bash array yields array_name[@] at index 0

Question

I am trying to store in a Look-Up-Table(sine) sin(x) values normalized between 0 and 255, with x being mapped from [0; 2Pi] to [0; 255], all integers. However when using trying to access the 0th or 256th index it shows up soemtimes as sine[@]

#! /bin/bash
function load_sin {
    sine=${1}[@]
    PI=3.14159
    for angle in {0..256}
    do  
        sine[$angle]=$(awk "BEGIN{ printf \"%8.0f \", ((sin($angle*($PI/128))*255))}")
    done
}
function sin {
    sine=${1}[@]
    angle=$(($2%256))
    echo "from sin function :sine[$angle] = ${sine[$angle]}"
}

declare -a sine
load_sin sine
for angle in {0..255}
do
    echo -n "from main scope : ${sine[$(($angle%255))]}  "
    sin sine $(($angle))
done

output is the following:

#from main scope :        0   from sin function :sine[0] = sine[@]
#from main scope :        6   from sin function :sine[1] =        6 
#from main scope :       13   from sin function :sine[2] =       13 
#...
#from main scope :      -25   from sin function :sine[252] =      -25 
#from main scope :      -19   from sin function :sine[253] =      -19 
#from main scope :      -13   from sin function :sine[254] =      -13 
#from main scope : sine[@]  from sin function :sine[255] =       -6

What I'd like to return is in every case: sine[255] = -6 and sine [0] = 0

...what do you expect sine=${1}[@] to mean or do? If you want an indirect reference to an array, that's what declare -n is for. — Charles Duffy
– Charles Duffy, Commented Feb 13, 2019 at 21:35
...but then, if this question evolves down to "how do I indirectly reference an array?", there are duplicates we can close it with. — Charles Duffy
– Charles Duffy, Commented Feb 13, 2019 at 21:37
Keep in mind that shell variables aren't local unless you make them so. When you set sine=${1}[@]}, you're changing ${sine[0]} because changing the first element of an array is what assigning to that array does. — Charles Duffy
– Charles Duffy, Commented Feb 13, 2019 at 21:39
@DavidC.Rankin Okay, but could you please elaborate on what is wrong? I'm a beginner bash scripting and I don't get what is wrong on line 3 and 11 — Soulthym
– Soulthym, Commented Feb 13, 2019 at 21:39

Charles Duffy · Accepted Answer · 2019-02-13 22:12:03Z

1

This happens because (1) bash variables are all global unless explicitly declared otherwise (so sine inside your function refers to the sine array declared outside of it); and (2) assigning a string to an array changes the first element of that array.

Thus, when you run:

sine=${1}[@]

...that's exactly identical to:

sine[0]=${1}[@]

So when you call load_sin sine, $1 is the string "sine", so the assignment assigns the string sine[@] to the first element of the array sine.

Compare to the following, which does proper indirect references and assignments (albeit as noops when sine is the exact variable name provided):

#!/usr/bin/env bash

load_sin() {
    # if our first argument is not "sine", make sine a reference to the variable thus-named
    [[ $1 != sine ]] && declare -n sine=$1

    while read -r angle value; do
      sine[$angle]=$value
    done < <(awk -v PI=3.14159 -v start=0 -v end=256 '
      BEGIN {
        for(angle=start; angle<end; angle++) {
          printf "%d %8.0f \n", angle, sin(angle*(PI/128))*255
        }
      }')
}

sin() {
    [[ $1 != sine ]] && declare -n sine=$1

    angle=$(($2%256))
    echo "from sin function :sine[$angle] = ${sine[$angle]}"
}

declare -a sine
load_sin sine
for angle in {0..255}
do
    printf '%s' "from main scope : ${sine[$(($angle%255))]}  "
    sin sine $(($angle))
done

edited Feb 13, 2019 at 22:12

answered Feb 13, 2019 at 21:39

Charles Duffy

299k43 gold badges441 silver badges496 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Soulthym Over a year ago

Wow thank you very much I had no idea it was passed as a string, that works perfectly

Charles Duffy Over a year ago

BTW, we could probably make load_sin much faster by running awk just once and printing all 256 values, instead of calling a separate awk once per value. Let me know if you're interested in that, and I might try to throw it together.

Charles Duffy Over a year ago

@Soulthym, ...so, I ended up doing that edit anyhow; hope you find it helpful.

Charles Duffy Over a year ago

Anything that's not built into the shell itself has a significant startup cost just to fork() off a subprocess and then call execve() to replace that forked-off copy of the shell with whatever the external program is (which needs to have its libraries linked and loaded, etc). It's not that big for a one-off, but if you're doing something in a loop -- even just calling /bin/true 256 times is more than half a second on my local box, and awk is larger than true with more dependencies.

Soulthym Over a year ago

well thank you, why not actually, I didn't know awk was that long to invoke

|

Collectives™ on Stack Overflow

bash array yields array_name[@] at index 0

1 Answer 1

8 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

8 Comments

Your Answer

Sign up or log in

Post as a guest

Related