How to Count Frequency of Characters in a String

How to count frequency of characters in a string?

You can use a java Map and map a char to an int. You can then iterate over the characters in the string and check if they have been added to the map, if they have, you can then increment its value.

For example:

HashMap<Character, Integer> map = new HashMap<Character, Integer>();
String s = "aasjjikkk";
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    Integer val = map.get(c);
    if (val != null) {
        map.put(c, new Integer(val + 1));
    }
    else {
       map.put(c, 1);
   }
}

At the end you will have a count of all the characters you encountered and you can extract their frequencies from that.

Alternatively, you can use Bozho's solution of using a Multiset and counting the total occurences.

Count the number of occurrences of a character in a string

str.count(sub[, start[, end]])

Return the number of non-overlapping occurrences of substring sub in the range [start, end]. Optional arguments start and end are interpreted as in slice notation.

>>> sentence = 'Mary had a little lamb'
>>> sentence.count('a')
4

Counting frequency of characters in a string using JavaScript

Here you go:

function getFrequency(string) {
    var freq = {};
    for (var i=0; i<string.length;i++) {
        var character = string.charAt(i);
        if (freq[character]) {
           freq[character]++;
        } else {
           freq[character] = 1;
        }
    }

    return freq;
};

Calculate the frequency of characters in a string (Java, Performance)

You don't need to explicitly initialize 26 entries in your frequency array (the default value is zero); you also don't need to keep the table of characters (it is sufficient to know the offset). That is, your code can eliminate c entirely and calculate each letter; like,

Scanner sc = new Scanner(System.in);
int[] f = new int[26];
System.out.println("Enter a string.");
String orig = sc.nextLine();
String k = orig.trim().toUpperCase();
System.out.println("Checking string = " + orig);
for (char ch : k.toCharArray()) {
    f[ch - 'A']++;
}
System.out.println("Char\tFreq");
for (int i = 0; i < f.length; i++) {
    if (f[i] != 0) {
        System.out.println((char) ('A' + i) + "\t" + f[i]);
    }
}

Counting Character Frequency in String(Java)

Well, count is an int[] with 256 slots:

int count[] = new int[MAX_CHAR]; // MAX_CHAR is 256

Your algorithm defines MAX_CHAR = 256, because it assumes the string consists only of 8-Bit ASCII characters.

[0, 0, ..., 0, 0] // 256 slots

Now you're iterating each character in string str and cast it to an integer (see type casting of primitives in Java). An A will be casted to 65 (ASCII table), a B to 66 and so on. The casted int is the slot to increment. So a string A would lead to an increment of the integer at index 65. Your question was primarily about

count[str.charAt(i)]++

That translates to this:

char c = str.charAt(i);    // c = A
int index = c;             // c = A, casted to an int = 65
count[index]++             // increments the int at position 65

Result:

[0, 0, ..., 1, ..., 0, 0]
            ^ index 65

The next A would increment the int at index 65 again:

[0, 0, ..., 2, ..., 0, 0]
            ^ index 65

count the frequency of each character using the dictionary in python

You can iterate over string and update the dictionary accordingly and also there's no need of any count variable.

test_str = input().lower()
dicx = {} 
  
for i in test_str: 
    if i in dicx: 
        dicx[i] += 1
    else: 
        dicx[i] = 1

print(dicx)

Counting frequency of characters in a string using JavaScript

Here you go:

function getFrequency(string) {
    var freq = {};
    for (var i=0; i<string.length;i++) {
        var character = string.charAt(i);
        if (freq[character]) {
           freq[character]++;
        } else {
           freq[character] = 1;
        }
    }

    return freq;
};

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