Why do these two loops return different results?

up vote
1
down vote

favorite

I have a loop for calculating the total hamming distance between two strings.

For the input:

nums = [4,14,2]

One version of the loop:

n, ans = len(nums), 0    

for i in range(32):

        bit, cnt = 1<<i, 0



        for num in nums:

            cnt += num & bit 



        ans += cnt * (n-cnt)



return ans

Gives the wrong result: -84

While an almost identical loop:

n, ans = len(nums), 0    

for i in range(32):

        bit, cnt = 1<<i, 0



        for num in nums:

            cnt += (num & bit) > 0  



        ans += cnt * (n-cnt)



return ans

Gives the correct answer 6. I can figure out why?

What does the '> 0' in the second loop do? I tried to understand it's effect using a simple test:

>>> i = -5 

>>> i += 1 

>>> i

-4

>>> i = -5 

>>> i += 1 > 0 

>>> i 

-4

and the '> 0' doesn't seem to do anything. Why are the two loops different?

asked Nov 12 at 5:50

Alex Kinman

64321227

4

> is greater than. In regards to your simple tests -5 += 1 > 0 is the same as -5 += True which evaluates to -5 += 1, the same as your first code block.
– Loocid
Nov 12 at 5:53

add a comment |

up vote
1
down vote

favorite

I have a loop for calculating the total hamming distance between two strings.

For the input:

nums = [4,14,2]

One version of the loop:

n, ans = len(nums), 0    

for i in range(32):

        bit, cnt = 1<<i, 0



        for num in nums:

            cnt += num & bit 



        ans += cnt * (n-cnt)



return ans

Gives the wrong result: -84

While an almost identical loop:

n, ans = len(nums), 0    

for i in range(32):

        bit, cnt = 1<<i, 0



        for num in nums:

            cnt += (num & bit) > 0  



        ans += cnt * (n-cnt)



return ans

Gives the correct answer 6. I can figure out why?

What does the '> 0' in the second loop do? I tried to understand it's effect using a simple test:

>>> i = -5 

>>> i += 1 

>>> i

-4

>>> i = -5 

>>> i += 1 > 0 

>>> i 

-4

and the '> 0' doesn't seem to do anything. Why are the two loops different?

asked Nov 12 at 5:50

Alex Kinman

64321227

4

> is greater than. In regards to your simple tests -5 += 1 > 0 is the same as -5 += True which evaluates to -5 += 1, the same as your first code block.
– Loocid
Nov 12 at 5:53

add a comment |

up vote
1
down vote

favorite

I have a loop for calculating the total hamming distance between two strings.

For the input:

nums = [4,14,2]

One version of the loop:

n, ans = len(nums), 0    

for i in range(32):

        bit, cnt = 1<<i, 0



        for num in nums:

            cnt += num & bit 



        ans += cnt * (n-cnt)



return ans

Gives the wrong result: -84

While an almost identical loop:

n, ans = len(nums), 0    

for i in range(32):

        bit, cnt = 1<<i, 0



        for num in nums:

            cnt += (num & bit) > 0  



        ans += cnt * (n-cnt)



return ans

Gives the correct answer 6. I can figure out why?

What does the '> 0' in the second loop do? I tried to understand it's effect using a simple test:

>>> i = -5 

>>> i += 1 

>>> i

-4

>>> i = -5 

>>> i += 1 > 0 

>>> i 

-4

and the '> 0' doesn't seem to do anything. Why are the two loops different?

asked Nov 12 at 5:50

Alex Kinman

64321227

I have a loop for calculating the total hamming distance between two strings.

For the input:

nums = [4,14,2]

One version of the loop:

n, ans = len(nums), 0    

for i in range(32):

        bit, cnt = 1<<i, 0



        for num in nums:

            cnt += num & bit 



        ans += cnt * (n-cnt)



return ans

Gives the wrong result: -84

While an almost identical loop:

n, ans = len(nums), 0    

for i in range(32):

        bit, cnt = 1<<i, 0



        for num in nums:

            cnt += (num & bit) > 0  



        ans += cnt * (n-cnt)



return ans

Gives the correct answer 6. I can figure out why?

