fopen file, read number with fscanf, then write back to same file with fprintf

Question

SoderlingPotro 2022-10-2

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1815895-fopen-file-read-number-with-fscanf-then-write-back-to-same-file-with-fprintf

评论： Walter Roberson 2022-10-12

I would like to open a txt file which has the number 200 in it, read it into MATLAB, add 20, then write it back to the same .txt file so it now contains the number 220.

This does exactly what I want, I can run it once and have the number 220 in the text file.

fileID = fopen('myfile.txt','r');
tmpSales = fscanf(fileID,'%f');
fclose(fileID);
tmpSales = tmpSales+20
fileID = fopen('myfile.txt','w');
fprintf(fileID,'%.2f',tmpSales)
fclose(fileID);

I cannot figure out how to do this without two calls to fopen, I've tried the r+ flag such as follows:

fileID = fopen('myfile.txt','r+');
tmpSales = fscanf(fileID,'%f');
tmpSales = tmpSales+20;
fprintf(fileID,'%.2f',tmpSales)
fclose(fileID);

But it produces this result: 200220.00

Any way to do it with just 1 call to fopen?

Thanks

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Walter Roberson 2022-10-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1815895-fopen-file-read-number-with-fscanf-then-write-back-to-same-file-with-fprintf#answer_1064965

编辑：Walter Roberson 2022-10-2

在 MATLAB Online 中打开

You need to rewind the file, or you need to fseek() to move the file pointer.

%prepare the initial file
filename = 'myfile.txt';
fileID = fopen(filename, 'w'); fprintf(fileID, '%g\n', 200); fclose(fileID);
%now do the work
add_to_file_content(filename);
dbtype(filename)
1     220.00
add_to_file_content(filename);
dbtype(filename)
1     240.00
function add_to_file_content(filename)
    fileID = fopen(filename,'r+');
    tmpSales = fscanf(fileID,'%f');
    tmpSales = tmpSales+20;
    frewind(fileID)
    fprintf(fileID,'%.2f',tmpSales);
    fclose(fileID);
end

2 个评论
显示无隐藏无

SoderlingPotro 2022-10-11

Thank you, I would have never figured this out.

Walter Roberson 2022-10-12

@SoderlingPotro

Note that the above code has a theoretical bug in the case where the result of adding 20 takes fewer printable characters than the input value. For example if there input were -13.00 then the computed value would be 7.00 which takes two fewer characters. When you frewind() or fseek() and write in new data, it does not somehow "shorten" the line it is on. The fprintf() operation there is not one of "replace line with this content", it is "write these characters starting at this location and stop when you run out of characters, even if you were in the middle of a line."

请先登录，再进行评论。

fopen file, read number with fscanf, then write back to same file with fprintf

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

2 个评论
显示无隐藏无

更多回答（0 个）

另请参阅

类别

标签

产品

版本

Community Treasure Hunt

fopen file, read number with fscanf, then write back to same file with fprintf

0 个评论 显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

2 个评论 显示 无隐藏 无

更多回答（0 个）

另请参阅

类别

标签

产品

版本

Community Treasure Hunt

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

2 个评论
显示无隐藏无