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Why does this if statement return true

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6

I've been thinking of some beginner mistakes and I ended up with the one on the if statement. I expanded a bit the code to this:

if (i = 1 && i == 0) {

    cout << i;

}

what I have seen is that the if statement returns true and it cout's i as 1. If i is 1 why the boolean expression returned true?

c++ if-statement

share|improve this question

edited 4 hours ago

πάντα ῥεῖ

75k1078147

asked 4 hours ago

TehMattGRTehMattGR

647

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  Guys, this is not a typo question. The OP wants to know why the if statement is entered with this code since i is set to 1.
  
  – NathanOliver
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You assign 1 to i, though it should never print from what I can see
  
  – JVApen
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Or does it assign the result of 1 && i == 0?
  
  – JVApen
  4 hours ago
  

  
  
  
  

add a comment | 

6

I've been thinking of some beginner mistakes and I ended up with the one on the if statement. I expanded a bit the code to this:

if (i = 1 && i == 0) {

    cout << i;

}

what I have seen is that the if statement returns true and it cout's i as 1. If i is 1 why the boolean expression returned true?

c++ if-statement

share|improve this question

edited 4 hours ago

πάντα ῥεῖ

75k1078147

asked 4 hours ago

TehMattGRTehMattGR

647

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  Guys, this is not a typo question. The OP wants to know why the if statement is entered with this code since i is set to 1.
  
  – NathanOliver
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You assign 1 to i, though it should never print from what I can see
  
  – JVApen
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Or does it assign the result of 1 && i == 0?
  
  – JVApen
  4 hours ago
  

  
  
  
  

add a comment | 

I've been thinking of some beginner mistakes and I ended up with the one on the if statement. I expanded a bit the code to this:

if (i = 1 && i == 0) {

    cout << i;

}

what I have seen is that the if statement returns true and it cout's i as 1. If i is 1 why the boolean expression returned true?

c++ if-statement

share|improve this question

edited 4 hours ago

πάντα ῥεῖ

75k1078147

asked 4 hours ago

TehMattGRTehMattGR

647

if (i = 1 && i == 0) {

    cout << i;

}

share|improve this question

edited 4 hours ago

πάντα ῥεῖ

75k1078147

asked 4 hours ago

TehMattGRTehMattGR

647

share|improve this question

edited 4 hours ago

πάντα ῥεῖ

75k1078147

asked 4 hours ago

TehMattGRTehMattGR

647

edited 4 hours ago

πάντα ῥεῖ

75k1078147

edited 4 hours ago

πάντα ῥεῖ

75k1078147

asked 4 hours ago

TehMattGRTehMattGR

647

asked 4 hours ago

TehMattGRTehMattGR

647

2 Answers
2

active

oldest

votes

16

This has to do with operator precedence.

if (i = 1 && i == 0)

is not

if ((i = 1) && (i == 0))

because both && and == have a higher precedence than =. What it really works out to is

if (i = (1 && (i == 0))

which assigns the result of 1 && (i == 0) to i. So, if i starts at 0 then i == 0 is true, so 1 && true is true (or 1), and then i gets set to 1. Then since 1 is true, you enter the if block and print the value you assigned to i.

share|improve this answer

answered 4 hours ago

NathanOliverNathanOliver

102k17148229

add a comment | 

5

Assuming your code actually looks like this:

#include <iostream>

using namespace std;



int main()  {

    int i = 0;

    if (i = 1 && i == 0) {

        cout << i;

    }

}

Then this:

if (i = 1 && i == 0) {

evaluates as

 if (i = (1 && i == 0)) {

and so i is set to 1.

share|improve this answer

answered 4 hours ago

Neil ButterworthNeil Butterworth

27.8k54983

add a comment | 

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16

This has to do with operator precedence.

if (i = 1 && i == 0)

is not

if ((i = 1) && (i == 0))

because both && and == have a higher precedence than =. What it really works out to is

if (i = (1 && (i == 0))

which assigns the result of 1 && (i == 0) to i. So, if i starts at 0 then i == 0 is true, so 1 && true is true (or 1), and then i gets set to 1. Then since 1 is true, you enter the if block and print the value you assigned to i.

share|improve this answer

answered 4 hours ago

NathanOliverNathanOliver

102k17148229

add a comment | 

16

This has to do with operator precedence.

if (i = 1 && i == 0)

is not

if ((i = 1) && (i == 0))

because both && and == have a higher precedence than =. What it really works out to is

if (i = (1 && (i == 0))

which assigns the result of 1 && (i == 0) to i. So, if i starts at 0 then i == 0 is true, so 1 && true is true (or 1), and then i gets set to 1. Then since 1 is true, you enter the if block and print the value you assigned to i.

share|improve this answer

answered 4 hours ago

NathanOliverNathanOliver

102k17148229

add a comment | 

This has to do with operator precedence.

if (i = 1 && i == 0)

is not

if ((i = 1) && (i == 0))

because both && and == have a higher precedence than =. What it really works out to is

if (i = (1 && (i == 0))

which assigns the result of 1 && (i == 0) to i. So, if i starts at 0 then i == 0 is true, so 1 && true is true (or 1), and then i gets set to 1. Then since 1 is true, you enter the if block and print the value you assigned to i.

share|improve this answer

answered 4 hours ago

NathanOliverNathanOliver

102k17148229

if (i = 1 && i == 0)

if ((i = 1) && (i == 0))

if (i = (1 && (i == 0))

share|improve this answer

answered 4 hours ago

NathanOliverNathanOliver

102k17148229

answered 4 hours ago

NathanOliverNathanOliver

102k17148229

answered 4 hours ago

NathanOliverNathanOliver

102k17148229

5

Assuming your code actually looks like this:

#include <iostream>

using namespace std;



int main()  {

    int i = 0;

    if (i = 1 && i == 0) {

        cout << i;

    }

}

Then this:

if (i = 1 && i == 0) {

evaluates as

 if (i = (1 && i == 0)) {

and so i is set to 1.

share|improve this answer

answered 4 hours ago

Neil ButterworthNeil Butterworth

27.8k54983

add a comment | 

5

Assuming your code actually looks like this:

#include <iostream>

using namespace std;



int main()  {

    int i = 0;

    if (i = 1 && i == 0) {

        cout << i;

    }

}

Then this:

if (i = 1 && i == 0) {

evaluates as

 if (i = (1 && i == 0)) {

and so i is set to 1.

share|improve this answer

answered 4 hours ago

Neil ButterworthNeil Butterworth

27.8k54983

add a comment | 

Assuming your code actually looks like this:

#include <iostream>

using namespace std;



int main()  {

    int i = 0;

    if (i = 1 && i == 0) {

        cout << i;

    }

}

Then this:

if (i = 1 && i == 0) {

evaluates as

 if (i = (1 && i == 0)) {

and so i is set to 1.

share|improve this answer

answered 4 hours ago

Neil ButterworthNeil Butterworth

27.8k54983

#include <iostream>

using namespace std;



int main()  {

    int i = 0;

    if (i = 1 && i == 0) {

        cout << i;

    }

}

if (i = 1 && i == 0) {

 if (i = (1 && i == 0)) {

share|improve this answer

answered 4 hours ago

Neil ButterworthNeil Butterworth

27.8k54983

answered 4 hours ago

Neil ButterworthNeil Butterworth

27.8k54983

answered 4 hours ago

Neil ButterworthNeil Butterworth

27.8k54983