Chemistry 311

Loops allow a program to repeat a block of code multiple times. In Python, loops are commonly used to process collections of data, perform repeated calculations, and automate tasks. Python provides two primary looping constructs: for loops and while loops.

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for loops

A for loop is used to iterate over a sequence of values, such as a list, a string, or a range of numbers. The loop runs once for each item in the sequence.

    for i in range(5):
        print(i)
      

In this example, the loop variable i takes on the values 0 through 4. The loop automatically stops when the sequence is exhausted.

for loops are ideal when:

• you know how many times the loop should run,
• you are iterating over an existing collection of items,
• each iteration is independent of the others.

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while loops

A while loop repeats a block of code as long as a condition remains true. The condition is checked before each iteration.

    count = 0
    while count < 5:
        print(count)
        count += 1
      

In this example, the loop continues until count reaches 5. Unlike a for loop, the programmer is responsible for updating the condition inside the loop.

while loops are useful when:

• you do not know in advance how many iterations are needed,
• the loop depends on a changing condition or user input,
• the loop should stop based on a logical test rather than a count.

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Choosing between for and while

In many cases, either type of loop could be used, but choosing the right one improves clarity and reduces errors.

• Use a for loop when iterating over a known sequence or range.
• Use a while loop when repetition depends on a condition that may change unpredictably.

Big idea: for loops are best for structured, count-controlled repetition, while while loops are best for condition-controlled repetition.