How to refer to the next reference in a for loop?

Hi I am trying to figure out this problem I'm having where I for example

I enter "123" I get [1,2,3] but the problem is when I enter
"1-23" I want to get [1,-2,3]

I cant figure out how to make it so that when it recognizes there is a negative
it would make the next one a negative (by multiplying it by one)

if I delete the first for it only works for positives

heres my code (sorry I'm a python beginner)

listy = 

for i in a:

    i + 1 = i * -1

for i in a:

    listy.append(i)

asked Nov 26 '18 at 8:17

Sam

334

i + 1 = i * -1 is illegal in Python. Please add a complete example.

– DYZ
Nov 26 '18 at 8:18

How do you obtain or define a? I see the answers assume it's an input string, but I want to clarify.

– Ken Y-N
Nov 26 '18 at 8:24

1

@DYZ: OP says they are a beginner; this could well be the complete example, to the extent of their ability. Be kind :) We require people to be proactive and try their best, not necessarily to be correct.

– Amadan
Nov 26 '18 at 8:24

stackoverflow.com/questions/14785495/… <- duplicate?

– timgeb
Nov 26 '18 at 8:27

@timgeb: No, the question is not about skipping, but modifiying a future element of the iterator (which is an XY question).

– Amadan
Nov 26 '18 at 8:28

|
show 1 more comment

Hi I am trying to figure out this problem I'm having where I for example

I enter "123" I get [1,2,3] but the problem is when I enter
"1-23" I want to get [1,-2,3]

I cant figure out how to make it so that when it recognizes there is a negative
it would make the next one a negative (by multiplying it by one)

if I delete the first for it only works for positives

heres my code (sorry I'm a python beginner)

listy = 

for i in a:

    i + 1 = i * -1

for i in a:

    listy.append(i)

asked Nov 26 '18 at 8:17

Sam

334

i + 1 = i * -1 is illegal in Python. Please add a complete example.

– DYZ
Nov 26 '18 at 8:18

How do you obtain or define a? I see the answers assume it's an input string, but I want to clarify.

– Ken Y-N
Nov 26 '18 at 8:24

1

@DYZ: OP says they are a beginner; this could well be the complete example, to the extent of their ability. Be kind :) We require people to be proactive and try their best, not necessarily to be correct.

– Amadan
Nov 26 '18 at 8:24

stackoverflow.com/questions/14785495/… <- duplicate?

– timgeb
Nov 26 '18 at 8:27

@timgeb: No, the question is not about skipping, but modifiying a future element of the iterator (which is an XY question).

– Amadan
Nov 26 '18 at 8:28

|
show 1 more comment

Hi I am trying to figure out this problem I'm having where I for example

I enter "123" I get [1,2,3] but the problem is when I enter
"1-23" I want to get [1,-2,3]

I cant figure out how to make it so that when it recognizes there is a negative
it would make the next one a negative (by multiplying it by one)

if I delete the first for it only works for positives

heres my code (sorry I'm a python beginner)

listy = 

for i in a:

    i + 1 = i * -1

for i in a:

    listy.append(i)

asked Nov 26 '18 at 8:17

Sam

334

Hi I am trying to figure out this problem I'm having where I for example

I enter "123" I get [1,2,3] but the problem is when I enter
"1-23" I want to get [1,-2,3]

I cant figure out how to make it so that when it recognizes there is a negative
it would make the next one a negative (by multiplying it by one)

if I delete the first for it only works for positives

heres my code (sorry I'm a python beginner)

listy = 

for i in a:

    i + 1 = i * -1

for i in a:

    listy.append(i)

python

asked Nov 26 '18 at 8:17

Sam

334

asked Nov 26 '18 at 8:17

Sam

334

asked Nov 26 '18 at 8:17

Sam

334

asked Nov 26 '18 at 8:17

Sam

334

asked Nov 26 '18 at 8:17

Sam

334

i + 1 = i * -1 is illegal in Python. Please add a complete example.

