GATE CSE 1993 | Question: 7.4

Question

What does the following code do?

var a, b: integer;
begin
    a:=a+b;
    b:=a-b;
    a:=a-b;
end;

• A. exchanges $a$ and $b$
• B. doubles $a$ and stores in $b$
• C. doubles $b$ and stores in $a$
• D. leaves $a$ and $b$ unchanged
• E. none of the above
   

Comments

typo mistake in a:a-b; it should be a:=a-b;

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@Raghav Chandra

Corrected 👍

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Level : easy : 

Answer 1

Answer is simply $A$ i.e. it swaps the values of the two.. Take any two values for $A$ and $B$. and perform the given operations over them.

Comments

@Kiyoshi for a=2 b=3?

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@halfcodeblood

what's the problem with that case ?

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@Shaik Masthan sir actually i was miscalculating the things. Now got it 

Thanks

Answer 2

(a)
initially a = 10 b = 5
a = 15
b =  10
a = 5

Comments

yeah thanks for the example got it

Answer 3

1. a= a+b

2. b= a-b= (a+b)-b= a (from 1)

3. a= a-b= (a+b) - (a) = b (from 1 and 2)

Hence the values are getting exchanged

Answer(A).

Answer 4

LET, a=1 ,b=2

a=a+b

a=3

b=a-b

b=1

a=a-b

a=2

 

a=2

b=1

 

so this programme swap the value of a and b