COMP26020p1 - Arrays, Strings, Command Line Arguments

class: center, middle

### COMP26020 Programming Languages and Paradigms Part 1: C Programming
***
# Arrays, Strings, Command Line Arguments

???

- Hello Everyone.
- In this video we are going to talk about arrays, strings and command line arguments in C

---
# Arrays

.leftcol[
```c
int array[4];     // declaration

array[0] = 10;    // write a slot
array[1] = 20;
array[2] = 30;
array[3] = 40;

int x = array[3];  // read a slot

```
]
.rightcol[
```c
printf("array[%d] contains %d\n", 0, array[0]);
printf("array[%d] contains %d\n", 1, array[1]);
printf("array[%d] contains %d\n", 2, array[2]);
printf("array[%d] contains %d\n", 3, array[3]);

```
.codelink[<a href="src/array.c" download>`05-c-arrays-strings-cmdline/array.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a><a href="https://www.programiz.com/online-compiler/6RZvghWOfCIPW" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]
]

Arrays are stored contiguously in memory:
<div style="text-align:center"><img src="include/arrays.svg" width=950 /></div>

???

- Let's start with arrays.
- They are declared with a statement starting with the type, followed by the name of the array variable, then between brackets the size of the array -- here 4 elements.
- You can write something in an array slot with the affectation symbol, equals, and the index to write between bracket
- For example here we write 10 in the first slot, 20 in the second one, and so on.
- You'll notice that the indexing starts at 0, the first slot is index 0.
- And you read a slot by indicating its index between brackets, for example here we read the fourth slot
- Something important to note is that arrays are stored contiguously in memory.
- So remember on x86-64 the size of int is 4 bytes.
- We declared an array with 4 elements, so we have something like this in memory.
- 16 contiguous bytes reserved by the compiler for the array, 4 bytes per slot.
- Now if we execute this program, we'll see the content of the array printed.

---

# Multi-Dimensional Arrays

```c
int my_2d_array[3][2];

my_2d_array[0][0] = 0;
my_2d_array[0][1] = 1;
my_2d_array[1][0] = 2;
my_2d_array[1][1] = 3;
my_2d_array[2][0] = 4;
my_2d_array[2][1] = 5;

printf("my_2d_array[%d][%d] = %d\n", 0, 0, my_2d_array[0][0]);
printf("my_2d_array[%d][%d] = %d\n", 0, 1, my_2d_array[0][1]);
printf("my_2d_array[%d][%d] = %d\n", 1, 0, my_2d_array[1][0]);
printf("my_2d_array[%d][%d] = %d\n", 1, 1, my_2d_array[1][1]);
printf("my_2d_array[%d][%d] = %d\n", 2, 0, my_2d_array[2][0]);
printf("my_2d_array[%d][%d] = %d\n", 2, 1, my_2d_array[2][1]);

```
.codelink[<a href="src/array-2d.c" download>`05-c-arrays-strings-cmdline/array-2d.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a><a href="https://www.programiz.com/online-compiler/5G4kTenDPNXMc" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]

???

- Arrays can be of multiple dimensions.
- To declare such an array you use several pairs of square brackets.
- For example on the slide we can see the declaration of a 3 by 2 array.
- We also refer to particular slots with pairs of square brackets.
- This is for a 2 dimensional array but you can have as many dimensions as you want.
- Arrays are still laid out contiguously in memory with the last dimension first, so we have array[0][0], followed by array[0][1], followed by array[1][0], and so on.
- Executing this program prints the content of the array.

---
# Arrays
- Static initialisation allows to omit the size (of the first dimension)

```c
int array[] = {1, 2, 3, 4, 5};
int array_2d[][2] = { {1, 2}, {3, 4}, {5, 6} };

printf("array[4] = %d\n", array[4]);
printf("array_2d[2][0] = %d\n", array_2d[2][0]);

```
.codelink[<a href="src/array-init-static.c" download>`05-c-arrays-strings-cmdline/array-init-static.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a><a href="https://www.programiz.com/online-compiler/2Yu8LVsiGt1AT" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]

???

- Rather than initialising the array after its declaration, we can also initialise it with its declaration
- This is called static initialisation
- If you use it, you can omit the size of the first dimension, as shown on this example.
- Here we create a one dimensional array with 5 slots, and a two dimensional array of size 3 by 2.
- If we execute it we print array[4] which is 5 and array_2d[2][0] which is 5.

