First Non-Repeating Character in C, C++, Java & Python – Code with Explanation & Examples in Short and Simple

   

C Program

#include <stdio.h>
#include <string.h>

int main() {
    char str[100];
    int freq[256] = {0};
    printf("Enter a string: ");
    gets(str);

    for (int i = 0; str[i]; i++) freq[(unsigned char)str[i]]++;

    for (int i = 0; str[i]; i++) {
        if (freq[(unsigned char)str[i]] == 1) {
            printf("First non-repeating character: %c\n", str[i]);
            return 0;
        }
    }
    printf("No non-repeating character found\n");
    return 0;
}

C Output

Input: programming

Output: First non-repeating character: p

C++ Program

#include <iostream>
#include <string>
using namespace std;

int main() {
    string str;
    int freq[256] = {0};
    cout << "Enter a string: ";
    getline(cin, str);

    for(char c : str) freq[(unsigned char)c]++;

    for(char c : str) {
        if(freq[(unsigned char)c] == 1) {
            cout << "First non-repeating character: " << c << endl;
            return 0;
        }
    }
    cout << "No non-repeating character found\n";
}

C++ Output

Input: swiss
Output: First non-repeating character: w

JAVA Program

import java.util.Scanner;

public class FirstNonRepeating {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a string: ");
        String str = sc.nextLine();
        int[] freq = new int[256];

        for (char c : str.toCharArray()) freq[c]++;

        for (char c : str.toCharArray()) {
            if (freq[c] == 1) {
                System.out.println("First non-repeating character: " + c);
                return;
            }
        }
        System.out.println("No non-repeating character found");
    }
}

JAVA Output

Input: level
Output: First non-repeating character: v

Python Program

s = input("Enter a string: ")
freq = {}

for ch in s:
    freq[ch] = freq.get(ch, 0) + 1

for ch in s:
    if freq[ch] == 1:
        print("First non-repeating character:", ch)
        break
else:
    print("No non-repeating character found")

Python Output

Input: success
Output: First non-repeating character: u

In-Depth Explanation

Example

If you input "swiss". The appearances of letters are:

s appears 3 times,

w appears once,

i appears once.

Scanning alphabetically: s is repeated, skip. The next one is w, once → result = w.

Real-Life Analogy

You are in a class and some students yell their names repeatedly. You need to find the first student who says their name just once. The first child saying their name without repeating is the "non-repeating" kid.

This analogy is similar to the way the program reads through each character and locates the solitary one that doesn't repeat over the string. 

Why It Matters

Identifying the first non-repeating character is a traditional string processing issue. It illustrates how to:

Efficiently count the occurrences of characters.

Maintain the order while verifying uniqueness.

Trade time complexity (O(n)) with space complexity (O(1)) using a frequency array or dictionary.

In interview problems, this is a very typical issue since it tests your skill to use hash maps, arrays, or frequency tables when solving problems.

Learning Insights

You learn the following from this problem:

How to scan a string while maintaining frequencies.

The need to check order + uniqueness simultaneously.

How various languages manage character arrays vs hash maps (C uses arrays, Java uses arrays, Python uses dictionaries).

You also learn real-world skills, such as:

Verifying usernames where uniqueness is important.

Identifying outliers in text logs (first unusual occurrence).

Locating unique values in data sets.

Real-World Usage

Imagine an online chat platform in which thousands of messages are being sent. To identify spam or strange text, the system might wish to highlight the first strange word that doesn't occur repeatedly like others. In the same way, in data compression schemes, unique characters hold particular significance since they can decide how encoding functions.

SEO-Optimized Conclusion Paragraph

The idea of identifying the first non-repeating character in a string is one of the most fundamental string problems for coding interviews and competitive programming. In C, C++, Java, or Python, the reasoning conveys the strength of frequency calculation and iteration order. It is commonly asked in technical interviews in companies such as TCS, Infosys, Wipro, and product-based companies to evaluate logical mindsets and handling of strings. Learning to solve the first non-repeating character problem not only improves your fundamentals in string manipulation but also enhances your skill in solving real-life text processing problems and algorithmic problems.