golang: read text file line by line of int strings

Question

I am dealing with an input file containing a list of integers as a string

10
..

I have choosen to read it line by line with ReadString('\n') method

The following code

line, error := inputReader.ReadString('\n')
lineStr := string(line)

console output (length and value)

lineStr %v 4
lineStr %v 10

lineStr as a length of "4", maybe because of rune encoding.

Then I have tried several way to convert it to simple integer but with no success.

Ex1

num, _ := strconv.ParseUint(lineStr, 0, 64)

ouputs a num of 0 (should be 10)

Ex2

num, _ := strconv.Atoi(lineStr)

ouputs a num of 0 (should be 10)

Ex3

num, _ := strconv.Atoi("10")

ouputs a num of 10 (ok)

Ex4

num, _ := strconv.ParseUint("10", 0, 64)

ouputs a num of 10 (ok)

string in literal are ok but string from file do not work, what is wrong ?

thanks in advance

Please, check the errors in the examples (and always). They will tell you why it returned 0 (the zero-value) instead of what you are expecting. — siritinga
– siritinga, Commented Mar 11, 2015 at 12:07
I will but this case was so trivial to watch it carefuly but thank you for advise — darul75
– darul75, Commented Mar 11, 2015 at 12:19

csabiftw · Accepted Answer · 2015-03-11 10:30:45Z

4

From the documentation: "ReadString reads until the first occurrence of delim in the input, returning a string containing the data up to and including the delimiter."

Because of this, i suggest you use a scanner, which handles this case like you would expect:

scanner := bufio.NewScanner(file)
for scanner.Scan() {
    lineStr := scanner.Text()
    num, _ := strconv.Atoi(lineStr)
    fmt.Println(lineStr, num)
}

answered Mar 11, 2015 at 10:30

csabiftw

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2 Comments

darul75 Over a year ago

I was re implementing it with a scanner and it works, thank you all

Dave C Over a year ago

Although I like using bufio.Scanner as well, for this specific case a simple fmt.Fscanln(r, &intvar) works well (perhaps wrapping the original reader with bufio.NewReader for more efficient reading depending on the source).

Arjan · Accepted Answer · 2015-03-11 10:28:40Z

1

If you look at the documentation of ReadString you will notice that the string returned will include the delimiter (in your case \n).

ReadString reads until the first occurrence of delim in the input, returning a string containing the data up to and including the delimiter.

And because the length of the line after reading 10 is 4 I would assume that the lines are delimited by \r\n. The easiest way to remove it is using one of the Trim functions (like TrimSpace).

answered Mar 11, 2015 at 10:28

Arjan

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1 Comment

darul75 Over a year ago

I was escaping '\n' but not '\r' thank you, trust me I was reading the documentation about delimiter included but forgot about carriage return char

Elwinar · Accepted Answer · 2015-03-11 10:33:41Z

Citing the documentation of bufio.Reader.ReadString (emphasis mine):

ReadString reads until the first occurrence of delim in the input, returning a string containing the data up to and including the delimiter.

Now, the documentation of strconv.ParseInt

The errors that ParseInt returns have concrete type *NumError and include err.Num = s. If s is empty or contains invalid digits, err.Err = ErrSyntax and the returned value is 0

Your problem is that the ReadString method return also the \n that terminate the line, which is an invalid character for the ParseInt function.

You can check the real error with the following snippet

i, err := strconv.ParseInt(line, 10, 64)
if err != nil {
    switch err.(*strconv.NumError).Err {
        case strconv.ErrSyntax:
            fmt.Println("syntax error")
        case strconv.ErrRange:
            fmt.Println("out of range value")
    }
}

Advice: use strings.TrimSpace to clean your input.

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