Assignment & Increment Operators (Bank Account Balance) C++

Question

I have to output the bank balance after a withdrawal.

int balance ;
int withdraw ;
cin >> balance;
cin >> withdraw;
cout << balance-= withdraw;

//What’s wrong with my answer?
//No match for ‘operator -= ‘
//What does this error message mean?

Answer 1

Try to print balance after you have subtracted withdrawal to it or just remove the equal sign.
_______________________
int balance, withdrawal;
cin >> balance;
cin >> withdrawal;
balance -= withdrawal;
cout << balance;
_______________________

---OR---
_______________________
int balance, withdrawal;
cin >> balance;
cin >> withdrawal;

cout << balance - withdrawal;
_______________________

Answer 2

*** DELETED ANSWER (WRONG CONCEPT COMPREHENSION) ***

(EDIT)
It turned out that right to left associativity of -= operator affects how the output is inserted to the output stream. This behaviour is changed by addition of parentheses to change priority of evaluation.

(CORRECTION)
Upon recent search I found that assignment and input/output operation in C++ are EXPRESSION and not STATEMENT. So I deleted my previous answer where I stated something wrong.

Answer 3

Ipang Angelo Arsenic
I didn't know that placing parentheses around it would also work. Thanks for the new knowledge/info.

Answer 4

The problem here (as already pointer out by Angelo ) is that compund assignment operator " -= " is right-left associative

Means your expression will be grouped from right to left, making it

(cout<< balance) -= (withdraw)

Simple fix is to override this behaviour by putting parentheses to show how the operands should be grouped.

cout<< (balance -= withdraw)

here is the fixed code 👇
https://code.sololearn.com/czh7JQvTd0HU/?ref=app

Answer 5

Because it reads
(cout << balance) -= withdraw

While you want
cout << (balance -= withdraw)

Answer 6

Ipang what you're saying is partially true (for example) in python but not in c/c++

There is practicaly no statement/expression in c/c++, a=2 is treated the same as a+2

Answer 7

Eve assignment operator returns the value of it's left operand after the assignment.

Answer 8

Angelo
a = 2 is not the same with a + 2, one thing to consider before placing those two in equal rank is that a + 2 in itself doesn't do anything to <a> unless the result is assigned to something.

And don't forget that a = 2 will be different to a + 2 when the declaration of <a> didn't initialize <a> to a certain value.

Answer 9

Thank you, Eve !

Answer 10

Ipang I know, I just took the first two operations that came in my mind

What I ment was that they are both functions, regardless of what they do

Answer 11

Tahiti what was the input you gave to get the faulty output ?

In any case, the code will print the final value of balance to the screen.

Answer 12

🤦‍♂️
Tahiti , what do you think your program is doing ?

Answer 13

I want to add here something, in c++ you can write it
cout << (balance-=withdrawal);
and it will work but in python it won't work in anyway. You have to write separately. Like --
balance -= withdrawal
print(balance)

Answer 14

You want to withdraw some money from your bank account.
The program takes two numbers as input, your account balance and the amount you want to withdraw. Calculate and output the remaining balance after the withdrawal.

Use the -= shorthand for easier calculation.

Answer 15

Arsenic
Your code gave output ‘0’.
But we need cout << balance;

Answer 16

Arsenic
cin >> balance;
cin >> withdraw;

Answer 17

Tahiti
That's correct
But you are just telling program to read user's input here. The user still have to give some input.

what was the input you gave at runtime to test this pice of code that gave you wrong output ?

Answer 18

Arsenic
//Here is exactly what I entered as code:

int balance ;
int withdraw ;
cin >> balance;
cin >> withdraw;
cout << balance-= withdraw;

// Error message: No match for ‘operator -= ‘

Answer 19

Arsenic
I’m not sure what you’re asking. The issue has been resolved, but the problem was with “cout 》balance -= withdraw; “

Answer 20

Pavan Chaitanya
Could you maybe create a new post for your question, include a link to the exercise, and also indicate which programming language you are using? Thank you, and good luck!