Array increment syntax

Question

static boolean permutation(String s, String t){
    if(s.length() != t.length()) return false;
    int [] letters = new int [128];
    char[] s_array = s.toCharArray();
    for (char c: s_array){
        letters[c]++;
    }
    for(int i = 0; i< t.length(); i++){
        int c = (int) t.charAt(i);
        letters[c]--;
        if ( letters [c] < 0 )return false; 
    }
    return true;
}

So this code snippet is from Cracking the Coding Interview and I was wondering what the letters[c]++ and letters[c]-- means.

Is that the same thing as letters[c++]?

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

So this code snippet is from Cracking the Coding Interview and I was wondering what the letters[c]++ and letters[c]-- means.

Is that the same thing as letters[c++]?

No. letters[c]++ is equivalent to

letters[c] = letters[c] + 1;

and letters[c]-- performs

letters[c] = letters[c] - 1;

while letters[c++] is equivalent to,

letters[c] = letters[c];
c = c + 1;

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jan 23, 2016 at 17:07

Elliott Frisch

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Comments

Rahmat Waisi · Accepted Answer · 2016-01-23 17:51:12Z

letters array is used to counting how much each letter in those two strings t and s has been repeated

each char has ascii-code so in this for-each loop , c convert to its ascii code , actually it casts from char to integer so it will be a index and ascii-code is unique for each character

in this statement letters[c]++; consider char c = 'A' so c is 65 according ascii-table , and in letters array 65th element will increase once, so until now we have one 'A' in our s string, and when loop ends we had counted repetition of each letter in string s.

for (char c: s_array){
    letters[c]++;
}

and other loop is similar , but it uses string t characters and for each character in t decreases once from it's repetition in string s to determine permutation of s and t

hope you understand what i said !

and for this question!

Is that the same thing as letters[c++]?

no they are different.consider char c ='A' this statement that you said will increase repetition of character B because ascii code of B is 66.

T.J. Crowder · Accepted Answer · 2016-01-23 17:07:19Z

0

Is that the same thing as letters[c++]?

Not even a little bit. :-) letters[c++] in those places would be a syntax error (since Java doesn't allow arbitrary expressions as statements), since it would be letters[c]; c = c + 1;

letters[c]++ is effectively:

letters[c] = letters[c] + 1;

E.g., it's increasing the value in letters[c] by one. It's not changing c at all.

-- works the same way, just decreasing.

answered Jan 23, 2016 at 17:07

T.J. Crowder

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Comments

Helping Hands · Accepted Answer · 2016-01-23 17:11:42Z

0

It's incrementing and decrementing values from the letters array. c is the index of the letters array you are incrementing; therefore letters[c++] would not work.

edited Jan 23, 2016 at 17:11

Helping Hands

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answered Jan 23, 2016 at 17:06

rkim

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Comments

Abdelhak · Accepted Answer · 2016-01-23 18:05:55Z

0

In this case are the same at this:

  letters[c] = letters[c] + 1;

And

  letters[c] = letters[c] - 1;

Because letters is an array of integer. But doesn't make sense when you use the ++ or -- operators on a value(when you have ex: an array of String).

edited Jan 23, 2016 at 18:05

answered Jan 23, 2016 at 17:08

Abdelhak

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2 Comments

Rahmat Waisi Over a year ago

letters[c]++ and letters[c] = letters[c] + 1; are same ! because letters[c] is an integer and we can use ` ++ or -- ` with an integer

Abdelhak Over a year ago

In this case yes but in general when you have an array of STRING NOT

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