C Program to Find a Non-Repeating Character
C Program to Find a Non-Repeating Character
A school text exercise needs to first nonrepeating character while preserving the buffer’s valid characters and final terminator.
A first nonrepeating character occurs exactly once in the entire string and has the earliest position among such characters.
Before this exercise, review strings and their terminator and array bounds. The worked program below applies those ideas to this specific problem.
Explain
Explain why first nonrepeating character uses this sequence of operations.
Trace
Trace the sample values and predict the printed result.
Apply
Try aabb, where every character repeats.
First Nonrepeating Character: follow the data
Given data
A school text exercise needs to first nonrepeating character while preserving the buffer’s valid characters and final terminator.
Rule
A first nonrepeating character occurs exactly once in the entire string and has the earliest position among such characters.
Observed result
First unique: s
Understand the operation
Count occurrences across the complete text, not only adjacent equal letters. Scanning candidate positions in order and stopping at the first count-one result implements the first requirement.
A C string ends at its first null character. Read only within its accessible storage and reserve a position for that terminator when producing a changed string. These examples use basic text bytes, not a Unicode grapheme-processing library.
Read the complete C program
The input values are written into this complete C17 example so the calculation can be reproduced without an unavailable service. Change the named data, rebuild and compare the actual output with your prediction.
C17 · complete program
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
const char text[] = "school";
char found = '\0';
size_t length = strlen(text);
for (size_t i = 0; i < length; i++) {
int occurrences = 0;
for (size_t j = 0; j < length; j++) if (text[j] == text[i]) occurrences++;
if (occurrences == 1) { found = text[i]; break; }
}
if (found != '\0') printf("First unique: %c\n", found); else puts("No unique character");
return 0;
}
Expected output
First unique: s
Follow the values through the code
C17 · code used in this explanation
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
const char text[] = "school";
char found = '\0';
size_t length = strlen(text);
for (size_t i = 0; i < length; i++) {
int occurrences = 0;
for (size_t j = 0; j < length; j++) if (text[j] == text[i]) occurrences++;
if (occurrences == 1) { found = text[i]; break; }
}
if (found != '\0') printf("First unique: %c\n", found); else puts("No unique character");
return 0;
}
The first candidate s occurs once in school; the repeated o does not affect that earlier valid result.
Compare the printed answer with the original text and the stated ordering or filtering rule. The presence of a terminator makes %s usable; it does not by itself make every earlier write fit.
Follow the worked calculation
Start with the sample
The first candidate s occurs once in school; the repeated o does not affect that earlier valid result.
Apply the rule
Compare the printed answer with the original text and the stated ordering or filtering rule. The presence of a terminator makes %s usable; it does not by itself make every earlier write fit.
Check the result
First unique: s
First unique: s
Try a small variation
C17 · code used in this explanation
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
const char text[] = "school";
char found = '\0';
size_t length = strlen(text);
for (size_t i = 0; i < length; i++) {
int occurrences = 0;
for (size_t j = 0; j < length; j++) if (text[j] == text[i]) occurrences++;
if (occurrences == 1) { found = text[i]; break; }
}
if (found != '\0') printf("First unique: %c\n", found); else puts("No unique character");
return 0;
}
Try aabb, where every character repeats.
Worked answer
The complete worked answer applies the same explicitly bounded operation to a small variation. Read its stored bytes and terminator separately from the visible output.
C17 · complete program
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
const char text[] = "aabb";
char found = '\0';
size_t length = strlen(text);
for (size_t i = 0; i < length; i++) {
int occurrences = 0;
for (size_t j = 0; j < length; j++) if (text[j] == text[i]) occurrences++;
if (occurrences == 1) { found = text[i]; break; }
}
if (found != '\0') printf("First unique: %c\n", found); else puts("No unique character");
return 0;
}
Expected output
No unique character
Recognize the important boundaries
The simple scan is quadratic in text length. A frequency table can improve repeated scanning when the supported character domain is defined.
An untrusted input string needs a verified buffer bound and a valid terminator. strlen is not a safe way to prove arbitrary raw storage has a terminator.
Check your understanding
Choose the answer, then compare the reasoning for every option.
Use the idea independently
A first nonrepeating character occurs exactly once in the entire string and has the earliest position among such characters.
Continue with character-array storage.
Continue with bounded line reading.
Language reference: WG14 C11 committee draft. These examples use the C17-compatible subset of the explained language features.
Reaching the recap and completing the quizzes are separate milestones.