reveal.js

# Arrays, Negative Numbers

---

CS 130 // 2025-09-15

# Review

## Compiling a `while` Statement

```python
i = 1
while i != 10:  
   print(i)
   i += 1
```

---

```mips
.text
	li $s0, 1
	li $t0, 10
	
start_loop:
	beq $s0, $t0, done

li $v0, 1 #print integer - no newline after!
	add $a0, $s0, $zero #copy $s0 into $a0
	syscall

addi $s0, $s0, 1
	b start_loop

done:
```

## Exercise

- Translate this into MIPS
- Hint: can you use `slt` here?

```python
sum = 0
i = 0

while i < 10:
    sum = sum + i
    i += 1

print(sum)
```

</div>

## Assignment

- [Assignment 2](../../assignments/assignment-2/)
  - Translate a Python program that has both an if statement and a loop

# Arrays

## What is an array?

<section style="text-align: left;">
     
An array is a series of data items stored in consecutive memory locations

- Python: list

- Java: array, ArrayList

- C/C++: array, Vector

</section>

## Example array program

```mips
.data
 
#arrays get stored in consecutive 32-bit memory addresses 
my_array: 1, 2, 3, 4
 
.text

la $s0, my_array #load the base address of the array

#Addresses go up by 4 because each byte has its own address
#So a word takes up 4 addresses
lw $t0, 0($s0) #load my_array[0]
lw $t1, 4($s0) #load my_array[1]
lw $t0, 8($s0) #load my_array[2]
lw $t0, 12($s0) #load my_array[3]
```

### New flavor of `lw`

```mips
lw $t1, 4($s0) #load my_array[1]
```

- Meaning
    - load the 32-bit value stored at the address `$s0`+4
- Good for accessing items in an array

## Arrays in memory

```mips
.data
 
#arrays get stored in consecutive 32-bit memory addresses 
my_array: 1, 2, 3, 4
```

![array in Mars memory](/Fall2025/CS130/assets/images/array-in-memory.png)

### Translating a program that uses an array

```python
    my_array = [10,6,0,27,92,18,42]
    i = 0

while i < 7:
        print(my_array[i])
        i = i + 1
```
</div>
<div>

```mips
.data

my_array: 10, 6, 0, 27, 92, 18, 42
newline: .asciiz "\n"

.text

la $s0, my_array #load the base address of the array
    li $t0, 0 #loop counter

loop:

slti $t1, $t0, 7 #check if loop counter < 7
    beq $t1, $zero, end #if not < 7, branch to end

#print the item from the array
    lw $a0, 0($s0) #load next value from array
    li $v0, 1 #print integer
    syscall

#print a newline
    la $a0, newline 
    li $v0, 4 #print string
    syscall

addi $t0, $t0, 1 #increment the loop counter
    addi $s0, $s0, 4 #move to the next word in the array

b loop

end:
```
<div>

## Exercise

- edit the above MIPS program to also find the sum of all the items in the array

# Review

## Base 2 (AKA Binary)

$1101_\text{two}$ means

`$$(1\cdot 8)+(1\cdot 4)+(0\cdot 2)+(1\cdot 1)$$`

`$$(1\cdot 2^3)+(1\cdot 2^2)+(0\cdot 2^1)+(1\cdot 2^0)$$`