overwrite a text file with other data

Ok so here is what im stuck on.

I have a text file called testdata.txt from wordpad. this file contains the data:

2 4 9 5

3 7 8 1

now i need to overwrite this file with the vector [3 6 9].

so now when i open the file testdata.txt from wordpad the data that shows should only be 3 6 9

Answer 1

[Geoff]

Let's say your value is in the variable called v:

v = [3 6 9];

You can use save which will output lots of extra decimal places:

save('testdata.txt', 'v', '-ascii');

Or use file I/O (this example assumes your vector is integer):

fileID = fopen('testdata.txt', 'w');
fprintf(fileID, '%d %d %d\n', v);
fclose(fileID);

[lowcalorie]

ok but when i use

fopen('testdata.txt', 'w');

my file identifier does not equal -1 if testdata.txt does not exist. see because if the file does not exist the file

ID will = -1 and then i have to re-ask the user to input a file that does exist.

another problem is that it also erases everything in my file which is not good because before i overwrite the file i need to do calculations on the data in the file and i cannot do this since this statement erases my data

[lowcalorie]

also my new data is always this stuff and not my 3 values

MATLAB 5.0 MAT-file, Platform: PCWIN, Created on: Thu Mar 15 00:13:02 2012 IM & xœãc``0 b6 æ€Ò À

å3Â13Cb^1% ä „¾ 6 xœãc``ð b6 æ b`…ò˜JƒÔdæ¥eæ¤20@Å3RsròõJ*J ú š§¹ $ xœãc``0 b6 æ€Ò À

å3Â1#CfÒ@ 8 ï - xœãc``° b6 æ€Ò À

å3133IŸÌ<0¤†Ý‘ ( /ðý ) xœãc``0 b6 æ€Ò À

å313˜faÈÍÌ3dòØ™ "yì % xœãc``0 b6 æ€Ò À

å3Â1 Cnfžâ "3á ) xœãc``0 b6 æ€Ò À

å313˜faÈM¬0dòØý˜ #r= & xœãc``0 b6 æ€Ò À

å3Â1 Cnb…åä #s0 # xœãc``0 b6 æ€Ò À

å3Â1#CfÒ 5 ˜ Û . xœãc``p b6 æ€Ò À

å3"av N,K-JLOeà„ʯ^Z owS 7 xœãc``ð b6 æ b`…ò˜J³q~iIZfN*ƒ T<#5''_¯¤¢ª ¨\; $ xœãc``0 b6 æ€Ò À

å3Â1#C%fÒb@ \ ö , xœãc``° b6 æ b`…ò‘0HMb^qyjƒ P$È ;ÿ 6 xœãc``ð b6 æ€Ò À

å313˜fd(Òœ@,Ä«W–Dõ{0Õì ™QD 0 xœãc``p b6 æ b`…ò‘0'¦¤å—–”–@Ô eò€4 m_ & xœãc``0 b6 æ b`…òᘑ! H i

à

[lowcalorie]

i fixed the first 2 problems so my only problem now is just getting this mess to go away and only have my 3 values show up in my file

[Geoff]

That 'mess' is the output from the save() function. I gave you two options. If you use save(), use it as I have shown (with the 'ascii' flag). If you use direct file I/O instead, you can get it to fail if you don't want to overwrite an existing file. You said you did want to overwrite, so I used the 'w' flag.

Answer 2

[Honglei Chen]

Use 'w' option to open the file

fid=fopen('testdata','w');
fprintf(fid,'%d %d %d',3,6,9);
fclose(fid);

For details, see

doc fopen