Strings in C

Overview

If you have not learnt C strings yet, the good news is that you only need a small subset for this assignment.

For the one-hot embedding stage, the main jobs are:

• store tokens as strings
• compare tokens to see whether two strings are the same
• sort strings into lexicographic order
• copy strings into your own arrays when needed

You do not need advanced text processing.

What a string is

In C, a string is a char array ending with the special character '\0', called the NUL byte .

For example:

char word[] = "cat";

This is stored as four characters:

'c' 'a' 't' '\0'

That final '\0' is what tells C where the string ends. It is written as '\0' because it is the character with numeric value zero.

This is not the same as NULL . '\0' is a byte inside a character array. NULL is used for pointers.

So if you store many tokens, a common pattern is:

char tokens[MAX_TOKENS][MAX_TOKEN_LENGTH + 1];

That means:

• each row stores one token string
• each token can use up to MAX_TOKEN_LENGTH visible characters
• one extra slot is needed for '\0'

Reading a string using scanf

To read in a string with scanf, we need to specify the maximum size of our char array. For example if stdin is “Hello World”, we would need to have atleast 6 chars of space allocated in our char array to store the visible characters and the NUL '/0' character of “Hello”.

We could use the array char str[6] with scanf("%5s ", str) to read {'H','e','l','l','o','\0'} into this array. If we tried to read “Hello! World”, scanf would fail as it would find 6 chars before reaching a whitespace character. To build this format string from the array size constants, the skeleton provides you with the constant TOKEN_STR_SCANF_FORMAT which you can use.

Important Warning

All the inbuilt string functions in C assume your char array is NUL '\0' terminated. If it isn’t, this will cause out of bounds memory access and will likely crash your program or cause weird behaviour. Be careful!

Compare and copy

The most important rule is this:

== does not compare string contents

In C, == on arrays or pointers does not mean “do these two words have the same letters?”. For strings, use functions from <string.h>.

Useful ones here are:

strcmp(a, b)

• returns 0 if the strings are equal
• returns a negative value if a comes before b
• returns a positive value if a comes after b

That is why strcmp is useful both for:

• checking whether two tokens are the same
• sorting tokens lexicographically

If you want to use the library sort for Stage 1 rather than writing your own sorting loop, see qsort and function pointers in C.

To copy a string into your own array, use:

strcpy(dest, src);

If you want the length of a string, use:

strlen(s);