Hash Tables

Group by Key

Build buckets keyed by a shared field, preserving the first-seen key order.

Algorithm

Canonical pairs (a,1), (b,2), (a,3), (c,4), (b,5) print {a: [1, 3], b: [2, 5], c: [4]}. The replay uses the same input in every language, so this Bash DSA implementation can be compared directly with the rest of the DSA track.

bucket map Each key owns a list. A new key creates a bucket; a repeated key appends to the existing bucket.

Basic Implementation

basic.sh
Replay: real traced execution (multi-file project)
#!/usr/bin/env bash
keys=(a b a c b)
values=(1 2 3 4 5)
declare -A groups=()
order=()
for i in "${!keys[@]}"; do
  key=${keys[$i]}
  value=${values[$i]}
  if [[ -z ${groups[$key]+set} ]]; then
    groups[$key]=$value
    order+=("$key")
  else
    groups[$key]="${groups[$key]}, $value"
  fi
done
parts=()
for key in "${order[@]}"; do
  parts+=("$key: [${groups[$key]}]")
done
joined=""
for part in "${parts[@]}"; do
  if [[ -n "$joined" ]]; then
    joined+=", "
  fi
  joined+="$part"
done
printf '{%s}\n' "$joined"

  1. pairs ← [(a, 1), (b, 2), (a, 3), (c, 4), (b, 5)]

    1#!/usr/bin/env bash2keys=(a b a c b)
    values this step[(a, 1), (b, 2), (a, 3), (c, 4), (b, 5)]pairs

  2. groups ← {}

    3values=(1 2 3 4 5)4declare -A groups=()5order=()
    values this step{}groups

  3. groups ← {a: [1]}

    3values=(1 2 3 4 5)4declare -A groups=()5order=()
    values this step{} → {a: [1]}groupsakey1value

  4. groups ← {a: [1], b: [2]}

    3values=(1 2 3 4 5)4declare -A groups=()5order=()
    values this step{a: [1]} → {a: [1], b: [2]}groupsbkey2value

  5. groups ← {a: [1, 3], b: [2]}

    3values=(1 2 3 4 5)4declare -A groups=()5order=()
    values this step{a: [1], b: [2]} → {a: [1, 3], b: [2]}groupsakey3value

  6. groups ← {a: [1, 3], b: [2], c: [4]}

    3values=(1 2 3 4 5)4declare -A groups=()5order=()
    values this step{a: [1, 3], b: [2]} → {a: [1, 3], b: [2], c: [4]}groupsckey4value

  7. groups ← {a: [1, 3], b: [2, 5], c: [4]}

    3values=(1 2 3 4 5)4declare -A groups=()5order=()
    values this step{a: [1, 3], b: [2], c: [4]} → {a: [1, 3], b: [2, 5], c: [4]}groupsbkey5value

  8. stdout ← {a: [1, 3], b: [2, 5], c: [4]}

    26done27printf '{%s}\n' "$joined"
    values this step{a: [1, 3], b: [2, 5], c: [4]}stdout{a: [1, 3], b: [2, 5], c: [4]}groups

  9. bucket 1 after collision ← c -> a, degradation risk ← long chains can degrade lookup toward O(n)

    26done27printf '{%s}\n' "$joined"
    values this stepa → c -> abucket 1 after collisionlong chains can degrade lookup toward O(n)degradation riskresize or rehash when load factor growsmitigationcnew key

Complexity

  • Time: O(n) average
  • Space: O(k + n) for buckets and values

Implementation notes

  • Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
  • The trace highlights the hash table state after each write and includes a collision contrast where one bucket chain grows, showing why long chains can degrade lookup and why real tables resize or rehash.