How to do a randomizer?

[SavageCabage619]

So im trying to make a script but i need it to give me a random mob / potion effect. How would I make an effect that gives a random mob or random effect?

[Jockerlight]

I don’t understand your question

[SavageCabage619]

1 hour ago, Jockerlight said:

I don’t understand your question

like this is one of the thing im trying to make

a script that whenever you step on a new block, a random mob will spawn

[AHelpingCandy]

on walk on cobblestone:
    chance of 5%:
        loop blocks in radius 5 around player:
            if loop-block is air:
                if block above loop-block is air:
                    add loop-block's location to {_locs::*}
                    set {_loc} to random element of {_locs::*}
                    spawn a zombie at {_loc}

[SavageCabage619]

56 minutes ago, Chloe30k said:

on walk on cobblestone:
    chance of 5%:
        loop blocks in radius 5 around player:
            if loop-block is air:
                if block above loop-block is air:
                    add loop-block's location to {_locs::*}
                    set {_loc} to random element of {_locs::*}
                    spawn a zombie at {_loc}

I tried doing 

on walk on block

but it don't work

i want my script to work on all blocks not just one, any ideas?