How to grab first number from each line of a .txt file and store it in a vector?

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Preyanka Dey 2020-2-9

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https://ww2.mathworks.cn/matlabcentral/answers/504553-how-to-grab-first-number-from-each-line-of-a-txt-file-and-store-it-in-a-vector

评论： Preyanka Dey 2020-2-10

input.txt

I want to grab first number from each line and store it in a vector. I am trying in following way. But, the written code is picking all numbers from the file. I have attached here the input.txt file. Can anybody please help me regarding this? Thank you.

My written code:

fid = fopen('input.txt');
matrix = fscanf(fid, '%d', [1,inf]);
disp(matrix)

Matlab Output:

1 3 4 8 2 -1 0 8 3 4 5 6 7 8 9 -2

I wanted to grab only(the first number from each line):

1 3 2 3 4 5 6 7 8

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Answer 1

Walter Roberson 2020-2-9

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https://ww2.mathworks.cn/matlabcentral/answers/504553-how-to-grab-first-number-from-each-line-of-a-txt-file-and-store-it-in-a-vector#answer_414666

在 MATLAB Online 中打开

fid = fopen('input.txt');
matrix = cell2mat( textscan(fid, '%f%*[^\n]') );   %second field skips to end of line
fclose(fid);

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Walter Roberson 2020-2-10

在 MATLAB Online 中打开

%*[^\n] is a single format specification.

The % signals that what follows is a format specification

The * signals that after the data indicated by what follows in the specification is read in, that that data is to be thrown away. You use * for parts of input that you know are there but you have no interest in the value of at the time.

The [] matches any number of characters in a row, each of which individually is consistent with the pattern inside the [] .

Typically what is inside [] indicates which character is to be matched. For example, [02468] would match any one of the characters 0, 2, 4, 6, or 8 . There are shortcuts to indicate ranges, such as [0-9] to indicate any character from the list 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9 . There are also shortcuts to indicate kinds of characters, such as [\d] indicates that the match is to be against a "digit". These special patterns are indicated by a \ before the character that specifies the "kind" to match against. \n is the special pattern for matching the newline character. So [\n] would be for matching any number of newline characters in a row.

However... if the very first character inside the [] is ^ then the test is reversed, and the input only matches for characters that do not meet the test inside the rest of the [] . Therefore the pattern [^\n] is to match any number of characters in a row, each of which is not the newline character.

Together with the * before that to discard input, the %*[^\n] means that you should start from the current character and read any number of characters until you hit a newline character, and you should then discard all of those characters. In other words, %*[^\n] is a pattern to ignore everything until the end of the current line.

Putting this together with what is before that, '%f%*[^\n]' means that on each line, you should skip any leading spaces, then read a number, then ignore everything to the end of the line. And then the next line you again look for a number and ignore whatever is after that on the line, and so on.

Your input file starts with

1
3 4 8

The first time through, the %f would match the 1 and store it to be returned. That would leave you positioned at the newline that is after the 1. Then %*[^\n] would match any number of characters that are not newline, but since you are at a newline already that would be no characters. The "no characters" would be discarded.

Then on the next line, the %f would match the 3 and store it to be returned. That would leave you positioned at space that is after the 3. Then %*[\n] would match characters that are not newlines, so that would match ' 4 8' . And then because of the * it would throw that away.

So the 1 got matched on the first line and stored; the 3 got matched on the second line and stored; the 4 and 8 got deliberately ignored and thrown away.

And so on.

Shuvashish Roy 2020-2-10