– DYZ
Nov 26 '18 at 8:18

How do you obtain or define a? I see the answers assume it's an input string, but I want to clarify.

– Ken Y-N
Nov 26 '18 at 8:24

1

@DYZ: OP says they are a beginner; this could well be the complete example, to the extent of their ability. Be kind :) We require people to be proactive and try their best, not necessarily to be correct.

– Amadan
Nov 26 '18 at 8:24

stackoverflow.com/questions/14785495/… <- duplicate?

– timgeb
Nov 26 '18 at 8:27

@timgeb: No, the question is not about skipping, but modifiying a future element of the iterator (which is an XY question).

– Amadan
Nov 26 '18 at 8:28

|
show 1 more comment

i + 1 = i * -1 is illegal in Python. Please add a complete example.

– DYZ
Nov 26 '18 at 8:18

How do you obtain or define a? I see the answers assume it's an input string, but I want to clarify.

– Ken Y-N
Nov 26 '18 at 8:24

1

@DYZ: OP says they are a beginner; this could well be the complete example, to the extent of their ability. Be kind :) We require people to be proactive and try their best, not necessarily to be correct.

– Amadan
Nov 26 '18 at 8:24

stackoverflow.com/questions/14785495/… <- duplicate?

– timgeb
Nov 26 '18 at 8:27

@timgeb: No, the question is not about skipping, but modifiying a future element of the iterator (which is an XY question).

– Amadan
Nov 26 '18 at 8:28

i + 1 = i * -1 is illegal in Python. Please add a complete example.

– DYZ
Nov 26 '18 at 8:18

How do you obtain or define a? I see the answers assume it's an input string, but I want to clarify.

– Ken Y-N
Nov 26 '18 at 8:24

@DYZ: OP says they are a beginner; this could well be the complete example, to the extent of their ability. Be kind :) We require people to be proactive and try their best, not necessarily to be correct.

– Amadan
Nov 26 '18 at 8:24

stackoverflow.com/questions/14785495/… <- duplicate?

– timgeb
Nov 26 '18 at 8:27

@timgeb: No, the question is not about skipping, but modifiying a future element of the iterator (which is an XY question).

– Amadan
Nov 26 '18 at 8:28

|
show 1 more comment

5 Answers
5

active

oldest

votes

You can't do it the way you imagine. One way is to remember the sign, and apply it when it matters:

a = "1-23"

listy = 

sign = 1

for i in a:

    if i == "-":

        sign = -1

    else:

        listy.append(int(i) * sign)

        sign = 1

The other is to use regexp to help you parse the string EDIT: as demonstrated beautifully in DYZ's answer :)

answered Nov 26 '18 at 8:23

Amadan

133k13147198

Thank you, this works for me!

– Sam
Nov 26 '18 at 15:40

add a comment |

Regular expressions can be used to extract single digits ("d"), optionally preceeded by a minus sign ("-?"):

import re

[int(x) for x in re.findall("-?d", "1-23")]

#[1, -2, 3]

[int(x) for x in re.findall("-?d", "123")]

#[1, 2, 3]

edited Nov 26 '18 at 8:29

answered Nov 26 '18 at 8:22

DYZ

28k62150

add a comment |

You can use a flag to check for negative numbers.

listy = 

a = input("numbers: ")

neg_flag = False



for i in a:

    if i == '-':

        neg_flag = True

    else:

        if neg_flag:

            listy.append("-"+i)

            neg_flag = False

    else:

        listy.append(i)



print(listy)

answered Nov 26 '18 at 8:25

bunbun

2,08532449

add a comment |

Assuming you don't want a regex solution, here's a solution using iter()

s='1-23'

res = 



iterator = iter(range(len(s)))

for i in iterator:

    if s[i] == '-':

        res.append(-1*int(s[i+1]))

        next(iterator, None)

    else:

        res.append(int(s[i]))



print(res)

