CODEWITHSUNDEEP
javascriptfor-loop
John
John

Reputation: 147

For Loop returning strange length

I have this javascript code:

var chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789',
  key = '',
  c;

for (i = 0; i < 32; i++) {
  c = Math.floor(Math.random() * chars.length + 1);
  key += chars.charAt(c)
}

console.log('return key length that needs to be 32...' + key.length);

I need to always return 32 key length..but i get random:

return key length that needs to be 32...31
return key length that needs to be 32...29
return key length that needs to be 32...32
return key length that needs to be 32...31
return key length that needs to be 32...30
return key length that needs to be 32...32
return key length that needs to be 32...31

I know that error is in: i<32; but javascript freezes when i add i=32; i try i<=32 but same results...so i know it is basic question but i don't know hot to return always 32 key length?

Upvotes: 0

Views: 57

Answers (4)

devd_102
devd_102

Reputation: 23

Sometimes math.random() return such value (Math.random() * chars.length + 1) becomes >= 64.

Math.random()*char.length+1 >= 64
Math.random()*char.length >= 63
Math.random()* >= 63/64 (in our case)
Math.random() >= 0.984375

If math.random returns value >= 0.984375 then c will be get 64 so length of key will be less than 32. You don't need to add 1 to solve this problem.

Upvotes: 1

phuzi
phuzi

Reputation: 13060

The problem is here Math.floor(Math.random() * chars.length + 1);

Math.random() returns a number between 0 and 1, inclusive. When it's 1 then Math.floor(1 * 32 + 1) gives 33 chars.charAt(33) is undefined though and results in nothing being added to key hence the random lengths.

Remove the + 1 and it should work as you expected.

Upvotes: 2

Colin
Colin

Reputation: 174

Try this 

  var chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789',
        key   = '',
        c;

    for (i=0;i<32;i++) {
        c = Math.floor(Math.random()*chars.length);
        key += chars.charAt(c);
    }

    console.log('return key length that needs to be 32...'+key.length);

Upvotes: 1

Barmar
Barmar

Reputation: 780808

The problem is the + 1 in this line:

c = Math.floor(Math.random() * chars.length + 1);

Valid indexes run from 0 to chars.length-1. The above assignment picks a random number from 1 to chars.length. So it ignores the first character in the string, and sometimes tries to access outside the string. In the latter case it chars.charAt(c) returns undefined, and appending that doesn't add to the length of the result.

var chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789',
  key = '',
  c;

for (i = 0; i < 32; i++) {
  c = Math.floor(Math.random() * chars.length);
  key += chars.charAt(c)
}

console.log('return key length that needs to be 32...' + key.length);

Upvotes: 4

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