Member Avatar for princessotes

Hi,
I have a list of positive digits in a file. All I want to do is to add the negative sign (-) to each digit. Is there a code for this. eg list of digits
1 correspond -1
2 correspond-2
3 correspond -3 etc

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  • Latest Post Latest Post by vegaseat

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If your file truly contains numbers and you want every value to be negative (without accidentally producing things like --3), parse each token as a number and write out its negative magnitude. Doing this line-by-line avoids loading the whole file into memory and correctly handles values that are already negative or have leading plus signs. Using parsing rather than string concatenation also prevents issues if the file contains spaces, tabs, or non-numeric tokens.

Example that forces every parsed value to be negative and writes a new file:

with open('input.txt', 'r', encoding='utf-8') as fin, open('output.txt', 'w', encoding='utf-8') as fout:
    for line in fin:
        tokens = line.split()
        if not tokens:
            fout.write('\n')
            continue
        out = []
        for t in tokens:
            try:
                n = int(t)
            except ValueError:
                try:
                    n = float(t)
                except ValueError:
                    out.append(t)  # leave non-numeric tokens unchanged
                else:
                    out.append(str(-abs(n)))
            else:
                out.append(str(-abs(n)))
        fout.write(' '.join(out) + '\n')

Notes:

  • Use -abs(n) to ensure the result is negative for both positive and already-negative inputs (abs).
  • For money or IDs with decimals that require exactness, use decimal.Decimal instead of float (decimal).
  • Prefer writing to a new file rather than editing in place to avoid data loss (reading and writing files).

Recommended Answers

All 4 Replies

Member Avatar for Gribouillis

The best way to do it is to read your file and write a new file with the negated digits. Could you be more specific about the content of your file (attach your file to a post ?).

Member Avatar for myle

Hi,
I have a list of positive digits in a file. All I want to do is to add the negative sign (-) to each digit. Is there a code for this. eg list of digits
1 correspond -1
2 correspond-2
3 correspond -3 etc

What about trying something similar to:

for i in range(1, 10):
    print("-" + str(i))

you may replace range(1, 10) with a list (simple solution) or a generator of your choice (more sophisticated).

EDIT: I am not sure I understood your question.

Edited by myle because: n/a
Member Avatar for JasonHippy

Do you mean something like this??

numarray = [1,2,-3,4,5,-25]

for n in numarray:
    print "The inverse of", num, "is", num * -1
    #print("The inverse of", num, "is", num*-1)
    # use the above line if you're using python 3.x!

The output from the program is:

>>>
The inverse of 1 is -1
The inverse of 2 is -2
The inverse of -3 is 3
The inverse of 4 is -4
The inverse of 5 is -5
The inverse of -25 is 25
>>>

Cheers for now,
Jas.

Edited by JasonHippy because: n/a
Member Avatar for vegaseat

Since your numeric data comes from a file you will have strings, so you might as well use concatenation ...

# the test data
data = """\
123
44
72
27"""

fname = "numdata.txt"

# write the test file
fout = open(fname, "w")
fout.write(data)
fout.close()

# read the test file
for line in open(fname):
    line = '-' + line.strip()
    print(line)

"""my result -->
-123
-44
-72
-27
"""

If you need to do calculations with the data, convert with float().

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