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Module 2

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Exercise: Previous

One tricky thing about command-line arguments is that our program always receives the arguments as strings. Even a program like this:

$ pidigits 2016
The 2016th digit of Pi is: 9

still receives that 2016 as a string: "2016". Sure, it happens to be text in which every character is numeric. But as far as the compiler is concerned, a string like "2016" is no different from any other string like "puppies" or "Biased Pluralism".

But that's no good. We want to be able to treat that number as an actual number, like an int, so we can do numbery things with it, like decide how many digits of Pi to calculate.

So how can we recover an integer that is "trapped inside" a string? There is a function called atoi which does just that!

atoi takes a string like "67" as input, and (attempts to) return the integer value trapped inside that string. For example:

• atoi("67") is 67
• atoi("123") is 123
• atoi("puppies") is 0
• atoi("puppies123") is 0

Notice that you must provide a string that is totally unamiguously an integer, otherwise you get back 0 as a "sentinal value" representing "Hey I dont know how to interpret this string you gave me."

In previous.c, we have attempted to write a program that receives an integer from the command-line, and then reports the number that is 1 less.

It should behave like this:

$ ./previous 67
The previous number is 66

Unfortunately, we forgot to convert our argument to an int using atoi. That's your job! Go ahead and fix the code so that the program works as intended.

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FYI, Where did the name "atoi" come from?