CS50 Week 1 Continued: Lecture

Tools

This is my code being printed out on the terminal with some suggestion coloring. Green spaces are added by the program to suggest they should be there. So if I write if(something) it will put a green space after the if.

Red spaces mean I have put those spaces in my code and style50 wants me to get rid of them. I personally don’t like this, because sometimes I add space so I can better understand what i’m writing and having all the code tightly bunched together is difficult for me to read. In sublime text you can change line padding so the lines are further apart so adding line breaks isn’t necessary, but I haven’t found a way to do this in the CS50 IDE yet.

Once you’ve made all the changes style50 will print out Looks good! I’ll try to follow the style guide moving forward but sometimes I feel like it misses out on the slick code you can write in C. For example if statements and for loops don’t require curly braces if the block of code to execute is only 1 line. Unfortunately style50 is gonna tell you to add the curly braces.

if (something)
    printf("doing something");
else
    printf("not doing something");

instead style50 wants this

if (something)
{
    printf("doing something");
}
else
{
    printf("not doing something");
}

Printing, Debugging

The printf function is like a typewriter to the terminal window. Using loops we can print a series of characters and even special characters which are instructions that can break lines, create spaces or tabs and even make beep noises to name a few.

Sometimes we get unexpected output from our program, and it is hard to just know where the error has occurred. We expect our programs to work, so this can be frustrating. The CS50 debugger is very helpful for tracking down issues, and I admit I have a hard time letting go of good old printf statements everywhere, but I think I need to start getting into the habit of using a debugger.

Let’s create an example of a program that compiles but outputs an unintended result:

In my case, the source code is test.c so i’m going to type debug50 test which will start the debugger program. A panel should slide open on the right side and start to output some information. Using test.c it looks like this:

Looking at the right panel you can see a section called local variables. This is helpful because we can see when the loop runs the first time it calls multiply, and at our breakpoint it is just about to return the result of num (900) multiplied by 2376000. We can see what the result variable is equal to just before it is returned back to the loop to be sent to the if/else statement.

Note that I don’t put the breakpoint right at the initialization of the variable result, but right after when i’m about to return it back to the caller. This is because the debugger won’t know what result equals until one step after the variable initialization. If you were to put the break point right on the same line as int result= num * 2376000 the first loop around would give you a weird nonsensical number and following that would give you the previous iteration’s value.

Now let’s say we don’t know when the error hits. We do know that we end up in the else block of our code so let’s toggle off the breakpoint on line 21 and put the breakpoint instead on line 14, within the else block. This saves us time because we don’t have to loop over and over through each iteration.

Now when we run debug50 test we get this:

When we hit the else block mynumber is -2147063296, a negative number. So going back to the terminal error message, signed integer overflow, when we originally ran our test program, it seems like we’ve exceeded the bounds of what an integer data type can handle. So since it can’t go any higher it wraps back to a negative value and up from there. To solve this issue we change all our relevant int types to long long types.

Now it works!

Strings, Arrays

The cs50 c library has several functions to get user input. These are:

get_char

get_double

get_float

get_int

get_long_long

get_string

The output to the terminal:

Some things to note, strlen is a function provided in the string.h library. It will return the length of characters in a sequence of them. So if my name is “John” it will return 4. There is a special character at the end of these sequences of characters, or strings, called a null character. It is represented like this: ‘\0′.

So the string “John” is an array of 4 chars (characters) and a null character. As a whole this would be called a Null-terminated string.

In the code you can also see we have the user’s name held in a string data type but printf outputs a character data type. This is known as type casting. We can go further and product the integer value for each character as well which maps to ascii to give us the character.

Since we have this fine control on how each character is printed to the screen, let’s try a small challenge. If a character is given that isn’t a capital letter let’s capitalize it before it is printed. How can we check for this?

If I use the expression

char[] x = "J";

I end up with a null terminated string, which means there is a hidden ‘\0’ you don’t see. Also keep in mind this is an array of type char. Now it is very subtle but if I use single quotes with a variable of type char…

char x = 'J';

Now I get just the ‘J’ character by itself, and also this is the integer 74 which is the integer that maps to that character. I can actually get K by just adding 1 to ‘J’.

printf("%c", 'J' + 1); // will output K

So back to the problem at hand. Here is the ascii map webpage I use to figure this kind of stuff out. We need to say:

given name[i] character in our loop
test if lower case (what is the range of lowercase #'s?)
if it is make it uppercase (integer distance from a to A?)

First let’s solve our conditional test. The ascii table shows us that lower case ‘a’ starts at integer value 97 and ends with ‘z’ at integer value 122.

char current_value = name[i]
bool is_lower_case = (name[i] >= 'a') && (name[i] <= 'z')
// bool is <cs50.h> data type for boolean. Int data type
// will work but not as verbose

Between lower case and upper case ascii chars are some other characters like ‘[‘ for brackets for example, so that can make it a little daunting at first but it’s not so bad when you convert everything to their number values.

For example,

‘a’ is 97

‘A’ is 65

If I said to you, “hey if I give you 97, return back 65,” you would just subtract some number from 97. So hopefully you know where this is going.

int magic_number = 97 - 65; // 32

Not the answer to life the universe and everything but now we know, subtract 32 from a lower case character to make it upper, and add 32 to an upper case character to make it lower. So let’s write this program up:

Cryptography