Definition df-if 4486

Description: Definition of the conditional operator for classes. The expression if(𝜑, 𝐴, 𝐵) is read "if 𝜑 then 𝐴 else 𝐵". See iftrue 4491 and iffalse 4494 for its values. In the mathematical literature, this operator is rarely defined formally but is implicit in informal definitions such as "let f(x)=0 if x=0 and 1/x otherwise".

An important use for us is in conjunction with the weak deduction theorem, which is described in the next section, beginning at dedth 4544. (Contributed by NM, 15-May-1999.)

Assertion

Ref	Expression
df-if	⊢ if(𝜑, 𝐴, 𝐵) = {𝑥 ∣ ((𝑥 ∈ 𝐴 ∧ 𝜑) ∨ (𝑥 ∈ 𝐵 ∧ ¬ 𝜑))}

Distinct variable groups:   𝜑,𝑥   𝑥,𝐴   𝑥,𝐵

Detailed syntax breakdown of Definition df-if

Step	Hyp	Ref	Expression
1		wph	. . 3 wff 𝜑
2		cA	. . 3 class 𝐴
3		cB	. . 3 class 𝐵
4	1, 2, 3	cif 4485	. 2 class if(𝜑, 𝐴, 𝐵)
5		vx	. . . . . . 7 setvar 𝑥
6	5	cv 1569	. . . . . 6 class 𝑥
7	6, 2	wcel 2147	. . . . 5 wff 𝑥 ∈ 𝐴
8	7, 1	wa 401	. . . 4 wff (𝑥 ∈ 𝐴 ∧ 𝜑)
9	6, 3	wcel 2147	. . . . 5 wff 𝑥 ∈ 𝐵
10	1	wn 3	. . . . 5 wff ¬ 𝜑
11	9, 10	wa 401	. . . 4 wff (𝑥 ∈ 𝐵 ∧ ¬ 𝜑)
12	8, 11	wo 861	. . 3 wff ((𝑥 ∈ 𝐴 ∧ 𝜑) ∨ (𝑥 ∈ 𝐵 ∧ ¬ 𝜑))
13	12, 5	cab 2741	. 2 class {𝑥 ∣ ((𝑥 ∈ 𝐴 ∧ 𝜑) ∨ (𝑥 ∈ 𝐵 ∧ ¬ 𝜑))}
14	4, 13	wceq 1570	1 wff if(𝜑, 𝐴, 𝐵) = {𝑥 ∣ ((𝑥 ∈ 𝐴 ∧ 𝜑) ∨ (𝑥 ∈ 𝐵 ∧ ¬ 𝜑))}

This definition is used by:  dfif2  4487  dfif6  4488  iffalse  4494  rabsnifsb  4683  bj-df-ifc  37628