Week 1, continued

A volunteer from the audience, Alana, will represent the [ say ] block from Scratch. David will give her an argument, a white sheet of paper, on which he writes the words hello, world. By handing her the argument, David is telling her what to say.

Now we have her put on a name tag that says printf. Alana writes the input given on the white paper on the touchscreen, simulating the effect of the printf function.

Another volunteer, Javier, will represent the GetString function. He takes a white sheet of paper and gets a name from the audience, and then brings it back.

Alana now gets a sheet of paper that has hello, __ and the sheet of paper from Javier. She uses the name on the second sheet of paper, Jonathan, to fill in the blank on the first sheet.

Recall that the CS50 Appliance is an operating system, called Ubuntu Linux, running in a window on your own computer, so everyone in the class has the same system to program in.

gedit is the text editor in which we write the code, and terminal is the blinking prompt where we type in commands. It is a CLI, or command-line interface, as opposed to a GUI, a graphical user interface.

Once we write code, recall that we need to enter commands like make hello that takes source code and converts it to object code, or zeroes and ones.

The object code is run with a command like ./hello, with the . representing the current directory, and the / separates the directory from the file name, so we are running the program named hello in the current folder.

1#include <stdio.h>
2
3int main(void)
4{
5    printf("hello, world\n");
6}

Let’s take a closer look at the above code.

You will learn to see the correct placement of these syntactical details as time goes on.

Source code is passed into a compiler that generates patterns of zeroes and ones.

make is not a compiler, but calls on clang, an actual compiler. Other compilers include Visual Studio and gcc. Let’s compile and run:

jharvard@appliance (~): make hello
clang -ggdb3 -O0 -std=c99 -Wall -Werror    hello.c  -lcs50 -lm -o hello

rm is the command to remove something, for example rm hello.

So now we try to compile with clang ourselves:

jharvard@appliance (~): clang hello.c
jharvard@appliance (~): ./hello
bash: ./hello: No such file or directory
jharvard@appliance (~):

bash is the name of the prompt we’re using, and it can’t find ./hello.

We run ls to see what is in the current directory, and see a.out. Back in the day, the default output for the compiler was decided to be a.out, so that is our compiled program.

To rename a file, we use mv, short for move.

jharvard@appliance (~): mv a.out hello
jharvard@appliance (~): ./hello
hello, world

Let’s add code to our program:

#include <stdio.h>

int main(void)
{
    printf("state your name: ");
    string s = GetString();
    printf("hello, world\n");
}

Now we get the same errors as last time:

jharvard@appliance (~): clang hello.c
hello.c:6:5: error: use of undeclared identifier 'string'; did you mean 'stdin'?
    string s = GetString();
    ^~~~~
    stdin
...

We need to include cs50.h, training wheels that include functions like GetString.

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    printf("state your name: ");
    string s = GetString();
    printf("hello, world\n");
}

But the compiler doesn’t know to combine the object code of both hello and the object code of cs50.h. So we have to say

jharvard@appliance (~): clang hello.c -lcs50

Now the program compiles and runs as a.out, but still says only hello, world. We quickly fix this bug:

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    printf("state your name: ");
    string s = GetString();
    printf("hello, %s\n", s);
}

And now we run this, with -o hello to output a program named hello rather than the default a.out.

jharvard@appliance (~): clang -o hello hello.c -lcs50

Since we don’t want to remember these, and other arguments, to clang we simply use make hello.

Recall that printf is in the library stdio.h. There’s \n, \r, \', \", \\, \0, and other escape sequences that are special expressions that start with a backslash. For example \" is useful in printing the double quotes without causing an error by ending the string passed to printf.

We also have placeholders like %c for printing a single character, %d for printing a decimal number, also written as %i, and %s for strings.

In C, we also have different types of variables that can store different types of things. char stores a character, float is for a real number, int is for an integer, and a long long is for a really long number, more than an int can hold.

In CS50.h we have two other data types, string, and bool, which can either be true or false. The library also contains GetChar, GetInt, GetString, etc, for getting a specific type of input from a user.

Conditions have the following structure:

if (condition)
{
    // do this
}

The // in line 3 marks a comment, a note to yourself that has no impact on the program.

There can also be two exclusive forks:

if (condition)
{
    // do this
}
else
{
    // do that
}

Or three:

if (condition)
{
    // do this
}
else if (condition)
{
    // do that
}
else
{
    // do this other thing
}

Incidentally, there are many other ways to write the same code, one being as follows:

if (condition) {
    // do this
} else if (condition) {
    // do that
} else {
    // do this other thing
}

We prefer that braces are on their own lines, among other things, and will guide you along the way to a standard CS50 style that note these distinctions of new lines and tabs, for consistency and readability.

Boolean expressions can be combined with && as "and", and || as "or":

if (condition && condition)
{
    // do this
}

if (condition || condition)
{
    // do this
}

Switches are an alternate way of expressing if and else if:

switch (expression)
{
    case i:
        // do this
        break;

    case j:
        // do that
        break;

    default:
        // do this other thing
        break;
}

Loops have various formats which we will return to:

for (initializations; condition; updates)
{
    // do this again and again
}

while (condition)
{
    // do this again and again
}

do
{
    // do this again and again
}
while (condition);

Variables in C have a particular type. Here, the first line creates a new variable of the type int, and the second assigns a value of 0 to it.

int counter;
counter = 0;

A more elegant way to write the above code:

int counter = 0;

Functions take arguments within parentheses:

string name = GetString();
printf("hello, %s\n", name);

As an aside, jailbreaking an iPhone or generally any phone, is doing something the company didn’t intend, such as installing applications from outside the App Store. This program, iUnlock.c, was written in C, and at the end of the day even advanced, familiar devices use the same code that we do in hello.c.

Let’s make another program, adder.c.

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    // ask user for input
    printf("Give me an integer: ");
    int x = GetInt();
    printf("Give me another integer: ");
    int y = GetInt();

    // do the math
    printf("The sum of %i and %i is %i!\n", x, y, x + y);
}

The last argument to printf in line 13, x + y, is simply what we want, the sum. Let’s compile and run it:

jharvard@appliance (~): make adder
clang -ggdb3 -O0 -std=c99 -Wall -Werror    adder.c  -lcs50 -lm -o adder
jharvard@appliance (~): ./adder
Give me an integer: 1
Giver me another integer: 2
The sum of 1 and 2 is 3!
jharvard@appliance (~):

We can be even fancier with conditions-0.c:

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    printf("I'd like an integer please: ");
    int n = GetInt();

    if (n > 0)
    {
        printf("You picked a positive number!\n");
    }
    else
    {
        printf("You picked a negative number!\n");
    }
}

This will print whether the number is positive or negative. Let’s try to compile:

jharvard@appliance (~): make conditions-0
make: Nothing to be done for 'conditions-0'.

So the mistake here was saving the file as conditions-0 rather than conditions-0.c. Now we compile and run, but when we type in 0:

jharvard@appliance (~): ./conditions-0
I'd like an integer please: 0
You picked a negative number!
jharvard@appliance (~):

So let’s add a third condition:

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    printf("I'd like an integer please: ");
    int n = GetInt();

    if (n > 0)
    {
        printf("You picked a positive number!\n");
    }
    else if (n < 0)
    {
        printf("You picked a negative number!\n");
    }
    else
    {
        printf("You picked zero!\n");
    }
}

jharvard@appliance (~): ./conditions-0
I'd like an integer please: 0
You picked zero!
jharvard@appliance (~):

We conclude with Saroo Brierley: Homeward Bound from Google, the story of a man who finds his home with the help Google Earth, testifying to the power of technology.