more-forloops

Module 2

Back to Class 1 Prep

Resource: More for-loops

Here, I want to briefly clear up something that you might find confusing.

When iterating over strings, a lot of CS50 examples use a for-loop like this:

for (int i = 0, n = strlen(my_string); i < n; i++) 
{
    char c = my_string[i];
    // and then do stuff with c
}

What makes this different from most "normal" for-loops you've seen? In this case, we initialize two variables rather than just one, namely:

• an int called i gets set to 0
• an int called n gets set to strlen(my_string)

We could have achieved the same outcome with this code:

for (int i = 0, i < strlen(my_string); i++) 
{
    char c = my_string[i];
    // and then do stuff with c
}

in which we initialize just one variable, i, and embed strlen(my_str) directly into the condition of the for-loop (the middle section of the three semicolons), rather than storing it in a variable n first, and then using n inside the condition.

But the second way is "worse", because it is less efficient. Do you remember why? The reason is that the code inside the condition gets executed every time around, and so we end up calling the strlen function a bunch of times, while the first for-loop only needs to call strlen once.

This explanation probably makes sense in a vague way, but nevertheless, you might still feel a little hazy on the details. The normal syntax for for-loops is already very dense and cryptic, so if you are like me, you probably just memorized

for (int i = 0; i < someAmount; i++)

and now anything that deviates from that standard vanilla version makes you think: Wait, what is this? That looks wrong. Can you do that?

In fact, using fancy for-loop tricks is not even necessary to solve this particular problem. If we want to loop over a string without having to call strlen multiple times, there is a much more straitforward way:

int n = strlen(my_string);
for (int i = 0; i < n; i++) 
{
    char c = my_string[i];
}

Here we have a "normal" loop, immediately preceeded by a typical, every-day variable declaration.

So hopefully the unfamiliarity of that fancy for-loop no longer seems as intimidating.

But, as long as we are here, the fancy version does provide a nice example of how to take advantage of the extremely flexible nature of for-loops for those times when we want to break out of the box of our vanilla version,

for (int i = 0; i < someAmount; i++)

This version does work fine most of the time, and is certainly very useful as a point of reference. But if we want to be cute, the for-loop construct gives us a lot of options:

• You can initialize one variable, or more than one, or none.
• You can name your variables whatever you want, just like a normal variable declaration
• Your condition can be anything, so long as it evaluates to true or false.
• Your modification step(s) can be anything, not just i++.

For example:

// loop backwards from 10 to 1
for (int i = 10; i != 0; i--)
{
    // do stuff
}

// count up by 3s
for (int i = 0; i < 100; i += 3)
{
    printf("%i is a multiple of 3\n", i);
}

printf("Two things are heading towards each other!\n");

// i goes up, j goes down (twice as fast), until they meet (or cross)
for (int i = 0, j = 100; i <= j; i++, j = j - 2)
{
    int distance = j - i;
    printf("The distance between the two things is %i\n", distance);
}
printf("And now the two things have crossed paths\n");