# [1, -2, 3]

By using s[i+1] we're going to the 'next' element in the array. And by using next(iterator, None) we're skipping 1 element in the array

Also, characters are transformed into ints. you can cast them back to str if you'd like.

edited Nov 26 '18 at 8:45

answered Nov 26 '18 at 8:31

Eran Moshe

1,408723

add a comment |

Set a conditional flag to keep track of whether the previous character was a '-':

a = "1-23"



listy = 



negative = False

for i in a:

    if i == '-':

        negative = True

        continue

    if negative == True:

        listy.append((int(i)*-1))

    else:

        listy.append(int(i))

    negative = False



print(listy)

edited Nov 26 '18 at 9:00

ntg

4,51643353

answered Nov 26 '18 at 8:23

Jay

15.3k2165122

add a comment |

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5 Answers
5

active

oldest

votes

5 Answers
5

active

oldest

votes

You can't do it the way you imagine. One way is to remember the sign, and apply it when it matters:

a = "1-23"

listy = 

sign = 1

for i in a:

    if i == "-":

        sign = -1

    else:

        listy.append(int(i) * sign)

        sign = 1

The other is to use regexp to help you parse the string EDIT: as demonstrated beautifully in DYZ's answer :)

answered Nov 26 '18 at 8:23

Amadan

133k13147198

Thank you, this works for me!

– Sam
Nov 26 '18 at 15:40

add a comment |

You can't do it the way you imagine. One way is to remember the sign, and apply it when it matters:

a = "1-23"

listy = 

sign = 1

for i in a:

    if i == "-":

        sign = -1

    else:

        listy.append(int(i) * sign)

        sign = 1

The other is to use regexp to help you parse the string EDIT: as demonstrated beautifully in DYZ's answer :)

answered Nov 26 '18 at 8:23

Amadan

133k13147198

Thank you, this works for me!

– Sam
Nov 26 '18 at 15:40

add a comment |

You can't do it the way you imagine. One way is to remember the sign, and apply it when it matters:

a = "1-23"

listy = 

sign = 1

for i in a:

    if i == "-":

        sign = -1

    else:

        listy.append(int(i) * sign)

        sign = 1

The other is to use regexp to help you parse the string EDIT: as demonstrated beautifully in DYZ's answer :)

answered Nov 26 '18 at 8:23

Amadan

133k13147198

You can't do it the way you imagine. One way is to remember the sign, and apply it when it matters:

a = "1-23"

listy = 

sign = 1

for i in a:

    if i == "-":

        sign = -1

    else:

        listy.append(int(i) * sign)

        sign = 1

The other is to use regexp to help you parse the string EDIT: as demonstrated beautifully in DYZ's answer :)

answered Nov 26 '18 at 8:23

Amadan

133k13147198

answered Nov 26 '18 at 8:23

Amadan

133k13147198

answered Nov 26 '18 at 8:23

Amadan

133k13147198

answered Nov 26 '18 at 8:23

Amadan

133k13147198

Thank you, this works for me!

– Sam
Nov 26 '18 at 15:40

add a comment |

Thank you, this works for me!

– Sam
Nov 26 '18 at 15:40

Thank you, this works for me!

– Sam
Nov 26 '18 at 15:40

add a comment |

Regular expressions can be used to extract single digits ("d"), optionally preceeded by a minus sign ("-?"):

import re

[int(x) for x in re.findall("-?d", "1-23")]

#[1, -2, 3]

[int(x) for x in re.findall("-?d", "123")]

#[1, 2, 3]

edited Nov 26 '18 at 8:29

answered Nov 26 '18 at 8:22

DYZ

28k62150

add a comment |

Regular expressions can be used to extract single digits ("d"), optionally preceeded by a minus sign ("-?"):

import re

[int(x) for x in re.findall("-?d", "1-23")]

#[1, -2, 3]

[int(x) for x in re.findall("-?d", "123")]

