Issues with running a data merge within an if command

Question

I've spent a few hours on this and looked around on the minecraft wiki and I cannot work out how to go about this. The aim is to add motion to a minecart a player is riding in when looking in a certain direction.

The following two commands work as intended:

execute as @e[type=minecart,distance=..18] run data merge entity @s {Motion:[-1.0d, 0.0d, 0.0d]}
execute if entity @p[team=red,distance=..20,y_rotation=0..90]

But I can't merge the two together to work as one.

Previous version command that used to work:

execute @a[team=red,r=200,rym=-64,ry=-26] ~ ~ ~ entitydata @e[type=minecart,r=1] {Motion:[4.5,0.04,4.5]}

Command so far, not working:

execute as @p if entity @p[distance=..20,y_rotation=0..359] run data merge entity @s[distance=..20] {Motion:[0.0,0.04,5.0]}

Apologies if I've missed something simple, I have learning difficulties that can make certain tasks difficult.

Answer 1

Instead of trying to fix your commands, I rewrote it completely, I hope this helps and can be applied to your case. This command works for me, it sets a minecart in motion whenever I'm in it:

/execute if data entity @s RootVehicle run data merge entity @e[type=minecart,distance=..0.4,limit=1] {Motion:[0.0,0.0,10.0]}

---

_{If you need to be absolutely, 100% sure that you move the minecart that you're riding, even if there's another one at the exact same position, then it gets complicated. I think the easiest way would be to loop over all minecarts in the area, each time copying the tag UUIDMost and UUIDLeast of the minecart that the player is riding (RootVehicle.UUIDMost, …) over to an arbitrary temporary storage (for example inside the tag tag of an item entity, or, in 1.15, the "storage" of the player, then trying to copy over the UUID parts of the current minecart in the loop, if that returns false (meaning that the same value was already stored there), then you found the minecart and apply motion to it.
I told you it's complicated. Sometimes being 99% sure is enough.}

Answer 2

If you wanna modify player data, I need to say you that this isn't possible anyway.
You can easily modify entity data, but playerdata can't be.
If you tell us what you're trying to do, maybe we could come up with another solution
for your problem.
I hope this helped you.