Learning Algorithms with JS, Python and Java 4: MaxChar

This is the fourth article of my attempts to follow Stephen Grider’s Udemy course in three different languages.

First I quote todays’s question:

— Directions
Given a string, return the character that is most
commonly used in the string.
— Examples
maxChar(“abcccccccd”) === “c”
maxChar(“apple 1231111”) === “1”

JavaScript:

function maxChar(str) {
    const charMap = {};
    let max = 0;
    let maxChar = '';

    for (let char of str) {
        if (charMap[char]) {
            charMap[char]++
        } else {
            charMap[char] = 1;
        }
    }

    for (let char in charMap) {
        if (charMap[char] > max) {
            max = charMap[char];
            maxChar = char;
        }
    }

    return maxChar;
}

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Stephen rewrites the first for loop as:

    for (let char of str) {
        charMap[char] = charMap[char] + 1 || 1;
    }

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This is possible because anObject['nonexistentKey'] returns undefined, undefined + 1 return NaN, which is falsy.

Python:

def max_char(str):
    char_map = {}
    max = 0
    max_char = ''

    for char in str:
        char_map.setdefault(char, 0)
        char_map[char] += 1

    for char in char_map:
        if char_map[char] > max:
            max = char_map[char]
            max_char = char

    return max_char

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In Python, you can’t just try char_map[char] += 1 as in JS, because it would throw a KeyError if the key doesn’t exist yet.

More concisely:

def max_char(str):
    char_map = {}

    for char in str:
        char_map.setdefault(char, 0)
        char_map[char] += 1

    return max(char_map, key=char_map.get)

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Source: https://stackoverflow.com/questions/268272/getting-key-with-maximum-value-in-dictionary
max(char_map, key=char_map.get) is the same as max(char_map.keys(), key=char_map.get).
key=char_map.get is needed to find the (key, value) tuple pair in the dictionary whose value is the largest. max({'a': 2, 'b': 1}) returns ‘b’.

Java:

import java.util.HashMap;
import java.util.Map;

public static char maxChar2(String str) {
    Map<Character, Integer> charMap = new HashMap<>();

    for (char chr : str.toCharArray()) {
        Integer chrCount = charMap.get(chr);
        if (chrCount == null) {
            chrCount = 0;
        }
        charMap.put(chr, chrCount + 1);
    }

    int max = 0;
    char maxChar = '\0';

    for (Map.Entry<Character, Integer> entry : charMap.entrySet()) {
        int value = entry.getValue();
        if (value > max) {
            max = value;
            maxChar = entry.getKey();
        }
    }

    return maxChar;
}

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Using Stream:

public static char maxChar(String str) {
    Map<Character, Long> charMap = str.chars()
            .mapToObj(i -> (char) i)
            .collect(Collectors.groupingBy(
                    c -> c, Collectors.counting()));

    return charMap.entrySet().stream()
            .collect(Collectors.groupingBy(
                    Map.Entry::getValue,
                    TreeMap::new,
                    Collectors.mapping(
                            Map.Entry::getKey, Collectors.toList())))
            .lastEntry()
            .getValue()
            .get(0);
}

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The return statement can be made simpler:

    return charMap.entrySet().stream()
            .max(Map.Entry.comparingByValue())
            .get()
            .getKey();

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References:
https://codereview.stackexchange.com/questions/133058/print-max-occurring-character-in-a-string
https://stackoverflow.com/questions/48280361/how-to-count-occurrences-for-each-value-in-multimap-java
https://stackoverflow.com/questions/42060294/using-java8-stream-to-find-the-highest-values-from-map

原文链接：Learning Algorithms with JS, Python and Java 4: MaxChar