Bash script - How to loop through rows in a CSV file - bash

Question

I am working with a huge CSV file (filename.csv) that contains a single column. From column 1, I wanted to read current row and compare it with the value of the previous row. If it is greater OR equal, continue comparing and if the value of the current cell is smaller than the previous row - divide the value of the current cell by the value of the previous cell and exit by printing the value of the division. For example from the following example: i wanted my bash script to divide 327 by 340 and print 0.961765 to the console and exit.
338
338
339
340
327
301
299
284
284
283
283
283
282
282
282
283
I tried it with the following awk and it works perfectly fine.
awk '$1 < val {print $1/val; exit} {val=$1}' filename.csv
However, since i want to include around 7 conditional statements (if-else's), I wanted to do it with a bit cleaner bash script and here is my approach. I am not that used to awk to be honest and that's why i prefer to use bash.
#!/bin/bash
FileName="filename.csv"
# Test when to stop looping
STOP=1
# to find the number of columns
NumCol=`sed 's/[^,]//g' $FileName | wc -c`; let "NumCol+=1"
# Loop until the current cell is less than the count+1
while [ "$STOP" -lt "$NumCol" ]; do
cat $FileName | cut -d, -f$STOP
let "STOP+=1"
done
How can we loop through the values and add conditional statements?
PS: the criteria for my if-else statement is (if the value ($1/val) is >=0.85 and <=0.9, print A, else if the value ($1/val) is >=0.7 and <=0.8, print B, if the value ($1/val) is >=0.5 and <=0.6 print C otherwise print D).

Answer 1

Here's one in GNU awk using switch, because I haven't used it in a while:
awk '
$1<p {
s=sprintf("%.1f",$1/p)
switch(s) {
case "0.9": # if comparing to values ranged [0.9-1.0[ use /0.9/
print "A" # ... in which case (no pun) you don't need sprintf
break
case "0.8":
print "B"
break
case "0.7":
print "c"
break
default:
print "D"
}
exit
}
{ p=$1 }' file
D
Other awks using if:
awk '
$1<p {
# s=sprintf("%.1f",$1/p) # s is not rounded anymore
s=$1/p
# if(s==0.9) # if you want rounding,
# print "A" # uncomment and edit all ifs to resemble
if(s~/0.9/)
print "A"
else if(s~/0.8/)
print "B"
else if(s~/0.7/)
print "c"
else
print "D"
exit
}
{ p=$1 }' file
D

Answer 2

This is an alternative approach,based on previous input data describing comparison of $1/val with fixed numbers 0.9 , 0.7 and 0.6.
This solution will not work with ranges like ($1/val) >=0.85 and <=0.9 as clarified later.
awk 'BEGIN{crit[0.9]="A";crit[0.7]="B";crit[0.6]="C"} \
$1 < val{ss=substr($1/val,1,3);if(ss in crit) {print crit[ss]} else {print D};exit}{val=$1}' file
A
This technique is based on checking if rounded value $1/val belongs to a predefined array loaded with corresponding messages.
Let me expand the code for better understanding:
awk 'BEGIN{crit[0.9]="A";crit[0.7]="B";crit[0.6]="C"} \ #Define the criteria array. Your criteria values are used as keys and values are the messages you want to print.
$1 < val{
ss=substr($1/val,1,3); #gets the first three chars of the result $1/val
if(ss in crit) { #checks if the first three chars is a key of the array crit declared in begin
print crit[ss] #if it is, print it's value
}
else {
print D #If it is not, print D
};
exit
}
{val=$1}' file
Using substr we get the first three chars of the result $1/val:
for $1/val = 0.961765 using substr($1/val,1,3) returns 0.9
If you want to make comparisons based on two decimals like 0.96 then change substr like substr($1/val,1,4).
In this case you need to accordingly provide the correct comparison entries in crit array i.e crit[0.96]="A"