Issue with fprintf in a loop

Here is my code :

close all; clear all; clc
g = 1.4;                                 % Specific heat ratio for air
beta = 0:pi/1800:pi/2;                % Range for shock wave angle in rad
m = 0;    
% theta 
for M1 = 1:0.5:10                        % Upstream Mach Number from 1 to 10
    m = m+1;
    p(m)=fprintf('%.2n' , M1);
    %     theta-beta-M relation
    A = ((M1^2)*((sin(beta)).^2))-1;    
    B = ((g+(cos(2*beta)))*M1^2)+2;
    theta = atan(2*cot(beta).*A./B);
    %      max. theta for a M1
    maxtheta(m) = max(theta);                   % max theta for the Mach No.
    maxbeta(m) = beta(theta==maxtheta(m));      % find the beta for max. theta
    plot(theta*180/pi,beta*180/pi,'-b')
    if M1<=5
        x(m)=(theta(beta==60*pi/180)*180/pi);
        if x(m)>0
            txt= sprintf('M1=%d',p(m));
            text(x(m),60,txt)
        end    
    end
    hold on
end
plot(maxtheta*180/pi,maxbeta*180/pi,'-r','Linewidth',1.5)
xlabel('\theta')
ylabel('\beta')
axis([0 50 0 90])

When i don't use the fprintf function, the M1 displayed on the graph have many zeros. SO i am trying to get rid of those useless zeros with fprintf. However when I use Fprintf in my loop it only returns 4 and not 1.0 ; 1.5 ; 2.0 and so on.

Could someone help me on that ?

Thank you

[Stephen23]

Note that the FPRINTF documentation explains that it "returns the number of bytes that fprintf writes". Obtaining the number of bytes is very unlikely to be useful to you. Instead you should format the SPRINTF text in a way that suits your needs.

[Adrien]

Hello,

Thank you for the answer. I am still not really comfortable with the %. And other types of formating. But the two comments below showed me the way

Have a good day

Answer 1

[Voss]

fprintf returns the number of bytes written. It's always zero in this case because '%.2n' is not a valid format.

Instead, use sprintf to construct txt, and use an appropriate format.

close all; clear all; clc
g = 1.4;                                 % Specific heat ratio for air
beta = 0:pi/1800:pi/2;                % Range for shock wave angle in rad
m = 0;    
                                         % theta 
for M1 = 1:0.5:10                        % Upstream Mach Number from 1 to 10
    m = m+1;
    %     theta-beta-M relation
    A = ((M1^2)*((sin(beta)).^2))-1;    
    B = ((g+(cos(2*beta)))*M1^2)+2;
    theta = atan(2*cot(beta).*A./B);
    %      max. theta for a M1
    maxtheta(m) = max(theta);                   % max theta for the Mach No.
    maxbeta(m) = beta(theta==maxtheta(m));      % find the beta for max. theta
    plot(theta*180/pi,beta*180/pi,'-b')
    if M1<=5
        x(m)=(theta(beta==60*pi/180)*180/pi);
        if x(m)>0
            txt= sprintf('M1=%.1f',M1);
            text(x(m),60,txt)
        end    
    end
    hold on
end
plot(maxtheta*180/pi,maxbeta*180/pi,'-r','Linewidth',1.5)
xlabel('\theta')
ylabel('\beta')
axis([0 50 0 90])

[Adrien]

Thank you for the answer !

But whats the difference between %.1f and %.99g ? (Showed on the post below)

[Voss]

%.1f is fixed-point with one digit after the decimal point.

%.99g is 99 significant digits with no trailing zeros.

sprintf('%.1f',1.1)

ans = '1.1'

sprintf('%.99g',1.1)

ans = '1.100000000000000088817841970012523233890533447265625'

https://www.mathworks.com/help/matlab/ref/sprintf.html#btf_bfy-1_sep_shared-formatSpec

[Adrien]

Oh i get it

Thanks

[Voss]

You're welcome! Any questions, let me know. Otherwise, please "Accept" this answer. Thanks!

[Walter Roberson]

%.99g is a quick shorthand that people sometimes use to mean "all of the decimal places in the number". But for that purpose it fails for some of the smaller numbers, which is why I tend to use %.999g myself.

The actual maximum number of decimal places needed is

E = eps(0);
Ed = sprintf('%.9999f', E)

Ed = '0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000494065645841246544176568792868221372365059802614324764425585682500675507270208751865299836361635992379796564695445717730926656710355939796398774796010781878126300713190311404527845817167848982103688718636056998730723050006387409153564984387312473397273169615140031715385398074126238565591171026658556686768187039560310624931945271591492455329305456544401127480129709999541931989409080416563324524757147869014726780159355238611550134803526493472019379026810710749170333222684475333572083243193609238289345836806010601150616980975307834227731832924790498252473077637592724787465608477820373446969953364701797267771758512566055119913150489110145103786273816725095583738973359899366480994116420570263709027924276754456522908753868250641971826553344726562500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000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00'

Ed = regexprep(Ed, '0+$', '');
L = length(Ed) - 2

L = 1074

fmt = sprintf('%%.%gf', L)

fmt = '%.1074f'

Ed2 = sprintf(fmt, E)

Ed2 = '0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000004940656458412465441765687928682213723650598026143247644255856825006755072702087518652998363616359923797965646954457177309266567103559397963987747960107818781263007131903114045278458171678489821036887186360569987307230500063874091535649843873124733972731696151400317153853980741262385655911710266585566867681870395603106249319452715914924553293054565444011274801297099995419319894090804165633245247571478690147267801593552386115501348035264934720193790268107107491703332226844753335720832431936092382893458368060106011506169809753078342277318329247904982524730776375927247874656084778203734469699533647017972677717585125660551199131504891101451037862738167250955837389733598993664809941164205702637090279242767544565229087538682506419718265533447265625'

strcmp(Ed, Ed2)

ans = logical
   1

1074 if you use a %f format. If you use a %g format the maximum needed is 754.

[Stephen23]

"But whats the difference between %.1f and %.99g ? (Showed on the post below)"

For your use-case %.9g would likely be better. Compare:

V = [1,1.1,1.9876];
fprintf('%.1f\n',V) % fixed-point, one fractional digit

1.0
1.1
2.0

fprintf('%.9g\n',V) % nine significant figures, no trailing zeros

1
1.1
1.9876