Counting Characters in C

Characters--the building blocks of strings--are stored in computers in a code called ASCII. Each symbol in the latin alphabet is assigned an integer value between 1-127 so that each character can be stored in one byte of memory. See the ASCII table for the specific assignments of characters to integers.

Strings in C and assembly language are implemented as arrays that can have a variable length. The length of the string is not explicitly encoded anywhere--there is no String.length property or method that can easily tell us the length of a string. Instead, strings have a special NULL terminator to mark the end of the string. The NULL terminator is a single byte that has the special value 0, which does not code for any printable characters in ASCII. For example, the string "NEIL" is shown below coded as a NULL-terminated ASCII string.

Character	ASCII
N	0x4E
E	0x45
I	0x49
L	0x4C
NULL TERMINATOR	0x00

The string "NEIL" is 5 bytes long (including the NULL terminator at the end), so it will not fit into a 32-bit (4-byte) integer. We can only tell how long the string is by counting the number of bytes starting at the beginning up to (and including) the NULL terminator.

If there is no String.length in C, how are we supposed to find out how many characters are in a string? The C library provides a function called strlen that we can call to count the number of characters in the string.

#include <string.h>

void main() {
  char s[] = "the quick brown fox jumped over the lazy dog";
  int s_length = strlen(s);
}

Part 1: Counting Occurrences of a Character

Write a program that loops through a string and counts all occurrences of a character. You can start by counting the occurrences of the character a. Make your program more versatile by making it possible to count the occurrences of any character.

Part 2(a): Converting One Character to Another -- Retabbing

Studies have shown that developers who use spaces to indent instead of tabs make 10% more money. Write a program that loops through a string and swaps all occurrences of the tab character (\t) for a space character.

Part 2(b): Retabbing v2

Modify your program from part 2(a) to copy characters from one string into a second string. When you encounter a tab character, don't copy it. Instead, insert four spaces in the second string.

Part 3: Checksumming

Write a program to add up the ASCII values of all characters in a string and print out the sum. For example, in the string NEIL above, the ASCII values are 0x4E, 0x45, 0x49, 0x4C. Their sum is 0x4E + 0x45 + 0x49 + 0x4C = 0x128.