What does the '> 0' in the second loop do? I tried to understand it's effect using a simple test:

>>> i = -5 

>>> i += 1 

>>> i

-4

>>> i = -5 

>>> i += 1 > 0 

>>> i 

-4

and the '> 0' doesn't seem to do anything. Why are the two loops different?

python

asked Nov 12 at 5:50

Alex Kinman

64321227

asked Nov 12 at 5:50

Alex Kinman

64321227

asked Nov 12 at 5:50

Alex Kinman

64321227

asked Nov 12 at 5:50

Alex Kinman

64321227

asked Nov 12 at 5:50

Alex Kinman

64321227

4

> is greater than. In regards to your simple tests -5 += 1 > 0 is the same as -5 += True which evaluates to -5 += 1, the same as your first code block.
– Loocid
Nov 12 at 5:53

add a comment |

4

> is greater than. In regards to your simple tests -5 += 1 > 0 is the same as -5 += True which evaluates to -5 += 1, the same as your first code block.
– Loocid
Nov 12 at 5:53

> is greater than. In regards to your simple tests -5 += 1 > 0 is the same as -5 += True which evaluates to -5 += 1, the same as your first code block.
– Loocid
Nov 12 at 5:53

add a comment |

3 Answers
3

active

oldest

votes

up vote
1
down vote

accepted

num & bit is not the same as num & bit > 0;

num & bit is a number (int), while num & bit > 0 is a boolean (bool);

in python, a True boolean is 1 when used as a number;

>>> i = -5 

>>> i += 2 

>>> i

-3

>>> i = -5 

>>> i += 2 > 0 

>>> i 

-4

as a rule of thumb, dont test with 1 or 0; test with a random number (2 is also bad but easy to read here);

edited Nov 12 at 6:18

answered Nov 12 at 5:57

Cyker

2,63553245

add a comment |

up vote
1
down vote

It's because some of the num&bit are bigger than 0, so (i.e there was one 8 in my run):

>>> a=8

>>> a>0

True

>>> int(a>0)

1

It's not 8!!!

So that's why.

answered Nov 12 at 5:55

U9-Forward

11.5k2834

add a comment |

up vote
0
down vote

In line

    cnt += num & bit

you are incrementing the value of cnt by 1,2,4,8 values.
while in line

    cnt += (num & bit) > 0

first you are validating the statement, which returns 0 or 1 then you are increamenting value by either 0 or 1.
You can cross check it by printing cnt value in both the codes.

answered Nov 12 at 6:16

Ankush Khobragade

134

add a comment |

Your Answer

StackExchange.ifUsing("editor", function () {
StackExchange.using("externalEditor", function () {
StackExchange.using("snippets", function () {
StackExchange.snippets.init();
});
});
}, "code-snippets");

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "1"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});

}
});

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53256521%2fwhy-do-these-two-loops-return-different-results%23new-answer', 'question_page');
}
);

Post as a guest

Name

Required, but never shown

3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

up vote
1
down vote

accepted

num & bit is not the same as num & bit > 0;

num & bit is a number (int), while num & bit > 0 is a boolean (bool);

in python, a True boolean is 1 when used as a number;

>>> i = -5 

>>> i += 2 

>>> i

-3

>>> i = -5 

>>> i += 2 > 0 

>>> i 

-4

as a rule of thumb, dont test with 1 or 0; test with a random number (2 is also bad but easy to read here);

edited Nov 12 at 6:18

answered Nov 12 at 5:57

Cyker

2,63553245

add a comment |

up vote
1
down vote

accepted

num & bit is not the same as num & bit > 0;

num & bit is a number (int), while num & bit > 0 is a boolean (bool);

in python, a True boolean is 1 when used as a number;

>>> i = -5 

>>> i += 2 

>>> i

-3

>>> i = -5 

>>> i += 2 > 0 

>>> i 

-4

as a rule of thumb, dont test with 1 or 0; test with a random number (2 is also bad but easy to read here);

edited Nov 12 at 6:18

answered Nov 12 at 5:57

Cyker

2,63553245

add a comment |

up vote
1
down vote

accepted

num & bit is not the same as num & bit > 0;

num & bit is a number (int), while num & bit > 0 is a boolean (bool);

in python, a True boolean is 1 when used as a number;

>>> i = -5 

>>> i += 2 

>>> i

-3

>>> i = -5 

>>> i += 2 > 0 

>>> i 

-4

as a rule of thumb, dont test with 1 or 0; test with a random number (2 is also bad but easy to read here);

edited Nov 12 at 6:18

answered Nov 12 at 5:57

Cyker

2,63553245

num & bit is not the same as num & bit > 0;

num & bit is a number (int), while num & bit > 0 is a boolean (bool);

in python, a True boolean is 1 when used as a number;