---

# Character arrays, i.e. Strings

- String in C: **array of characters** that ends with the special `\0`
  end of string character marker

.leftcol[
```c
char string1[6] = "hello";
char string2[] = "hello";
char string3[6];

string3[0] = 'h';
string3[1] = 'e';
string3[2] = 'l';
string3[3] = 'l';
string3[4] = 'o';
string3[5] = '\0'; // Important here
```
]
.rightcol[
```c
printf("%s\n", string1);
printf("%s\n", string2);
printf("%s\n", string3);
//
```
.codelink[<a href="src/string.c" download>`05-c-arrays-string-cmdline/array-init-static.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a><a href="https://www.programiz.com/online-compiler/6fkusPbXRyJDq" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]
]

???

- Let's talk bit about strings.
- In C a string is simply a character array that is terminated with a special character, anti slash 0, marking the end of the string.
- This character is used by many functions to know where a given string ends.
- For example when we call printf on a string, it prints on the console all the characters of the array until it encounters the termination character.
- We can declare it like an array.
- It can be done statically, and we use the double quotes to indicate a string.
- The compiler will automatically include the termination character for these.
- However be careful to account for it when setting the size of the array, even if hello only has 5 characters, we need a sixth for the anti slash 0.
- We can also fill an array after its declaration.
- If we run this code, it prints 3 times hello.

---
# Command Line Arguments

???

- Now that we know about arrays and strings we can see how to access command line arguments.
- Command line arguments are strings passed on the command line when we invoke a program, separated with spaces
- For example here we execute the program gcc with 3 arguments: test.c, dash o, and program

---
# Command Line Arguments

- `main` parameters:
  - `argc`
  - `argv`: array containing the command line arguments

```c
int main(int argc, char **argv) { // 'char ** argv' means 'char argv[][]'

printf("Number of command line arguments: %d\n", argc);

// This is a bit dangerous...
    printf("argument 0: %s\n", argv[0]);
    printf("argument 1: %s\n", argv[1]);
    printf("argument 2: %s\n", argv[2]);

return 0;
}
```
.codelink[<a href="src/cmdline-args.c" download>`05-c-arrays-string-cmdline/cmdline-args.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a><a href="https://www.programiz.com/online-compiler/5BTs3Dw6aFmVs" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]

???

- the main function can be declared with 2 parameters
- argc, an integer indicating the number of command line arguments
- and argv, a bi-dimensional character array, in other words an array of strings, that contains the arguments
- Here is an example of a simple program simply printing the number of command line arguments and the first 3 of these arguments.
- Don't be scared by the double star before argv, it is just another way to declare a 2 dimensional char array.
- Let's try it out
- I compile the program and call it with 3 arguments
- You'll notice that the program name is the first member of the argument string array so for this particular invocation we don't see the last argument.
- Finally, not that this program is not ideal: what happens if this code is called with less than 3 arguments
- We get a memory error
- This bug cannot be caught by the compiler
- The program is still trying to print something but it's leaking weird values from the program memory
- That's quite dangerous, there could be important things like a password in there
- We'll remediate this in the next lecture regarding control flow

---
# Command Line Arguments

- Arguments are strings, to convert to integer use `atoi`:

```c
#include <stdio.h>
#include <stdlib.h> // needed for atoi

/* Sum the two integers passed as command line integers */
int main(int argc, char **argv) {
    int a, b;

// Once again, dangerous!
    // We'll fix that in the next lecture regarding control flow
    a = atoi(argv[1]);
    b = atoi(argv[2]);

printf("%d + %d = %d\n", a, b, a+b);

return 0;
}
```
.codelink[<a href="src/atoi.c" download>`05-c-arrays-string-cmdline/atoi.c`</a> <a href="https://github.com/olivierpierre/comp26020-devcontainer" target="_blank" style="text-decoration: none"><img src="include/gh-logo.svg" style="height: 1em" ></a><a href="https://www.programiz.com/online-compiler/9Q1TCHWr3jDTl" target="_blank" style="text-decoration: none"><img src="include/programiz-logo.png" style="height: 1em"></a>]

- To convert a string to a `double` use `atof`

???

- Command line arguments are strings, so if you want to pass numbers you need to covert them.
- You can do this with the atoi function.
- To use it you need to include stdlib.h
- Then you can pass a string as parameter and if successful atoi returns its conversion as an integer.
- We can try this program that sums two integers passed as command line parameters.
- To convert a string to a double you can call atof

---
# Summary

- Arrays
- Strings
- Command line arguments

----
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<div style="text-align:center"><img src="include/qr-code.png" height=150 /></div>

???
- Let's recap.
- In this video we talked about arrays
- We also saw strings, that are nothing more than character arrays with a termination character
- And we learn how to access command line parameters.
- In the next video, we will talk about control flow, that is conditionals, and loops