Modular•6mo ago

Less Ugly way to concatenate strings

I have been working on a plotting library in mojo that is based on gnuplot. The code works but the string parts are extremely ugly:

...
    fn plotArray(inout self, *data: Float32, lineType:Int32=0, lineColor:Int32=7, title:String="Plot") raises:
        with open("m1v1tf01_temp.dat", "w") as f:
            var string: String = ""
            for dat in data:
                string += FloatToString(dat) + ",\n"
            f.write(string)
        self.gnuplot_gateway.callGNUPlotCommand("plot \"m1v1tf01_temp.dat\" title " + title + " lt " + IntToString(lineType) + " lc " + IntToString(lineColor)) # <<< Ugly strings
...

...
    fn plotArray(inout self, *data: Float32, lineType:Int32=0, lineColor:Int32=7, title:String="Plot") raises:
        with open("m1v1tf01_temp.dat", "w") as f:
            var string: String = ""
            for dat in data:
                string += FloatToString(dat) + ",\n"
            f.write(string)
        self.gnuplot_gateway.callGNUPlotCommand("plot \"m1v1tf01_temp.dat\" title " + title + " lt " + IntToString(lineType) + " lc " + IntToString(lineColor)) # <<< Ugly strings
...

So does anyone know a good way to concatenate strings? Does mojo have f-strings yet?

3 Replies

toasty•6mo ago

I made a naive/simple sprintf function if you want to give it a try. https://github.com/thatstoasty/gojo/blob/main/gojo/fmt/fmt.mojo You could also try an external_call to sprintf. It's been awhile, but I do recall the external call also working for me

fn sprintf[
    *T: AnyType
](s: Pointer[c_char], format: Pointer[c_char], *args: *T) -> c_int:
    """Libc POSIX `sprintf` function
    Reference: https://man7.org/linux/man-pages/man3/fprintf.3p.html
    Fn signature: int sprintf(char *restrict s, const char *restrict format, ...).

    Args: s: A pointer to a buffer to store the result.
        format: A pointer to a C string containing the format.
        args: The optional arguments.
    Returns: The number of bytes written or -1 in case of failure.
    """
    return external_call[
        "sprintf", c_int, Pointer[c_char], Pointer[c_char]  # FnName, RetType  # Args
    ](s, format, args)

fn sprintf[
    *T: AnyType
](s: Pointer[c_char], format: Pointer[c_char], *args: *T) -> c_int:
    """Libc POSIX `sprintf` function
    Reference: https://man7.org/linux/man-pages/man3/fprintf.3p.html
    Fn signature: int sprintf(char *restrict s, const char *restrict format, ...).

    Args: s: A pointer to a buffer to store the result.
        format: A pointer to a C string containing the format.
        args: The optional arguments.
    Returns: The number of bytes written or -1 in case of failure.
    """
    return external_call[
        "sprintf", c_int, Pointer[c_char], Pointer[c_char]  # FnName, RetType  # Args
    ](s, format, args)

Could also use a string builder, if you're constructing a large string. For smaller strings, maybe just sprintf or concat would be better. https://github.com/thatstoasty/gojo/blob/main/gojo/strings/builder.mojo

Hammad Ali•6mo ago

So Mojo, doesn't have str.format and f-strings?

toasty•6mo ago

Not at the moment

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Less Ugly way to concatenate strings