Szathmáry László honlapja @ DEIK | Nim2 / increment a variable

Similar to Python.

nim> var x = 0
nim> ++x
Error: undeclared identifier: '++'
nim> x++
Error: expression expected, but found '[EOF]'

++x and x++ won't work. What can you use instead?

var x = 0

x += 1
echo x # 1

x += 2
echo x # 3

inc(x)
echo x # 4

x = 10
echo x # 10

x -= 1
echo x # 9

x -= 4
echo x # 5

dec(x)
echo x # 4

More examples:

var a = 0

inc(a)
echo a # 1

a.inc()
echo a # 2

a.inc
echo a # 3

a.inc(5)
echo a # 8

How does inc() work?

By default, arguments are passed by value (like in C), i.e. a copy is made about the argument. inc() modifies the variable it received, which means that inc() must receive the variable by reference. Here is how to do it in Nim:

Nim: pass by reference
#include <stdio.h> void my_inc(int p) { p += 1; } int main() { int x = 0; my_inc(&x); printf("%d\n", x); // 1 return 0; }	# here "var" means that the argument is passed by reference # if you modify it, it'll be visible on the caller's side # You can think of "n" that it's an alias on the "x" variable. proc my_inc(n: var int) = n += 1 var x = 0 my_inc(x) echo x # 1

In Nim how you pass an argument to a procedure (by value or by reference) is indicated on the formal parameter list of the procedure.

In C, you decide it on the caller's side. Either you pass a variable or its memory address.

Exercise

Improve my_inc() so that it can accept the value of incrementation. Expected behaviour:

var x = 0

x.my_inc()
echo x # 1

x.my_inc(4) # !!! new thing !!!
echo x # 5

my_inc(x)
echo x # 6

x.my_inc
echo x # 7