Create an array of size n from user input n

Question

For this assignment i need to be able to make an array of size n based on a user inputed value from zero to fifty. So far this is what ive done below. If you have any advice for the question overall that would be very helpful too.

a) Prompt the user for an integer in the range of 0 to 50. If the user inputs 0 the program stops.

b) Otherwise, the program stores the numbers from 0 up to the input value into an array of words in memory, i.e. initializes the array with values from 0 up to N where N is the value that user has inputted.

c) The program then adds the value of all items of the array together (up to N) by loading them from the main memory then add them up, then prints out the sum with the message "The sum of integers from 0 to N is:". For example, if the user gave 5 as the input the program prints out "The sum of integers from 0 to 5 is 15".

Submit your work as a zip file as specified in the syllabus to eLearning by the due date.

.data 
userPrompt : .asciiz "enter in an integer from zero to fifty  "
zeroMessage : .asciiz " you have entered a zero , the program will close "
incorrectEntry : .asciiz " you have entered in a value greater than 50 , 
this is an incorrect value"

InputVal : .word

upperLim : .word 50

Array : .space InputVal



.text 

main: 
    addi $t7  , $zero , 50

    li $v0, 4 # load for printing of strings 
    la $a0, userPrompt
    syscall

# take in user input and move the read in number to a temp
    li $v0, 5 
    la $t0 , InputVal   
    syscall


   # Store int A into memory 
    move $t0 , $v0
    beq $t0 , $0 , numbersEqual


    la $t1 , upperLim

    li $v0 , 1
    move $a0 , $t1
    syscall


    slt $t3 ,$t0 , $t1
    sw $t0 , InputVal

    #beq $t3 , $0 , ELSE





ELSE :
    li $v0 , 4 
    la $a0 , incorrectEntry
    syscall
    li $v0 , 10
    syscall


numbersEqual: 

    li $v0 , 4 
    la $a0 , zeroMessage
    syscall
    li $v0 , 10
    syscall

You can't store values from 0 to N into array of size N. Just try it for N=2: now you have array of size 2 { [..], [..] } .. now try to put values 0, 1, 2 into that (without some compression scheme). Can't be done. You need array of N+1 size. — Ped7g

Ped7g Ped7g · Accepted Answer · 2017-09-23T23:45:51

The assembly language is symbolic language for machine code, and machine code is what your CPU can execute.

After you run assembler to compile your source, you get machine code, which is sort of final. Then you execute that machine code.

The .word 0x12345678 is not instruction of CPU, but directive of your assembler, it tells it to reserve whole word of memory at that place of machine code, and store value of 0x12345678 there. I'm not sure what .word without value does, whether it will reserve at least one word, you should rather do InputVal: .word 0 to be sure.

In the final machine code there is no ".word", that's not CPU instruction, there will be just those 4 bytes with their respective values forming word value, at some address, which was known to the assembler as symbol InputVal (that's also not part of machine code any more in this textual form, any instruction using this memory address has only the proper address encoded in it as numeric value, and the executable binary may contain some sort of relocation table for OS to patch addresses properly after loading the machine code to target memory before execution).

Now this may sounds like stating the obvious, but it's important for you to understand the difference, what is available when the CPU is already executing your machine code, and what is available during compilation by assembler (code not running yet).

Array : .space InputVal will not work as you wish, because InputVal is symbol of memory address. So the directive .space will either reserve bazillion bytes (value of memory address of InputVal), or more likely the compilation will fail. What you want is content of memory at InputVal, but that's not known yet, because the code didn't run, user didn't enter anything, it's still just assembly step. So the value is not know, you may just as write Array: .space 0. But that will reserve no space.

Unless you want to delve into dynamic memory allocation, there's simple trick to resolve such situation, which will work in your particular case. If you will read the task, the N is valid only when inputted value is from 0 to 50 (anything else is user error and you can exit). So for maximum N=50 you will need array of 51 values, and you will never need more.

So you can avoid all the dynamic memory allocation (at runtime) by simply doing:

       .align 4   # make sure the reserved space is word-aligned
Array: .space (51*4)

This will reserve 204 bytes (51 words) of memory in the data area for your machine code, with symbol Array pointing at the first byte of it.

Now you have memory reserved, and you can store values into it, and use it. If you would want during runtime to change your mind, and use 52 words of memory, you are out of luck with such code. Then you need either to code dynamic allocation, or increment the hard-sized fixed buffer during compilation time.

Also your code has always 51 words available in that Array, so it's up to your code to use only the inputted N+1 values out of them. If user enters N=5, then you should work only over 6 words (24 bytes), ignoring the remaining reserved words).

So each loop in your code will be like for (i = 0; i <= N; ++i) Array[i] = i;, where N is the value entered by user during runtime (store it to InputVal reserved memory, so you can access it whenever you need it), not known during compilation.

Create an array of size n from user input n

1 Answers