11B. Additional Features of For-Loops

Declaring a variable in a for-loop heading

The first part of a for-loop heading can create a new variable. But a variable created there can only be used in the for-loop heading and body. For example for(int i = 0; i < 5; i++) { printf("%d\n", i); } creates variable i that can only be used in this for-loop. You are not allowed to say for(int i = 0; i < 5; i++) { printf("%d\n", i); } printf("%d\n", i); since that tries to use i outside of the for-loop. Typically, a variable that is controlled in a for-loop heading is only intended to be used within the loop, and it is a good idea to limit its scope. If you need to use the loop control variable after the loop, declare it before the loop.

Omitting part of a for-loop heading

Every for-loop heading has exactly two semicolons. But you can omit any (or all) of the three parts that are separated by semicolons. If the first part is empty, no initialization is done. If the second part is empty, it is assumed to be true, so the loop keeps going until stopped some other way. If the third part is empty, no update is added to the end of the body. For example, for(; k < n; k++) { ... } assumes that k has already been initialized. But do not omit any of the three parts without a good reason. Each of the three parts of a for-loop heading has a purpose. Instead of int k = 0; for(; k < n; k++) { ... } write int k; for(k = 0; k < n; k++) { ... } The standards for this course require you not to omit any part of a for-loop heading without good reason.