C PROGRAMMING PROGRAM • STRINGS

Count the Frequency of Every Character

Learn how to count the frequency of every character using a clear C program.

IntermediateStringsASCII tableFrequency array

PROBLEM UNDERSTANDING

Input and expected output

Sample input

hello

Sample output

e : 1
h : 1
l : 2
o : 1

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

int main(void)
{
    char text[300];
    int frequency[256] = {0};

    fgets(text, sizeof text, stdin);
    for (int index = 0; text[index] != '\0' && text[index] != '\n'; index++)
        frequency[(unsigned char) text[index]]++;
    for (int value = 0; value < 256; value++)
        if (frequency[value] > 0) printf("%c : %d\n", value, frequency[value]);
    return 0;
}

CURRENT STEP

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EXPECTED OUTPUT FOR THE SAMPLE

e : 1
h : 1
l : 2
o : 1

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Use each unsigned character code as an index into a 256-entry frequency array.
Display the computed result.

Use each unsigned character code as an index into a 256-entry frequency array.

EFFICIENCY

Time and space complexity

Time complexity

O(n)

Auxiliary space

O(1)

DEBUGGING CHECKLIST

Common mistakes

Check this

Use the correct format specifier for every variable.

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Initialize variables before using their values.

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Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.