#[1, 2, 3]

edited Nov 26 '18 at 8:29

answered Nov 26 '18 at 8:22

DYZ

28k62150

add a comment |

Regular expressions can be used to extract single digits ("d"), optionally preceeded by a minus sign ("-?"):

import re

[int(x) for x in re.findall("-?d", "1-23")]

#[1, -2, 3]

[int(x) for x in re.findall("-?d", "123")]

#[1, 2, 3]

edited Nov 26 '18 at 8:29

answered Nov 26 '18 at 8:22

DYZ

28k62150

Regular expressions can be used to extract single digits ("d"), optionally preceeded by a minus sign ("-?"):

import re

[int(x) for x in re.findall("-?d", "1-23")]

#[1, -2, 3]

[int(x) for x in re.findall("-?d", "123")]

#[1, 2, 3]

edited Nov 26 '18 at 8:29

answered Nov 26 '18 at 8:22

DYZ

28k62150

edited Nov 26 '18 at 8:29

answered Nov 26 '18 at 8:22

DYZ

28k62150

answered Nov 26 '18 at 8:22

DYZ

28k62150

answered Nov 26 '18 at 8:22

DYZ

28k62150

add a comment |

You can use a flag to check for negative numbers.

listy = 

a = input("numbers: ")

neg_flag = False



for i in a:

    if i == '-':

        neg_flag = True

    else:

        if neg_flag:

            listy.append("-"+i)

            neg_flag = False

    else:

        listy.append(i)



print(listy)

answered Nov 26 '18 at 8:25

bunbun

2,08532449

add a comment |

You can use a flag to check for negative numbers.

listy = 

a = input("numbers: ")

neg_flag = False



for i in a:

    if i == '-':

        neg_flag = True

    else:

        if neg_flag:

            listy.append("-"+i)

            neg_flag = False

    else:

        listy.append(i)



print(listy)

answered Nov 26 '18 at 8:25

bunbun

2,08532449

add a comment |

You can use a flag to check for negative numbers.

listy = 

a = input("numbers: ")

neg_flag = False



for i in a:

    if i == '-':

        neg_flag = True

    else:

        if neg_flag:

            listy.append("-"+i)

            neg_flag = False

    else:

        listy.append(i)



print(listy)

answered Nov 26 '18 at 8:25

bunbun

2,08532449

You can use a flag to check for negative numbers.

listy = 

a = input("numbers: ")

neg_flag = False



for i in a:

    if i == '-':

        neg_flag = True

    else:

        if neg_flag:

            listy.append("-"+i)

            neg_flag = False

    else:

        listy.append(i)



print(listy)

answered Nov 26 '18 at 8:25

bunbun

2,08532449

answered Nov 26 '18 at 8:25

bunbun

2,08532449

answered Nov 26 '18 at 8:25

bunbun

2,08532449

answered Nov 26 '18 at 8:25

bunbun

2,08532449

add a comment |

Assuming you don't want a regex solution, here's a solution using iter()

s='1-23'

res = 



iterator = iter(range(len(s)))

for i in iterator:

    if s[i] == '-':

        res.append(-1*int(s[i+1]))

        next(iterator, None)

    else:

        res.append(int(s[i]))



print(res)

# [1, -2, 3]

By using s[i+1] we're going to the 'next' element in the array. And by using next(iterator, None) we're skipping 1 element in the array

Also, characters are transformed into ints. you can cast them back to str if you'd like.

edited Nov 26 '18 at 8:45

answered Nov 26 '18 at 8:31

Eran Moshe

1,408723

add a comment |

Assuming you don't want a regex solution, here's a solution using iter()

s='1-23'

res = 



iterator = iter(range(len(s)))

for i in iterator:

    if s[i] == '-':

        res.append(-1*int(s[i+1]))

        next(iterator, None)

    else:

        res.append(int(s[i]))



print(res)

# [1, -2, 3]

By using s[i+1] we're going to the 'next' element in the array. And by using next(iterator, None) we're skipping 1 element in the array

