How does this function work "abcdefghijklmnopqrstuvwxyz" [rand()%26]?

Question

To generate random letters of the alphabet

"abcdefghijklmnopqrstuvwxyz" [rand()%26]

How does this work?

Answer 1

In fact this expression

"abcdefghijklmnopqrstuvwxyz" [rand()%26]

is equivalent to

char *p = "abcdefghijklmnopqrstuvwxyz";

p[rand()%26]

That is a random character is selected from a string literal by using the subscript operator.

The string literal used in the original expression is implicitly converted to pointer to its first element.

The expression rand()%26 yields a random value (remainder) in the range [0, 25]

Answer 2

A string literal is an array expression, and as such you can subscript it like any other array expression¹.

It's equivalent to writing:

const char alphabet[] = "abcdefghijklmnopqrstuvwxyz";
int idx = rand() % 26;
char c = alphabet[idx];

rand() % 26 returns a (pseudo)random value in the range [0..25], and that's used to index into the alphabet array.

You don't often see literals be subscripted like that because it's hard to read and not always obvious.

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1. Strictly speaking, the expression "decays" to type char *, and that's what the [] operator is applied to.

Answer 3

String literals in C are really arrays (that as usual decay to pointers to their first element). And you can index those arrays like any other array (which is what's happening here).