the condition i == 3 is found to be true and the continue ‘outer statement causes execution to be transferred to the next iteration of the outer loop on line 2. The variable i is incremented to 4 and the execution continues.
    • When the value of j increments to 2, then the 2nd iteration of the inner loop gets skipped and continue ‘inner causes the execution to be transferred to the next iteration of the inner loop on line 4. The variable j is incremented to 3 and the execution continues.
    i j output


    1
    1
    2
    3
    4

    1 * 1 = 1
    1 * 3 = 3
    1 * 4 = 4

    2 1
    2
    3
    4
    2 * 1 = 2
    2 * 3 = 6
    2 * 4 = 8
    3    
    4 1
    2
    3
    4
    4 * 1 = 4
    4 * 3 = 12
    4 * 4 = 16