>>> i = -5 

>>> i += 2 

>>> i

-3

>>> i = -5 

>>> i += 2 > 0 

>>> i 

-4

as a rule of thumb, dont test with 1 or 0; test with a random number (2 is also bad but easy to read here);

edited Nov 12 at 6:18

answered Nov 12 at 5:57

Cyker

2,63553245

edited Nov 12 at 6:18

answered Nov 12 at 5:57

Cyker

2,63553245

answered Nov 12 at 5:57

Cyker

2,63553245

answered Nov 12 at 5:57

Cyker

2,63553245

add a comment |

up vote
1
down vote

It's because some of the num&bit are bigger than 0, so (i.e there was one 8 in my run):

>>> a=8

>>> a>0

True

>>> int(a>0)

1

It's not 8!!!

So that's why.

answered Nov 12 at 5:55

U9-Forward

11.5k2834

add a comment |

up vote
1
down vote

It's because some of the num&bit are bigger than 0, so (i.e there was one 8 in my run):

>>> a=8

>>> a>0

True

>>> int(a>0)

1

It's not 8!!!

So that's why.

answered Nov 12 at 5:55

U9-Forward

11.5k2834

add a comment |

up vote
1
down vote

It's because some of the num&bit are bigger than 0, so (i.e there was one 8 in my run):

>>> a=8

>>> a>0

True

>>> int(a>0)

1

It's not 8!!!

So that's why.

answered Nov 12 at 5:55

U9-Forward

11.5k2834

It's because some of the num&bit are bigger than 0, so (i.e there was one 8 in my run):

>>> a=8

>>> a>0

True

>>> int(a>0)

1

It's not 8!!!

So that's why.

answered Nov 12 at 5:55

U9-Forward

11.5k2834

answered Nov 12 at 5:55

U9-Forward

11.5k2834

answered Nov 12 at 5:55

U9-Forward

11.5k2834

answered Nov 12 at 5:55

U9-Forward

11.5k2834

add a comment |

up vote
0
down vote

In line

    cnt += num & bit

you are incrementing the value of cnt by 1,2,4,8 values.
while in line

    cnt += (num & bit) > 0

first you are validating the statement, which returns 0 or 1 then you are increamenting value by either 0 or 1.
You can cross check it by printing cnt value in both the codes.

answered Nov 12 at 6:16

Ankush Khobragade

134

add a comment |

up vote
0
down vote

In line

    cnt += num & bit

you are incrementing the value of cnt by 1,2,4,8 values.
while in line

    cnt += (num & bit) > 0

first you are validating the statement, which returns 0 or 1 then you are increamenting value by either 0 or 1.
You can cross check it by printing cnt value in both the codes.

answered Nov 12 at 6:16

Ankush Khobragade

134

add a comment |

up vote
0
down vote

In line

    cnt += num & bit

you are incrementing the value of cnt by 1,2,4,8 values.
while in line

    cnt += (num & bit) > 0

first you are validating the statement, which returns 0 or 1 then you are increamenting value by either 0 or 1.
You can cross check it by printing cnt value in both the codes.

answered Nov 12 at 6:16

Ankush Khobragade

134

In line

    cnt += num & bit

you are incrementing the value of cnt by 1,2,4,8 values.
while in line

    cnt += (num & bit) > 0

first you are validating the statement, which returns 0 or 1 then you are increamenting value by either 0 or 1.
You can cross check it by printing cnt value in both the codes.

answered Nov 12 at 6:16

Ankush Khobragade

134

answered Nov 12 at 6:16

Ankush Khobragade

134

answered Nov 12 at 6:16

Ankush Khobragade

134

answered Nov 12 at 6:16

Ankush Khobragade

134

add a comment |

draft saved

draft discarded

Thanks for contributing an answer to Stack Overflow!

Please be sure to answer the question. Provide details and share your research!

But avoid …

Asking for help, clarification, or responding to other answers.

Making statements based on opinion; back them up with references or personal experience.

To learn more, see our tips on writing great answers.

Some of your past answers have not been well-received, and you're in danger of being blocked from answering.

Please pay close attention to the following guidance:

Please be sure to answer the question. Provide details and share your research!

But avoid …

Asking for help, clarification, or responding to other answers.

Making statements based on opinion; back them up with references or personal experience.

To learn more, see our tips on writing great answers.

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

Post as a guest

Name

Required, but never shown

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

Name

Required, but never shown

Name

Required, but never shown

This page is only for reference, If you need detailed information, please check here

搜尋此網誌

Vfrdtyky