Extract variables from child process output string

Question

I am building an application that uses a child process to make mathematical calculations:

var child = exec('get_hrv -M -R rr.txt', function(error, stdout) {
    if (error !== null) {
        console.log('exec error: ' + error);
    } else {
        res.send(stdout);       
    }
});

The output (stdout) looks like this:

NN/RR    = 0.992424
AVNN     = 789.443
SDNN     = 110.386
SDANN    = 0
SDNNIDX  = 110.386
rMSSD    = 73.5775
pNN50    = 36.9231
TOT PWR  = 12272.8
ULF PWR  = 788.161
VLF PWR  = 4603.59
LF PWR   = 4221.97
HF PWR   = 2659.05
LF/HF    = 1.58777

Everything is working as it should, however, this output is a string and I need the values it contains assigned to separate integer variables, e.g. like this:

var extractedVars = {
    NN/RR: 0.992424
    AVNN: 789.443
    SDNN: 110.386
    SDANN: 0
    SDNNIDX: 110.386
    rMSSD: 73.5775
    pNN50: 36.9231
    TOT PWR: 12272.8
    ULF PWR: 788.161
    VLF PWR: 4603.59
    LF PWR: 4221.97
    HF PWR: 2659.05
    LF/HF: 1.58777
}

How can this be done? Regex?

No, I am using a shell script from Physionet HRV Toolkit (physionet.org/tutorials/hrv-toolkit). — Jan Gunzenhauser
– Jan Gunzenhauser, Commented Mar 9, 2015 at 21:59

Barış Uşaklı · Accepted Answer · 2015-03-10 23:49:59Z

1

You can try something like this

var lines = dataFromProc.split(require('os').EOL);
var data = {};
lines.forEach(function (line) {
    var parts = line.split('=');
    if (parts[1]) {
        data[parts[0].trim()] = parseFloat(parts[1].trim());
    }
});
console.log(data);

edited Mar 10, 2015 at 23:49

answered Mar 9, 2015 at 21:46

Barış Uşaklı

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1 Comment

zamnuts Over a year ago

This assumes line feeds are CRLF ("\r\n"). Assuming get_hrv uses the platform's EOL character, require('os').EOL will be more true. See stackoverflow.com/a/14063413/1481489

Henrik Karlsson · Accepted Answer · 2015-03-09 22:07:17Z

1

Since you mentioned regular expressions I wrote a solution using regexp. I'm not sure I would use this solution since the others probably are easier to read, but here it goes anyway. The key goes into capture group 0 and the value into group 1.

var regexp = /([0-9A-Za-z\/ ?]*)= ([0-9\.]*)/;
var extractedVars = {}
data.split(os.EOL).forEach(function (line) {
    var res = regexp.exec(line);
    extractedVars[res[1].trim()] = parseFloat(res[2]);
});

The expression on regexr

answered Mar 9, 2015 at 22:07

Henrik Karlsson

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2 Comments

Jan Gunzenhauser Over a year ago

is there a difference between your solution and the others in terms of performance?

Henrik Karlsson Over a year ago

Looks like the regex one is a little bit faster on chrome at least jsperf.com/regexp-vs-split-so , although it's probably not worth the harder to read code IMO.

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