Bash Script Binary Search

Question

Write a bash script to do a binary search. Read student names and grades from a file into an array. Prompt the user for a student name. Find the name in the array and displa

Answer 1

I think it's best to use a generic binary search function then to code your own for your particular case.

Binary search function in bash

# Returns the largest i for which `command i` succeeds (exits with a null exit code)
function dichotomic_search {

  min=$1
  max=$2
  command=$3

  while [ $min -lt $max ]; do
    # Compute the mean between min and max, rounded up to the superior unit
    current=`expr '(' "$min" + "$max" + 1 ')' / 2`
    if $command $current
      then min=$current
      else max=`expr $current - 1`
    fi
  done

  echo $min
}

It calls the function given as its last argument repetitively using binary search to find the last value for which it returns true. More explanations on Github

Binary search through a bash array

In your case, you would use it like that:

#!/usr/bin/env bash

source dichotomic.sh
arr=(Ann:C Bob:A Cindy:B Dean:E Emily:A Karen:A Zob:A)

function is_smaller {
  element=$(echo ${arr[$2]} | cut -f1 -d :)
  if [[ "$element" > "$1" ]]
    then false
    else true
  fi
}

read target
highest_index=`expr ${#arr[@]} - 1`
index=$(dichotomic_search 0 $highest_index "is_smaller $target")
echo "${arr[$index]}"