Having difficulty defining conditions to call certain functions and error messages - bash

Question

I'm writing a piece of code which will use data from a file that I've already made in order to work out the average value of the file, the minimum value, maximum value and then finally displaying all values at once.
I'm very new to unix so I'm trying to learn it but I just cant seem to crack where I need to go with my code in order for it to gain functionality.
I've got the basics of the code but I need to find a way to call the functions using the year, which is stored in a directory corresponding to that year which is making me think I'm going to have problems calling from the file as I'm using a sed function to only take line 4 of that file rather than the year.
I also need to figure out how to set error messages and status to the script if they have not stated (Year) (One of the 4 commands), the year doesnt correspond to one available in the tree and the keyword is invalid.
Any help or even pointers towards good material to learn these things would be great.
Here is my current code:
#!/bin/bash
#getalldata() {
#find . -name "ff_*" -exec sed -n '4p' {} \;
#}
#Defining where the population configuration file is which contains all the data
popconfile.txt=$HOME/testarea
#Function to find the average population of all of the files
averagePopulation()
{
total=0
list=$(cat popconfile.txt)
for var in "${list[#]}"
do
total=$((total + var))
done
average=$((total/$(wc -l popconfile.txt)))
echo "$average"
}
#Function to find the maximum population from all the files
maximumPopulation()
{
max=1
for in `cat popconfile.txt`
do
if [[ $1 > "$max" ]]; then
max=$1
echo "$max"
fi
done
}
#Function to find the minimum population from all the files
minimumPopulation()
{
min=1000000
for in `cat popconfile.txt`
do
if [[ $1 < "$min" ]]; then
max=$1
echo "$min"
fi
done
}
#Function to show all of the results in one place
function showAll()
{
echo "$min"
echo "$max"
echo "$average"
}
Thanks!

Answer 1

Assuming your popconfile.txt format is
cat popconfile.txt
150
10
45
1000
34
87
You might be able to simplify your code with :
for i in $(cat popconfile.txt);do
temp[$i]=$i
done
pop=(${temp[*]})
min=${pop[0]}
max=${pop[$((${#pop[*]}-1))]}
for ((j=0;j<${#pop[*]};j++));do
sum=$(($sum+${pop[$j]}))
done
average=$(($sum/${#pop[*]}))
echo "maximum="$max
echo "minimum="$min
echo "average="$average
Be aware though that the average here or in your code is calculated with integer mathematics, so you're loosing all decimals.