Control Flow Patterns

Foreach with Next and Last

next skips to the next item, and last stops the loop early.

loop control Loop control statements change how the current loop continues.

Foreach with Next and Last

stop
foreach_next_last.pl
Replay: real traced execution (multi-file project)
use strict;
use warnings;

my $stop = 5;
my @values = (1, 2, 3, 4, 5, 6);
my $kept = 0;
my $seen = 0;

foreach my $value (@values) {
    if ($value > $stop) {
        last;
    }
    if ($value % 2 == 0) {
        next;
    }
    $kept = $kept + $value;
    $seen = $seen + 1;
}

print "stop=$stop\n";
print "seen=$seen\n";
print "kept=$kept\n";

use strict;
use warnings;

my $stop = 4;
my @values = (1, 2, 3, 4, 5, 6);
my $kept = 0;
my $seen = 0;

foreach my $value (@values) {
    if ($value > $stop) {
        last;
    }
    if ($value % 2 == 0) {
        next;
    }
    $kept = $kept + $value;
    $seen = $seen + 1;
}

print "stop=$stop\n";
print "seen=$seen\n";
print "kept=$kept\n";

use strict;
use warnings;

my $stop = 6;
my @values = (1, 2, 3, 4, 5, 6);
my $kept = 0;
my $seen = 0;

foreach my $value (@values) {
    if ($value > $stop) {
        last;
    }
    if ($value % 2 == 0) {
        next;
    }
    $kept = $kept + $value;
    $seen = $seen + 1;
}

print "stop=$stop\n";
print "seen=$seen\n";
print "kept=$kept\n";

  1. $stop ← 5, @values ← 6, $kept ← 0, $seen ← 0

    4my $stop→ 5 = 5; #@stop=4, 65my @values→ 6 = (1, 2, 3, 4, 5, 6);6my $kept→ 0 = 0;7my $seen→ 0 = 0;

  2. $kept ← 1, $seen ← 1

    pass 1 of 6
    9foreach my $value1 (@values6) {10    if ($value > $stop) {11        last;12    }13    if ($value % 2 == 0) {14        next;15    }16    $kept→ 1 = $kept + $value1;17    $seen→ 1 = $seen + 1;18}
    All 6 passes — pass 1 is the card above
    pass$value$stop$kept$seen
    11—0 → 10 → 1
    22———
    33—1 → 41 → 2
    44———
    55—4 → 92 → 3
    665——

  3. if ($value % 2 == 0)

    pass 1 of 2
    12}13if ($value2 % 2 == 0) {14    next;15}

  4. if ($value % 2 == 0)

    pass 2 of 2
    12}13if ($value4 % 2 == 0) {14    next;15}

  5. if ($value > $stop)

    9foreach my $value (@values) {10    if ($value6 > $stop5) {11        last;12    }

  6. print "stop=$stop ";

    20print "stop=$stop5\n";21print "seen=$seen3\n";22print "kept=$kept9\n";
    outputstop=5
seen=3
kept=9

  1. $stop ← 4, @values ← 6, $kept ← 0, $seen ← 0

    4my $stop→ 4 = 4;5my @values→ 6 = (1, 2, 3, 4, 5, 6);6my $kept→ 0 = 0;7my $seen→ 0 = 0;

  2. $kept ← 1, $seen ← 1

    pass 1 of 5
    9foreach my $value1 (@values6) {10    if ($value > $stop) {11        last;12    }13    if ($value % 2 == 0) {14        next;15    }16    $kept→ 1 = $kept + $value1;17    $seen→ 1 = $seen + 1;18}
    All 5 passes — pass 1 is the card above
    pass$value$stop$kept$seen
    11—0 → 10 → 1
    22———
    33—1 → 41 → 2
    44———
    554——

  3. if ($value % 2 == 0)

    pass 1 of 2
    12}13if ($value2 % 2 == 0) {14    next;15}

  4. if ($value % 2 == 0)

    pass 2 of 2
    12}13if ($value4 % 2 == 0) {14    next;15}

  5. if ($value > $stop)

    9foreach my $value (@values) {10    if ($value5 > $stop4) {11        last;12    }

  6. print "stop=$stop ";

    20print "stop=$stop4\n";21print "seen=$seen2\n";22print "kept=$kept4\n";
    outputstop=4
seen=2
kept=4

  1. $stop ← 6, @values ← 6, $kept ← 0, $seen ← 0

    4my $stop→ 6 = 6;5my @values→ 6 = (1, 2, 3, 4, 5, 6);6my $kept→ 0 = 0;7my $seen→ 0 = 0;

  2. $kept ← 1, $seen ← 1

    pass 1 of 6
    9foreach my $value1 (@values6) {10    if ($value > $stop) {11        last;12    }13    if ($value % 2 == 0) {14        next;15    }16    $kept→ 1 = $kept + $value1;17    $seen→ 1 = $seen + 1;18}
    All 6 passes — pass 1 is the card above
    pass$value$kept$seen
    110 → 10 → 1
    22——
    331 → 41 → 2
    44——
    554 → 92 → 3
    66——

  3. if ($value % 2 == 0)

    pass 1 of 3
    12}13if ($value2 % 2 == 0) {14    next;15}
    All 3 passes — pass 1 is the card above
    pass$value
    12
    24
    36

  4. print "stop=$stop ";

    20print "stop=$stop6\n";21print "seen=$seen3\n";22print "kept=$kept9\n";
    outputstop=6
seen=3
kept=9

Keep Odd Values Until Stop

  1. $stop starts at 5.
  2. The loop visits values from 1 through 6.
  3. Even values use next, so they are skipped.
  4. Values above 5 use last, so the loop stops.
  5. The kept total is 1 + 3 + 5, which is 9. | Value | Action | Seen | Kept | | --- | --- | --- | --- | | 1 | add | 1 | 1 | | 2 | next | 1 | 1 | | 3 | add | 2 | 4 | | 4 | next | 2 | 4 | | 5 | add | 3 | 9 | | 6 | last | 3 | 9 |

Exercise: foreach_next_last.pl

Use next to skip even values, last after a stop value, and print seen and kept totals