Common Lisp’s DO loop

Introducing the Most General Looping Form in Common Lisp

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The DO loop is the most general form of a loop in Common Lisp.

I want to go with you through the intricacies of this looping command, so that you can write a DO loop by yourself in future.

Loops have mostly variables, which have an initial value and get modified in every round of the loop.

In a normal DO loop, these variables are set and updated in parallel (they can’t see each other).

A very typical loop in C languages is the for loop.

C’s for loop

Let’s take a typical for loop in C and translate it into the DO loop in Common Lisp.

#include <stdio.h>

int main() {
    // Loop from 1 to 10
    for (int i = 1; i <= 10; i++) {
        printf("The current value of i is: %d\n", i);
    }
    
    return 0;
}

The initial value for i is bound to i. Then, the exit condition i <= 10 is given, followed by the instruction how to modify i in every round: i++ which is i = i + 1 .
And then comes the instruction which should be executed for every round: To print the value of i.

General DO Loop Form