C Program to Remove Non-Alphabet Characters

A school text exercise needs to keep alphabetic characters while preserving the buffer’s valid characters and final terminator.

Compact retained characters toward the front of a writable string, then terminate the new logical result.

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 keep alphabetic characters uses this sequence of operations.
Trace
Trace the sample values and predict the printed result.
Apply
Use text containing no alphabetic characters and produce a valid empty string.

Keep Alphabetic Characters: follow the data

Given data
A school text exercise needs to keep alphabetic characters while preserving the buffer’s valid characters and final terminator.
Rule
Compact retained characters toward the front of a writable string, then terminate the new logical result.
Observed result
AshaRao

A guided model of this example. The illustration explains the process; run the complete C program to check its output.

Understand the operation

read examines each original character; write advances only when a character is retained. Because write never overtakes read, unread source characters are not destroyed.

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) {
  char text[] = "Asha-7 Rao!";
  size_t write = 0;
  for (size_t read = 0; text[read] != '\0'; read++) if (isalpha((unsigned char)text[read])) text[write++] = text[read];
  text[write] = '\0';
  puts(text);
  return 0;
}

Expected output

AshaRao

Run in C compiler →

Follow the values through the code

C17 · code used in this explanation

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
  char text[] = "Asha-7 Rao!";
  size_t write = 0;
  for (size_t read = 0; text[read] != '\0'; read++) if (isalpha((unsigned char)text[read])) text[write++] = text[read];
  text[write] = '\0';
  puts(text);
  return 0;
}

Letters A,s,h,a,R,a,o are retained, while the hyphen, digit, space and exclamation mark are removed.

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
Letters A,s,h,a,R,a,o are retained, while the hyphen, digit, space and exclamation mark are removed.
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.
3
Check the result
AshaRao
```
AshaRao
```

A guided model of this example. The illustration explains the process; run the complete C program to check its output.

Try a small variation

C17 · code used in this explanation

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
  char text[] = "Asha-7 Rao!";
  size_t write = 0;
  for (size_t read = 0; text[read] != '\0'; read++) if (isalpha((unsigned char)text[read])) text[write++] = text[read];
  text[write] = '\0';
  puts(text);
  return 0;
}

Use text containing no alphabetic characters and produce a valid empty string.

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) {
  char text[] = "123!?";
  size_t write = 0;
  for (size_t read = 0; text[read] != '\0'; read++) if (isalpha((unsigned char)text[read])) text[write++] = text[read];
  text[write] = '\0';
  puts(text);
  return 0;
}

Expected output

Run in C compiler →

Recognize the important boundaries

Removing separators can change meaning: a name cleaner should not automatically concatenate words unless that is the task. isalpha follows the active locale and operates on valid byte arguments.

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.

Choose the answer, then compare the reasoning for every option.

Use the idea independently

Compact retained characters toward the front of a writable string, then terminate the new logical result.

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.

Next lesson: C Program to Add Two Binary Strings →

C Program to Remove Non-Alphabet Characters