# Lecture 12 – Sep 30th, 2019

## Setup

2. Create a directory and cd into it.
3. Copy ~steve/ex/parse_int.c into the directory.

1. Read through parse_int.c and try to understand what it is doing. Looking at the man page for getchar(3) and isdigit(3) is going to be helpful.
2. Compile parse_int and run it. It’s expecting input from stdin, so you can either type into the terminal while it’s running. It’ll receive input one line at a time so be sure to hit return/enter and if you want to signal the end of the input, press control-d (on an otherwise blank line). You can also run $echo 12345 | ./parse_int. 3. Edit parse_int.c and change num_value so that it returns 0 on input '0', 1 on input '1', and so on up to 9 on input ‘9’. You can assume the input is in ASCII (take a look at the ascii(7) man page for handy tables of values) and in particular, ‘0’–‘9’ are consecutive values. Thus ch - '0' is probably useful to you. 4. Modify the behavior of parse_int.c so that it reads a positive integer (in decimal) one character at a time (until a non-digit is read). Print out the value of the integer times 7. $ echo 0 | ./parse_int
0 * 7 = 0
$echo 1 | ./parse_int 1 * 7 = 7$ echo 12345 | ./parse_int
12345 * 7 = 86415


Recall, $539$ means $5\cdot10^2 + 3\cdot10^1 + 9\cdot10^0 = (5\cdot10^1 + 3\cdot10^0)\cdot10 + 9.$ But notice that the number in the parentheses is just $53$. Therefore, if val is our current value, then after reading in another digit, we need to multiply val by 10 and add the value of the digit.

5. Modify the behavior of parse_int.c again so that instead of reading just one integer, it reads two integers, separated by a +, adds them together, and prints out the result.
\$ echo 32527+1256289 | ./parse_int
32527 + 1256289 = 1288816

6. Modify parse_int.c one more time so that if the two numbers are separated by +, it adds them; by -, it subtracts the second from the first; by *, it multiples them; and by /, it divides the first by the second. What happens if you try to divide by 0? Handle that gracefully.