Lec 05 - Loops

Slides:

CS1010 AY24/25 S1 Lecture 5
Lecture Slides

Loops

for loop

The for loop in C has the following syntax:

for (<initialize>; <condition>; <update>) {
    <body>
}

while loop

The while loop looks like this:

<initialize>
while (<condition>) {
    <body>
    <update>
}

Note that there is no restriction <update> must execute after <body> in a while loop. But in a for loop, <udpate> is always executed after <body>.

do-while loop

The do-while loop looks similar to the while loop, except that the body of the loop is guaranteed to be executed at least once.

<initialize>
do {
    <body>
    <update>
} while (<condition>);

Similar to the while loop, the <body> component and <update> componenet in the loop do not have to be in order.

When doing paper questions about the loop, always think about whether it will become an infinite loop or not.

Loop Invariant

A loop invariant is an assertion that is true before the loop, after each iteration of the loop, and after the loop.

Argue that an invariant is true

  • it is true before entering the loop.

  • it is true at the end of the first iteration of the loop

  • if it is true at the end of the kk-th iteration of the loop, then it is true at the end of the k+1k+1-th iteration.

  • it is true when we exit the loop.

Loop invariant, however, is not unique. We can write down infinitely many loop invariants. A good invariant, however, will lead us to an assertion that we want to see (e.g., relating product to n). We can derive other invariants in our code (such as { n != 0 } below) that do not contribute to the reasoning of the behavior of our loop. Such invariants should be avoided.

In the past year final papers, you may see that the loop invariant doesn't need to contain exact expression, it can contain some formative words, like "m is the minimum element in the list", etc.

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