Also, characters are transformed into ints. you can cast them back to str if you'd like.

edited Nov 26 '18 at 8:45

answered Nov 26 '18 at 8:31

Eran Moshe

1,408723

add a comment |

Assuming you don't want a regex solution, here's a solution using iter()

s='1-23'

res = 



iterator = iter(range(len(s)))

for i in iterator:

    if s[i] == '-':

        res.append(-1*int(s[i+1]))

        next(iterator, None)

    else:

        res.append(int(s[i]))



print(res)

# [1, -2, 3]

By using s[i+1] we're going to the 'next' element in the array. And by using next(iterator, None) we're skipping 1 element in the array

Also, characters are transformed into ints. you can cast them back to str if you'd like.

edited Nov 26 '18 at 8:45

answered Nov 26 '18 at 8:31

Eran Moshe

1,408723

Assuming you don't want a regex solution, here's a solution using iter()

s='1-23'

res = 



iterator = iter(range(len(s)))

for i in iterator:

    if s[i] == '-':

        res.append(-1*int(s[i+1]))

        next(iterator, None)

    else:

        res.append(int(s[i]))



print(res)

# [1, -2, 3]

By using s[i+1] we're going to the 'next' element in the array. And by using next(iterator, None) we're skipping 1 element in the array

Also, characters are transformed into ints. you can cast them back to str if you'd like.

edited Nov 26 '18 at 8:45

answered Nov 26 '18 at 8:31

Eran Moshe

1,408723

edited Nov 26 '18 at 8:45

answered Nov 26 '18 at 8:31

Eran Moshe

1,408723

answered Nov 26 '18 at 8:31

Eran Moshe

1,408723

answered Nov 26 '18 at 8:31

Eran Moshe

1,408723

add a comment |

Set a conditional flag to keep track of whether the previous character was a '-':

a = "1-23"



listy = 



negative = False

for i in a:

    if i == '-':

        negative = True

        continue

    if negative == True:

        listy.append((int(i)*-1))

    else:

        listy.append(int(i))

    negative = False



print(listy)

edited Nov 26 '18 at 9:00

ntg

4,51643353

answered Nov 26 '18 at 8:23

Jay

15.3k2165122

add a comment |

Set a conditional flag to keep track of whether the previous character was a '-':

a = "1-23"



listy = 



negative = False

for i in a:

    if i == '-':

        negative = True

        continue

    if negative == True:

        listy.append((int(i)*-1))

    else:

        listy.append(int(i))

    negative = False



print(listy)

edited Nov 26 '18 at 9:00

ntg

4,51643353

answered Nov 26 '18 at 8:23

Jay

15.3k2165122

add a comment |

Set a conditional flag to keep track of whether the previous character was a '-':

a = "1-23"



listy = 



negative = False

for i in a:

    if i == '-':

        negative = True

        continue

    if negative == True:

        listy.append((int(i)*-1))

    else:

        listy.append(int(i))

    negative = False



print(listy)

edited Nov 26 '18 at 9:00

ntg

4,51643353

answered Nov 26 '18 at 8:23

Jay

15.3k2165122

Set a conditional flag to keep track of whether the previous character was a '-':

a = "1-23"



listy = 



negative = False

for i in a:

    if i == '-':

        negative = True

        continue

    if negative == True:

        listy.append((int(i)*-1))

    else:

        listy.append(int(i))

    negative = False



print(listy)

edited Nov 26 '18 at 9:00

ntg

4,51643353

answered Nov 26 '18 at 8:23

Jay

15.3k2165122

edited Nov 26 '18 at 9:00

ntg

4,51643353

edited Nov 26 '18 at 9:00

ntg

4,51643353

edited Nov 26 '18 at 9:00

ntg

4,51643353

answered Nov 26 '18 at 8:23

Jay

15.3k2165122

answered Nov 26 '18 at 8:23

Jay

15.3k2165122

answered Nov 26 '18 at 8:23

Jay

15.3k2165122

add a comment |

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